libabigail

The upstream code repository of Libabigail contains several tools written using the library.  They are maintained and released as part of the project.  All tools come with a bash-completion script.

abidiff compares the Application Binary Interfaces (ABI) of two shared libraries in ELF format.  It emits a meaningful report describing the differences between the two ABIs.

This tool can also compare the textual representations of the ABI of two ELF binaries (as emitted by abidw) or an ELF binary against a textual representation of another ELF binary.

For a comprehensive ABI change report that includes changes about function and variable sub-types, the two input shared libraries must be accompanied with their debug information in DWARF format. Otherwise, only ELF symbols that were added or removed are reported.

abidiff [options] <first-shared-library> <second-shared-library>

abidiff loads two default suppression specifications files, merges their content and use it to filter out ABI change reports that might be considered as false positives to users.

• Default system-wide suppression specification file

  It’s located by the optional environment variable LIBABIGAIL_DEFAULT_SYSTEM_SUPPRESSION_FILE.  If that environment variable is not set, then abidiff tries to load the suppression file $libdir/libabigail/libabigail-default.abignore.  If that file is not present, then no default system-wide suppression specification file is loaded.

• Default user suppression specification file.

  It’s located by the optional environment LIBABIGAIL_DEFAULT_USER_SUPPRESSION_FILE.  If that environment variable is not set, then abidiff tries to load the suppression file $HOME/.abignore.  If that file is not present, then no default user suppression specification is loaded.

The exit code of the abidiff command is either 0 if the ABI of the binaries being compared are equal, or non-zero if they differ or if the tool encountered an error.

In the later case, the exit code is a 8-bits-wide bit field in which each bit has a specific meaning.

The first bit, of value 1, named ABIDIFF_ERROR means there was an error.

The second bit, of value 2, named ABIDIFF_USAGE_ERROR means there was an error in the way the user invoked the tool.  It might be set, for instance, if the user invoked the tool with an unknown command line switch, with a wrong number or argument, etc.  If this bit is set, then the ABIDIFF_ERROR bit must be set as well.

The third bit, of value 4, named ABIDIFF_ABI_CHANGE means the ABI of the binaries being compared are different.

The fourth bit, of value 8, named ABIDIFF_ABI_INCOMPATIBLE_CHANGE means the ABI of the binaries compared are different in an incompatible way.  If this bit is set, then the ABIDIFF_ABI_CHANGE bit must be set as well.  If the ABIDIFF_ABI_CHANGE is set and the ABIDIFF_INCOMPATIBLE_CHANGE is NOT set, then it means that the ABIs being compared might or might not be compatible.  In that case, a human being needs to review the ABI changes to decide if they are compatible or not.

Note that, at the moment, there are only a few kinds of ABI changes that would result in setting the flag ABIDIFF_ABI_INCOMPATIBLE_CHANGE. Those ABI changes are either:

• the removal of the symbol of a function or variable that has been defined and exported.
• the modification of the index of a member of a virtual function table (for C++ programs and libraries).

With time, when more ABI change patterns are found to always constitute incompatible ABI changes, we will adapt the code to recognize those cases and set the ABIDIFF_ABI_INCOMPATIBLE_CHANGE accordingly.  So, if you find such patterns, please let us know.

The remaining bits are not used for the moment.

1. Detecting a change in a sub-type of a function:

   $ cat -n test-v0.cc
            1      // Compile this with:
            2      //   g++ -g -Wall -shared -o libtest-v0.so test-v0.cc
            3
            4      struct S0
            5      {
            6        int m0;
            7      };
            8
            9      void
           10      foo(S0* /*parameter_name*/)
           11      {
           12        // do something with parameter_name.
           13      }
   $
   $ cat -n test-v1.cc
            1      // Compile this with:
            2      //   g++ -g -Wall -shared -o libtest-v1.so test-v1.cc
            3
            4      struct type_base
            5      {
            6        int inserted;
            7      };
            8
            9      struct S0 : public type_base
           10      {
           11        int m0;
           12      };
           13
           14      void
           15      foo(S0* /*parameter_name*/)
           16      {
           17        // do something with parameter_name.
           18      }
   $
   $ g++ -g -Wall -shared -o libtest-v0.so test-v0.cc
   $ g++ -g -Wall -shared -o libtest-v1.so test-v1.cc
   $
   $ ../build/tools/abidiff libtest-v0.so libtest-v1.so
   Functions changes summary: 0 Removed, 1 Changed, 0 Added function
   Variables changes summary: 0 Removed, 0 Changed, 0 Added variable
   
   1 function with some indirect sub-type change:
   
     [C]'function void foo(S0*)' has some indirect sub-type changes:
           parameter 0 of type 'S0*' has sub-type changes:
             in pointed to type 'struct S0':
               size changed from 32 to 64 bits
               1 base class insertion:
                 struct type_base
               1 data member change:
                'int S0::m0' offset changed from 0 to 32
   $

2. Detecting another change in a sub-type of a function:

   $ cat -n test-v0.cc
            1      // Compile this with:
            2      //   g++ -g -Wall -shared -o libtest-v0.so test-v0.cc
            3
            4      struct S0
            5      {
            6        int m0;
            7      };
            8
            9      void
           10      foo(S0& /*parameter_name*/)
           11      {
           12        // do something with parameter_name.
           13      }
   $
   $ cat -n test-v1.cc
            1      // Compile this with:
            2      //   g++ -g -Wall -shared -o libtest-v1.so test-v1.cc
            3
            4      struct S0
            5      {
            6        char inserted_member;
            7        int m0;
            8      };
            9
           10      void
           11      foo(S0& /*parameter_name*/)
           12      {
           13        // do something with parameter_name.
           14      }
   $
   $ g++ -g -Wall -shared -o libtest-v0.so test-v0.cc
   $ g++ -g -Wall -shared -o libtest-v1.so test-v1.cc
   $
   $ ../build/tools/abidiff libtest-v0.so libtest-v1.so
   Functions changes summary: 0 Removed, 1 Changed, 0 Added function
   Variables changes summary: 0 Removed, 0 Changed, 0 Added variable
   
   1 function with some indirect sub-type change:
   
     [C]'function void foo(S0&)' has some indirect sub-type changes:
           parameter 0 of type 'S0&' has sub-type changes:
             in referenced type 'struct S0':
               size changed from 32 to 64 bits
               1 data member insertion:
                 'char S0::inserted_member', at offset 0 (in bits)
               1 data member change:
                'int S0::m0' offset changed from 0 to 32
   
   
   $

3. Detecting that functions got removed or added to a library:

   $ cat -n test-v0.cc
            1      // Compile this with:
            2      //   g++ -g -Wall -shared -o libtest-v0.so test-v0.cc
            3
            4      struct S0
            5      {
            6        int m0;
            7      };
            8
            9      void
           10      foo(S0& /*parameter_name*/)
           11      {
           12        // do something with parameter_name.
           13      }
   $
   $ cat -n test-v1.cc
            1      // Compile this with:
            2      //   g++ -g -Wall -shared -o libtest-v1.so test-v1.cc
            3
            4      struct S0
            5      {
            6        char inserted_member;
            7        int m0;
            8      };
            9
           10      void
           11      bar(S0& /*parameter_name*/)
           12      {
           13        // do something with parameter_name.
           14      }
   $
   $ g++ -g -Wall -shared -o libtest-v0.so test-v0.cc
   $ g++ -g -Wall -shared -o libtest-v1.so test-v1.cc
   $
   $ ../build/tools/abidiff libtest-v0.so libtest-v1.so
   Functions changes summary: 1 Removed, 0 Changed, 1 Added functions
   Variables changes summary: 0 Removed, 0 Changed, 0 Added variable
   
   1 Removed function:
     'function void foo(S0&)'    {_Z3fooR2S0}
   
   1 Added function:
     'function void bar(S0&)'    {_Z3barR2S0}
   
   $

abipkgdiff compares the Application Binary Interfaces (ABI) of the ELF binaries contained in two software packages.  The software package formats currently supported are Deb, RPM, tar archives (either compressed or not) and plain directories that contain binaries.

For a comprehensive ABI change report that includes changes about function and variable sub-types, the two input packages must be accompanied with their debug information packages that contain debug information in DWARF format.

abipkgdiff [option] <package1> <package2>

package1 and package2 are the packages that contain the binaries to be compared.

abipkgdiff loads two default suppression specifications files, merges their content and use it to filter out ABI change reports that might be considered as false positives to users.

• Default system-wide suppression specification file

  It’s located by the optional environment variable LIBABIGAIL_DEFAULT_SYSTEM_SUPPRESSION_FILE.  If that environment variable is not set, then abipkgdiff tries to load the suppression file $libdir/libabigail/libabigail-default.abignore.  If that file is not present, then no default system-wide suppression specification file is loaded.

• Default user suppression specification file.

  It’s located by the optional environment LIBABIGAIL_DEFAULT_USER_SUPPRESSION_FILE.  If that environment variable is not set, then abipkgdiff tries to load the suppression file $HOME/.abignore.  If that file is not present, then no default user suppression specification is loaded.

In addition to those default suppression specification files, abipkgdiff will also look inside the packages being compared and if it sees a file that ends with the extension .abignore, then it will consider it as a suppression specification and it will combine it to the default suppression specification that might be already loaded.

The user might as well use the --suppressions option (that is documented further below) to provide a suppression specification.

The exit code of the abipkgdiff command is either 0 if the ABI of the binaries compared are equal, or non-zero if they differ or if the tool encountered an error.

In the later case, the value of the exit code is the same as for the abidiff tool.

kmidiff compares the binary Kernel Module Interfaces of two Linux Kernel trees.  The binary KMI is the interface that the Linux Kernel exposes to its modules.  The trees we are interested in here are the result of the build of the Linux Kernel source tree.

And example of how to build your kernel if you want to compare it to another one using kmidiff is:

git clone -b v4.5 git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git linux/v4.5
cd linux/v4.5
make allyesconfig all

Then install the modules into a directory, for instance, the build/modules sub-directory of the your kernel source tree:

mkdir build/modules
make modules_install INSTALL_MOD_DIR=build/modules

Then construct a list of interfaces exported by the kernel, that you want to compare:

cat > kmi-whitelist << EOF
[kernel_4.5_kmi_whitelist]
 init_task
 schedule
 dev_queue_xmit
 __kmalloc
 printk
EOF

Suppose you’ve done something similar for the v4.6 branch of the Linux kernel, you now have these two directories: linux/v4.5 and linux/v4.6. Their modules are present under the directories linux/v4.5/build/modules and linux/v4.6/build/modules.

To Comparing their KMI kmidiff needs to know where to find the vmlinux binaries and their associated modules.  Here would be what the command line looks like:

kmidiff                                     \
  --kmi-whitelist  linux/v4.6/kmi-whitelist \
  --vmlinux1       linux/v4.5/vmlinux       \
  --vmlinux2       linux/v4.6/vmlinux       \
                   linux/v4.5/build/modules \
                   linux/v4.6/build/modules

More generally, kmidiff is invoked under the form:

kmidiff [options] <first-modules-dir> <second-modules-dir>

By default, kmidiff compares all the interfaces (exported functions and variables) between the Kernel and its modules.  In practice, though, users want to compare a subset of the those interfaces.

Users can then define a “white list” of the interfaces to compare. Such a white list is a just a file in the “INI” format that looks like:

[kernel_version_x86_64_whitelist]
  function1_name
  function2_name
  global_variable1_name
  ....

Note that the name of the section (the name that is between the two brackets) of that INI file just has to end with the string “whitelist”.  So you can define the name you want, for instance [kernel_46_x86_64_whitelist].

Then each line of that whitelist file is the name of an exported function or variable.  Only those interfaces along with the types reachable from their signatures are going to be compared by kmidiff recursively.

Note that kmidiff compares the interfaces exported by the vmlinux binary and by the all of the compiled modules.

• --help | -h

  Display a short help about the command and exit.

• --version | -v

  Display the version of the program and exit.

• --verbose

  Display some verbose messages while executing.

• --debug-info-dir1 | --d1 <di-path1>

  For cases where the debug information for the binaries of the first Linux kernel is split out into separate files, tells kmidiff where to find those separate debug information files.

  Note that di-path must point to the root directory under which the debug information is arranged in a tree-like manner.  Under Red Hat based systems, that directory is usually <root>/usr/lib/debug.

• --debug-info-dir2 | --d2 <di-path2>

  Like --debug-info-dir1, this options tells kmidiff where to find the split debug information for the binaries of the second Linux kernel.

• --vmlinux1 | --l1 <path-to-first-vmlinux>

  Sets the path to the first vmlinux binary to consider.  This has to be the uncompressed vmlinux binary compiled with debug info.

• --vmlinux2 | --l2 <path-to-first-vmlinux>

  Sets the path to the second vmlinux binary to consider.  This has to be the uncompressed vmlinux binary compiled with debug info.

• --kmi-whitelist | -w <path-to-interface-whitelist>

  Set the path to the white list of interfaces to compare while comparing the Kernel Module Interface of the first kernel against the one of the second kernel.

  If this option is not provided, all the exported interfaces of the two kernels are compared.  That takes a lot of times and is not necessarily meaningful because many interface are probably meant to see their reachable types change.

  So please, make sure you always use this option unless you really know what you  are doing.

• --suppressions | --suppr <path-to-suppressions>

  Use a suppression specification file located at path-to-suppressions.  Note that this option can appear multiple times on the command line.  In that case, all of the provided suppression specification files are taken into account.

  Please note that, by default, if this option is not provided, then the default suppression specification files are loaded .

• --impacted-interfaces | -i

  Tell what interfaces got impacted by each individual ABI change.

• --full-impact | -f

  Emit a change report that shows the full impact of each change on exported interfaces.  This is the default kind of report emitted by tools like abidiff or abipkgdiff.

• --show-bytes

  Show sizes and offsets in bytes, not bits.  This option is activated by default.

• --show-bits

  Show sizes and offsets in bits, not bytes.  By default, sizes and offsets are shown in bytes.

• --show-hex

  Show sizes and offsets in hexadecimal base.  This option is activated by default.

• --show-dec

  Show sizes and offsets in decimal base.

abidw reads a shared library in ELF format and emits an XML representation of its ABI to standard output.  The emitted representation includes all the globally defined functions and variables, along with a complete representation of their types.  It also includes a representation of the globally defined ELF symbols of the file.  The input shared library must contain associated debug information in DWARF format.

When given the --linux-tree option, this program can also handle a Linux kernel tree.  That is, a directory tree that contains both the vmlinux binary and Linux kernel modules.  It analyses those Linux kernel binaries and emits an XML representation of the interface between the kernel and its module, to standard output.  In this case, we don’t call it an ABI, but a KMI (Kernel Module Interface).  The emitted KMI includes all the globally defined functions and variables, along with a complete representation of their types.  The input binaries must contain associated debug information in DWARF format.

abidw [options] [<path-to-elf-file>]

As of the version 4 of the DWARF specification, Alternate debug information is a GNU extension to the DWARF specification.  It has however been proposed for inclusion into the upcoming version 5 of the DWARF standard.  You can read more about the GNU extensions to the DWARF standard here.

abicompat checks that an application that links against a given shared library is still ABI compatible with a subsequent version of that library.  If the new version of the library introduces an ABI incompatibility, then abicompat hints the user at what exactly that incompatibility is.

abicompat [options] [<application> <shared-library-first-version> <shared-library-second-version>]

The exit code of the abicompat command is either 0 if the ABI of the binaries being compared are equal, or non-zero if they differ or if the tool encountered an error.

In the later case, the exit code is a 8-bits-wide bit field in which each bit has a specific meaning.

The first bit, of value 1, named ABIDIFF_ERROR means there was an error.

The second bit, of value 2, named ABIDIFF_USAGE_ERROR means there was an error in the way the user invoked the tool.  It might be set, for instance, if the user invoked the tool with an unknown command line switch, with a wrong number or argument, etc.  If this bit is set, then the ABIDIFF_ERROR bit must be set as well.

The third bit, of value 4, named ABIDIFF_ABI_CHANGE means the ABI of the binaries being compared are different.

The fourth bit, of value 8, named ABIDIFF_ABI_INCOMPATIBLE_CHANGE means the ABI of the binaries compared are different in an incompatible way.  If this bit is set, then the ABIDIFF_ABI_CHANGE bit must be set as well.  If the ABIDIFF_ABI_CHANGE is set and the ABIDIFF_INCOMPATIBLE_CHANGE is NOT set, then it means that the ABIs being compared might or might not be compatible.  In that case, a human being needs to review the ABI changes to decide if they are compatible or not.

The remaining bits are not used for the moment.

• Detecting a possible ABI incompatibility in a new shared library version:

  $ cat -n test0.h
       1  struct foo
       2  {
       3    int m0;
       4
       5    foo()
       6      : m0()
       7    {}
       8  };
       9
      10  foo*
      11  first_func();
      12
      13  void
      14  second_func(foo&);
      15
      16  void
      17  third_func();
  $
  
  $ cat -n test-app.cc
       1  // Compile with:
       2  //  g++ -g -Wall -o test-app -L. -ltest-0 test-app.cc
       3
       4  #include "test0.h"
       5
       6  int
       7  main()
       8  {
       9    foo* f = first_func();
      10    second_func(*f);
      11    return 0;
      12  }
  $
  
  $ cat -n test0.cc
       1  // Compile this with:
       2  //  g++ -g -Wall -shared -o libtest-0.so test0.cc
       3
       4  #include "test0.h"
       5
       6  foo*
       7  first_func()
       8  {
       9    foo* f = new foo();
      10    return f;
      11  }
      12
      13  void
      14  second_func(foo&)
      15  {
      16  }
      17
      18  void
      19  third_func()
      20  {
      21  }
  $
  
  $ cat -n test1.h
       1  struct foo
       2  {
       3    int  m0;
       4    char m1; /* <-- a new member got added here! */
       5
       6    foo()
       7    : m0(),
       8      m1()
       9    {}
      10  };
      11
      12  foo*
      13  first_func();
      14
      15  void
      16  second_func(foo&);
      17
      18  void
      19  third_func();
  $
  
  $ cat -n test1.cc
       1  // Compile this with:
       2  //  g++ -g -Wall -shared -o libtest-1.so test1.cc
       3
       4  #include "test1.h"
       5
       6  foo*
       7  first_func()
       8  {
       9    foo* f = new foo();
      10    return f;
      11  }
      12
      13  void
      14  second_func(foo&)
      15  {
      16  }
      17
      18  /* Let's comment out the definition of third_func()
      19     void
      20     third_func()
      21     {
      22     }
      23  */
  $

  • Compile the first and second versions of the libraries: libtest-0.so and libtest-1.so:

    $ g++ -g -Wall -shared -o libtest-0.so test0.cc
    $ g++ -g -Wall -shared -o libtest-1.so test1.cc

  • Compile the application and link it against the first version of the library, creating the test-app binary:

    $ g++ -g -Wall -o test-app -L. -ltest-0.so test-app.cc

  • Now, use abicompat to see if libtest-1.so is ABI compatible with app, with respect to the ABI of libtest-0.so:

    $ abicompat test-app libtest-0.so libtest-1.so
    ELF file 'test-app' might not be ABI compatible with 'libtest-1.so' due to differences with 'libtest-0.so' below:
    Functions changes summary: 0 Removed, 2 Changed, 0 Added functions
    Variables changes summary: 0 Removed, 0 Changed, 0 Added variable
    
    2 functions with some indirect sub-type change:
    
      [C]'function foo* first_func()' has some indirect sub-type changes:
        return type changed:
          in pointed to type 'struct foo':
            size changed from 32 to 64 bits
            1 data member insertion:
              'char foo::m1', at offset 32 (in bits)
      [C]'function void second_func(foo&)' has some indirect sub-type changes:
        parameter 0 of type 'foo&' has sub-type changes:
          referenced type 'struct foo' changed, as reported earlier
    
    $

  • Now use the weak mode of abicompat, that is, providing just the application and the new version of the library:

    $ abicompat --weak-mode test-app libtest-1.so
    functions defined in library
        'libtest-1.so'
    have sub-types that are different from what application
        'test-app'
    expects:
    
      function foo* first_func():
        return type changed:
          in pointed to type 'struct foo':
            size changed from 32 to 64 bits
            1 data member insertion:
              'char foo::m1', at offset 32 (in bits)
    
    $

abilint parses the native XML representation of an ABI as emitted by abidw.  Once it has parsed the XML representation of the ABI, abilint builds and in-memory model from it.  It then tries to save it back to an XML form, to standard output.  If that read-write operation succeeds chances are the input XML ABI representation is meaningful.

Note that the main intent of this tool to help debugging issues in the underlying Libabigail library.

Note also that abilint can also read an ELF input file, build the in-memory model for its ABI, and serialize that model back into XML to standard output.  In that case, the ELF input file must be accompanied with its debug information in the DWARF format.

abilint [options] [<abi-file1>]

• --help

  Display a short help message and exits.

• –version | -v

  Display the version of the program and exit.

• --debug-info-dir <path>

  When reading an ELF input file which debug information is split out into a separate file, this options tells abilint where to find that separate debug information file.

  Note that path must point to the root directory under which the debug information is arranged in a tree-like manner.  Under Red Hat based systems, that directory is usually <root>/usr/lib/debug.

  Note also that this option is not mandatory for split debug information installed by your system’s package manager because then abidiff knows where to find it.

• --diff

  For XML inputs, perform a text diff between the input and the memory model saved back to disk.  This can help to spot issues in the handling of the XML format by the underlying Libabigail library.

• --noout

  Do not display anything on standard output.  The return code of the command is the only way to know if the command succeeded.

• --suppressions | suppr <path-to-suppression-specifications-file>

  Use a suppression specification file located at path-to-suppression-specifications-file.  Note that this option can appear multiple times on the command line.  In that case, all of the provided suppression specification files are taken into account.  ABI artifacts matched by the suppression specifications are suppressed from the output of this tool.

• --headers-dir | --hd <headers-directory-path-1>

  Specifies where to find the public headers of the first shared library that the tool has to consider.  The tool will thus filter out types that are not defined in public headers.

• --stdin | --

  Read the input content from standard input.

• --tu

  Expect the input XML to represent a single translation unit.

fedabipkgdiff compares the ABI of shared libraries in Fedora packages.  It’s a convenient way to do so without having to manually download packages from the Fedora Build System.

fedabipkgdiff knows how to talk with the Fedora Build System to find the right packages versions, their associated debug information and development packages, download them, compare their ABI locally, and report about the possible ABI changes.

Note that by default, this tool reports ABI changes about types that are defined in public header files found in the development packages associated with the packages being compared.  It also reports ABI changes about functions and global variables whose symbols are defined and exported in the ELF binaries found in the packages being compared.

fedabipkgdiff [option] <NVR> ...

fedabipkgdiff loads two default suppression specifications files, merges their content and use it to filter out ABI change reports that might be considered as false positives to users.

• Default system-wide suppression specification file

  It’s located by the optional environment variable LIBABIGAIL_DEFAULT_SYSTEM_SUPPRESSION_FILE.  If that environment variable is not set, then fedabipkgdiff tries to load the suppression file $libdir/libabigail/libabigail-default.abignore.  If that file is not present, then no default system-wide suppression specification file is loaded.

• Default user suppression specification file.

  It’s located by the optional environment LIBABIGAIL_DEFAULT_USER_SUPPRESSION_FILE.  If that environment variable is not set, then fedabipkgdiff tries to load the suppression file $HOME/.abignore.  If that file is not present, then no default user suppression specification is loaded.

Note that a build is a specific version and release of an RPM package. It’s specified by its the package name, version and release. These are specified by the Fedora Naming Guidelines

The exit code of the abipkgdiff command is either 0 if the ABI of the binaries compared are equivalent, or non-zero if they differ or if the tool encountered an error.

In the later case, the value of the exit code is the same as for the abidiff tool.

Below are some usage examples currently supported by fedabipkgdiff.

1. Compare the ABI of binaries in a local package against the ABI of the latest stable package in Fedora 23.

   Suppose you have built just built the httpd package and you want to compare the ABI of the binaries in this locally built package against the ABI of the binaries in the latest http build from Fedora 23.  The command line invocation would be:

   $ fedabipkgdiff --from fc23 ./httpd-2.4.18-2.fc24.x86_64.rpm

2. Compare the ABI of binaries in two local packages.

   Suppose you have built two versions of package httpd, and you want to see what ABI differences between these two versions of RPM files. The command line invocation would be:

   $ fedabipkgdiff path/to/httpd-2.4.23-3.fc23.x86_64.rpm another/path/to/httpd-2.4.23-4.fc24.x86_64.rpm

   All what fedabipkgdiff does happens on local machine without the need of querying or downloading RPMs from Koji.

3. Compare the ABI of binaries in the latest build of the httpd package in Fedora 23 against the ABI of the binaries in the latest build of the same package in 24.

   In this case, note that neither of the two packages are available locally.  The tool is going to talk with the Fedora Build System, determine what the versions and releases of the latest packages are, download them and perform the comparison locally. The command line invocation would be:

   $ fedabipkgdiff --from fc23 --to fc24 httpd

4. Compare the ABI of binaries of two builds of the httpd package, designated their versions and releases.

   If we want to do perform the ABI comparison for all the processor architectures supported by Fedora the command line invocation would be:

   $ fedabipkgdiff httpd-2.8.14.fc23 httpd-2.8.14.fc24

   But if we want to perform the ABI comparison for a specific architecture, say, x86_64, then the command line invocation would be:

   $ fedabipkgdiff httpd-2.8.14.fc23.x86_64 httpd-2.8.14.fc24.x86_64

5. If the use wants to also compare the sub-packages of a given package, she can use the –all-subpackages option.  The first command of the previous example would thus look like:

   $ fedabipkgdiff --all-subpackages httpd-2.8.14.fc23 httpd-2.8.14.fc24

An ABI artifact is a relevant part of the ABI of a shared library or program.  Examples of ABI artifacts are exported types, variables, functions, or ELF symbols exported by a shared library.

The set of ABI artifact for a binary is called an ABI Corpus.

A change in the diff report is considered harmful if it might cause ABI compatibility issues.  That is, it might prevent an application dynamically linked against a given version of a library to keep working with the changed subsequent versions of the same library.

A change in the diff report is considered harmless if it will not cause any ABI compatibility issue.  That is, it will not prevent an application dynamically linked against given version of a library to keep working with the changed subsequent versions of the same library.

By default, abidiff filters harmless changes from the diff report.

A suppression specification file is a way for a user to instruct abidiff, abipkgdiff or any other relevant libabigail tool to avoid emitting reports for changes involving certain ABI artifacts.

It contains directives (or specifications) that describe the set of ABI artifacts to avoid emitting change reports about.

Its syntax is based on a simplified and customized form of Ini File Syntax.  For instance, to specify that change reports on a type named FooPrivateType should be suppressed, one could write this suppression specification:

[suppress_type]
  name = FooPrivateType

If we want to ensure that only change reports about structures named FooPrivateType should be suppressed, we could write:

[suppress_type]
  type_kind = struct
  name = FooPrivateType

But we could also want to suppress change reports avoid typedefs named FooPrivateType.  In that case we would write:

[suppress_type]
  type_kind = typedef
  name = FooPrivateType

Or, we could want to suppress change reports about all struct which names end with the string “PrivateType”:

[suppress_type]
  type_kind = struct
  name_regexp = ^.*PrivateType

Let’s now look at the generic syntax of suppression specification files.

More generally, the format of suppression lists is organized around the concept of property.  Every property has a name and a value, delimited by the = sign.  E.g:

name = value

Leading and trailing white spaces are ignored around property names and values.

The value of some properties might be a regular expression.  In that case, they must comply with the syntax of extended POSIX regular expressions. Note that Libabigail uses the regular expression engine of the GNU C Library.

When trying to match a string that contains a * character, like in the pointer type int*, one must be careful to notice that the character * is a special character in the extended POSIX regular expression syntax.  And that character must be escaped for the regular expression engine.  Thus the regular expression that would match the string int* in a suppression file should be

int\\*

Wait; but then why the two \ characters?  Well, because the \ character is a special character in the Ini File Syntax used for specifying suppressions.  So it must be escaped as well, so that the Ini File parser leaves a \ character intact in the data stream that is handed to the regular expression engine.  Hence the \\ targeted at the Ini File parser.

So, in short, to escape a character in a regular expression, always prefix the character with the \\ sequence.

Suppression specifications can be applied at two different points of the processing pipeline of libabigail.

In the default operating mode called “late suppression mode”, suppression specifications are applied to the result of comparing the in-memory internal representations of two ABIs.  In this mode, if an ABI artifact matches a suppression specification, its changes are not mentioned in the ABI change report.  The internal representation of the “suppressed” changed ABI artifact is still present in memory; it is just not mentioned in the ABI change report.  The change report can still mention statistics about the number of changed ABI artifacts that were suppressed.

There is another operating mode called the “early suppression mode” where suppression specifications are applied during the construction of the in-memory internal representation of a given ABI.  In that mode, if an ABI artifact matches a suppression specification, no in-memory internal representation is built for it.  As a result, no change about the matched ABI artifact is going to be mentioned in the ABI change report and no statistic about the number of suppressed ABI changes is available.  Also, please note that because suppressed ABI artifacts are removed from the in-memory internal representation in this mode, the amount memory used by the internal representation is potentially smaller than the memory consumption in the late suppression mode.

Properties are then grouped into arbitrarily named sections that shall not be nested.  The name of the section is on a line by itself and is surrounded by square brackets, i.e:

[section_name]
property1_name = property1_value
property2_name = property2_value

A section might or might not have properties.  Sections that expect to have properties and which are found nonetheless empty are just ignored.  Properties that are not recognized by the reader are ignored as well.

Each different section can be thought of as being a directive to suppress ABI change reports for a particular kind of ABI artifact.

This directive prevents a given tool from loading a file (binary or not) if its file name matches certain properties.  Thus, if the tool is meant to compare the ABIs of two files, and if the directive prevents it from loading either one of the files, then no comparison is performed.

Note that for the [suppress_file] directive to work, at least one of the following properties must be provided:

The potential properties of this sections are listed below:

• file_name_regexp

  Usage:

  file_name_regexp = <regular-expression>

  Prevents the system from loading the file which name matches the regular expression specified as value of this property.

• file_name_not_regexp

  Usage:

  file_name_not_regexp = <regular-expression>

  Prevents the system from loading the file which name does not match the regular expression specified as value of this property.

• label

  Usage:

  label = <some-value>

  Define a label for the section.  A label is just an informative string that might be used by the tool to refer to a type suppression in error messages.

This directive suppresses report messages about a type change.

Note that for the [suppress_type] directive to work, at least one of the following properties must be provided:

If none of the following properties are provided, then the [suppress_type] directive is simply ignored.

The potential properties of this sections are listed below:

• file_name_regexp

  Usage:

  file_name_regexp = <regular-expression>

  Suppresses change reports about ABI artifacts that are defined in a binary file which name matches the regular expression specified as value of this property.

• file_name_not_regexp

  Usage:

  file_name_not_regexp = <regular-expression>

  Suppresses change reports about ABI artifacts that are defined in a binary file which name does not match the regular expression specified as value of this property.

• soname_regexp

  Usage:

  soname_regexp = <regular-expression>

  Suppresses change reports about ABI artifacts that are defined in a shared library which SONAME property matches the regular expression specified as value of this property.

• soname_not_regexp

  Usage:

  soname_not_regexp = <regular-expression>

  Suppresses change reports about ABI artifacts that are defined in a shared library which SONAME property does not match the regular expression specified as value of this property.

• name_regexp

  Usage:

  name_regexp = <regular-expression>

  Suppresses change reports involving types whose name matches the regular expression specified as value of this property.

• name

  Usage:

  name = <a-value>

  Suppresses change reports involving types whose name equals the value of this property.

• type_kind

  Usage:

  type_kind = class | struct | union | enum |

  array | typedef | builtin

  Suppresses change reports involving a certain kind of type.  The kind of type to suppress change reports for is specified by the possible values listed above:

  • class: suppress change reports for class types. Note that

    even if class types don’t exist for C, this value still triggers the suppression of change reports for struct types, in C.  In C++ however, it should do what it suggests.

  • struct: suppress change reports for struct types in C or C++.

    Note that the value class above is a super-set of this one.

  • union: suppress change reports for union types.
  • enum: suppress change reports for enum types.
  • array: suppress change reports for array types.
  • typedef: suppress change reports for typedef types.
  • builtin: suppress change reports for built-in (or native) types.  Example of built-in types are char, int, unsigned int, etc.

• source_location_not_in

  Usage:

  source_location_not_in = <list-of-file-paths>

  Suppresses change reports involving a type which is defined in a file which path is NOT listed in the value list-of-file-paths.  Note that the value is a comma-separated list of file paths e.g, this property

  source_location_not_in = libabigail/abg-ir.h, libabigail/abg-dwarf-reader.h

  suppresses change reports about all the types that are NOT defined in header files whose path end up with the strings libabigail/abg-ir.h or libabigail/abg-dwarf-reader.h.

• source_location_not_regexp

  Usage:

  source_location_not_regexp = <regular-expression>

  Suppresses change reports involving a type which is defined in a file which path does NOT match the regular expression provided as value of the property. E.g, this property

  source_location_not_regexp = libabigail/abg-.*\\.h

  suppresses change reports involving all the types that are NOT defined in header files whose path match the regular expression provided a value of the property.

• has_data_member_inserted_at

  Usage:

  has_data_member_inserted_at = <offset-in-bit>

  Suppresses change reports involving a type which has at least one data member inserted at an offset specified by the property value offset-in-bit.  The value offset-in-bit is either:

  • an integer value, expressed in bits, which denotes the offset of the insertion point of the data member, starting from the beginning of the relevant structure or class.
  • the keyword end which is a named constant which value equals the offset of the end of the of the structure or class.
  • the function call expression offset_of(data-member-name) where data-member-name is the name of a given data member of the relevant structure or class.  The value of this function call expression is an integer that represents the offset of the data member denoted by data-member-name.
  • the function call expression offset_after(data-member-name) where data-member-name is the name of a given data member of the relevant structure or class.  The value of this function call expression is an integer that represents the offset of the point that comes right after the region occupied by the data member denoted by data-member-name.

• has_data_member_inserted_between

  Usage:

  has_data_member_inserted_between = {<range-begin>, <range-end>}

  Suppresses change reports involving a type which has at least one data mber inserted at an offset that is comprised in the range between range-begin`` and range-end.  Please note that each of the lues range-begin and range-end can be of the same form as the has_data_member_inserted_at property above.

  Usage examples of this properties are:

  has_data_member_inserted_between = {8, 64}

  or:

  has_data_member_inserted_between = {16, end}

  or:

  has_data_member_inserted_between = {offset_after(member1), end}

• has_data_members_inserted_between

  Usage:

  has_data_members_inserted_between = {<sequence-of-ranges>}

  Suppresses change reports involving a type which has multiple data member inserted in various offset ranges.  A usage example of this property is, for instance:

  has_data_members_inserted_between = {{8, 31}, {72, 95}}

  This usage example suppresses change reports involving a type which has data members inserted in bit offset ranges [8 31] and [72 95]. The length of the sequence of ranges or this has_data_members_inserted_between is not bounded; it can be as long as the system can cope with.  The values of the boundaries of the ranges are of the same kind as for the has_data_member_inserted_at property above.

  Another usage example of this property is thus:

  has_data_members_inserted_between =
    {
         {offset_after(member0), offset_of(member1)},
         {72, end}
    }

• accessed_through

  Usage:

  accessed_through = <some-predefined-values>

  Suppress change reports involving a type which is referred to either directly or through a pointer or a reference.  The potential values of this property are the predefined keywords below:

  • direct

    So if the [suppress_type] contains the property description:

    accessed_through = direct

    then changes about a type that is referred-to directly (i.e, not through a pointer or a reference) are going to be suppressed.

  • pointer

    If the accessed_through property is set to the value pointer then changes about a type that is referred-to through a pointer are going to be suppressed.

  • reference

    If the accessed_through property is set to the value reference then changes about a type that is referred-to through a reference are going to be suppressed.

  • reference-or-pointer

    If the accessed_through property is set to the value reference-or-pointer then changes about a type that is referred-to through either a reference or a pointer are going to be suppressed.

  For an extensive example of how to use this property, please check out the example below about suppressing change reports about types accessed either directly or through pointers.

• drop

  Usage:

  drop = yes | no

  If a type is matched by a suppression specification which contains the “drop” property set to “yes” (or to “true”) then the type is not even going to be represented in the internal representation of the ABI being analyzed.  This property makes its enclosing suppression specification to be applied in the early suppression specification mode.  The net effect is that it potentially reduces the memory used to represent the ABI being analyzed.

  Please note that for this property to be effective, the enclosing suppression specification must have at least one of the following properties specified: name_regexp, name, name_regexp, source_location_not_in or source_location_not_regexp.

• label

  Usage:

  label = <some-value>

  Define a label for the section.  A label is just an informative string that might be used by a tool to refer to a type suppression in error messages.

• changed_enumerators

  Usage:

  changed_enumerators = <list-of-enumerators>

  Suppresses change reports involving changes in the value of enumerators of a given enum type.  This property is applied if the type_kind property is set to the value enum, at least.  The value of the changed_enumerators is a comma-separated list of the enumerators that the user expects to change.  For instance:

  changed_enumerators = LAST_ENUMERATORS0, LAST_ENUMERATOR1

This directive suppresses report messages about changes on a set of functions.

Note that for the [suppress_function] directive to work, at least one of the following properties must be provided:

If none of the following properties are provided, then the [suppress_function] directive is simply ignored.

The potential properties of this sections are:

• label

  Usage:

  label = <some-value>

  This property is the same as the label property defined above.

• file_name_regexp

  Usage:

  file_name_regexp = <regular-expression>

  Suppresses change reports about ABI artifacts that are defined in a binary file which name matches the regular expression specified as value of this property.

• file_name_not_regexp

  Usage:

  file_name_not_regexp = <regular-expression>

  Suppresses change reports about ABI artifacts that are defined in a binary file which name does not match the regular expression specified as value of this property.

• soname_regexp

  Usage:

  soname_regexp = <regular-expression>

  Suppresses change reports about ABI artifacts that are defined in a shared library which SONAME property matches the regular expression specified as value of this property.

• soname_not_regexp

  Usage:

  soname_not_regexp = <regular-expression>

  Suppresses change reports about ABI artifacts that are defined in a shared library which SONAME property does not match the regular expression specified as value of this property.

• name

  Usage:

  name = <some-value>

  Suppresses change reports involving functions whose name equals the value of this property.

• name_regexp

  Usage:

  name_regexp = <regular-expression>

  Suppresses change reports involving functions whose name matches the regular expression specified as value of this property.

  Let’s consider the case of functions that have several symbol names. This happens when the underlying symbol for the function has aliases.  Each symbol name is actually one alias name.

  In this case, if the regular expression matches the name of at least one of the aliases names, then it must match the names of all of the aliases of the function for the directive to actually suppress the diff reports for said function.

• name_not_regexp

  Usage:

  name_not_regexp = <regular-expression>

  Suppresses change reports involving functions whose names don’t match the regular expression specified as value of this property.

  The rules for functions that have several symbol names are the same rules as for the name_regexp property above.

• change_kind

  Usage:

  change_kind = <predefined-possible-values>

  Specifies the kind of changes this suppression specification should apply to.  The possible values of this property as well as their meaning are listed below:

  • function-subtype-change

    This suppression specification applies to functions that which have at least one sub-type that has changed.

  • added-function

    This suppression specification applies to functions that have been added to the binary.

  • deleted-function

    This suppression specification applies to functions that have been removed from the binary.

  • all

    This suppression specification applies to functions that have all of the changes above.  Note that not providing the change_kind property at all is equivalent to setting it to the value all.

• parameter

  Usage:

  parameter = <function-parameter-specification>

  Suppresses change reports involving functions whose parameters match the parameter specification indicated as value of this property.

  The format of the function parameter specification is:

  ' <parameter-index> <space> <type-name-or-regular-expression>

  That is, an apostrophe followed by a number that is the index of the parameter, followed by one of several spaces, followed by either the name of the type of the parameter, or a regular expression describing a family of parameter type names.

  If the parameter type name is designated by a regular expression, then said regular expression must be enclosed between two slashes; like /some-regular-expression/.

  The index of the first parameter of the function is zero. Note that for member functions (methods of classes), the this is the first parameter that comes after the implicit “this” pointer parameter.

  Examples of function parameter specifications are:

  '0 int

  Which means, the parameter at index 0, whose type name is int.

  '4 unsigned char*

  Which means, the parameter at index 4, whose type name is unsigned char*.

  '2 /^foo.*&/

  Which means, the parameter at index 2, whose type name starts with the string “foo” and ends with an ‘&’.  In other words, this is the third parameter and it’s a reference on a type that starts with the string “foo”.

• return_type_name

  Usage:

  return_type_name = <some-value>

  Suppresses change reports involving functions whose return type name equals the value of this property.

• return_type_regexp

  Usage:

  return_type_regexp = <regular-expression>

  Suppresses change reports involving functions whose return type name matches the regular expression specified as value of this property.

• symbol_name

  Usage:

  symbol_name = <some-value>

  Suppresses change reports involving functions whose symbol name equals the value of this property.

• symbol_name_regexp

  Usage:

  symbol_name_regexp = <regular-expression>

  Suppresses change reports involving functions whose symbol name matches the regular expression specified as value of this property.

  Let’s consider the case of functions that have several symbol names. This happens when the underlying symbol for the function has aliases.  Each symbol name is actually one alias name.

  In this case, the regular expression must match the names of all of the aliases of the function for the directive to actually suppress the diff reports for said function.

• symbol_version

  Usage:

  symbol_version = <some-value>

  Suppresses change reports involving functions whose symbol version equals the value of this property.

• symbol_version_regexp

  Usage:

  symbol_version_regexp = <regular-expression>

  Suppresses change reports involving functions whose symbol version matches the regular expression specified as value of this property.

• drop

  Usage:

  drop = yes | no

  If a function is matched by a suppression specification which contains the “drop” property set to “yes” (or to “true”) then the function is not even going to be represented in the internal representation of the ABI being analyzed.  This property makes its enclosing suppression specification to be applied in the early suppression specification mode.  The net effect is that it potentially reduces the memory used to represent the ABI being analyzed.

  Please note that for this property to be effective, the enclosing suppression specification must have at least one of the following properties specified: name_regexp, name, name_regexp, source_location_not_in or source_location_not_regexp.

This directive suppresses report messages about changes on a set of variables.

Note that for the [suppress_variable] directive to work, at least one of the following properties must be provided:

If none of the following properties are provided, then the [suppres_variable] directive is simply ignored.

The potential properties of this sections are:

• label

  Usage:

  label = <some-value>

  This property is the same as the label property defined above.

• file_name_regexp

  Usage:

  file_name_regexp = <regular-expression>

  Suppresses change reports about ABI artifacts that are defined in a binary file which name matches the regular expression specified as value of this property.

• file_name_not_regexp

  Usage:

  file_name_not_regexp = <regular-expression>

  Suppresses change reports about ABI artifacts that are defined in a binary file which name does not match the regular expression specified as value of this property.

• soname_regexp

  Usage:

  soname_regexp = <regular-expression>

  Suppresses change reports about ABI artifacts that are defined in a shared library which SONAME property matches the regular expression specified as value of this property.

• soname_not_regexp

  Usage:

  soname_not_regexp = <regular-expression>

  Suppresses change reports about ABI artifacts that are defined in a shared library which SONAME property does not match the regular expression specified as value of this property.

• name

  Usage:

  name = <some-value>

  Suppresses change reports involving variables whose name equals the value of this property.

• name_regexp

  Usage:

  name_regexp = <regular-expression>

  Suppresses change reports involving variables whose name matches the regular expression specified as value of this property.

• change_kind

  Usage:

  change_kind = <predefined-possible-values>

  Specifies the kind of changes this suppression specification should apply to.  The possible values of this property as well as their meaning are the same as when it’s used in the [suppress_function] section.

• symbol_name

  Usage:

  symbol_name = <some-value>

  Suppresses change reports involving variables whose symbol name equals the value of this property.

• symbol_name_regexp

  Usage:

  symbol_name_regexp = <regular-expression>

  Suppresses change reports involving variables whose symbol name matches the regular expression specified as value of this property.

• symbol_version

  Usage:

  symbol_version = <some-value>

  Suppresses change reports involving variables whose symbol version equals the value of this property.

• symbol_version_regexp

  Usage:

  symbol_version_regexp = <regular-expression>

  Suppresses change reports involving variables whose symbol version matches the regular expression specified as value of this property.

• type_name

  Usage:

  type_name = <some-value>

  Suppresses change reports involving variables whose type name equals the value of this property.

• type_name_regexp

  Usage:

  type_name_regexp = <regular-expression>

  Suppresses change reports involving variables whose type name matches the regular expression specified as value of this property.

; or # ASCII character at the beginning of a line indicates a comment.  Comment lines are ignored.

1.

Suppressing change reports about types.

Suppose we have a library named libtest1-v0.so which contains this very useful code:

$ cat -n test1-v0.cc
     1  // A forward declaration for a type considered to be opaque to
     2  // function foo() below.
     3  struct opaque_type;
     4
     5  // This function cannot touch any member of opaque_type.  Hence,
     6  // changes to members of opaque_type should not impact foo, as far as
     7  // ABI is concerned.
     8  void
     9  foo(opaque_type*)
    10  {
    11  }
    12
    13  struct opaque_type
    14  {
    15    int member0;
    16    char member1;
    17  };
$

Let’s change the layout of struct opaque_type by inserting a data member around line 15, leading to a new version of the library, that we shall name libtest1-v1.so:

$ cat -n test1-v1.cc
     1  // A forward declaration for a type considered to be opaque to
     2  // function foo() below.
     3  struct opaque_type;
     4
     5  // This function cannot touch any member of opaque_type;  Hence,
     6  // changes to members of opaque_type should not impact foo, as far as
     7  // ABI is concerned.
     8  void
     9  foo(opaque_type*)
    10  {
    11  }
    12
    13  struct opaque_type
    14  {
    15    char added_member; // <-- a new member got added here now.
    16    int member0;
    17    char member1;
    18  };
$

Let’s compile both examples.  We shall not forget to compile them with debug information generation turned on:

$ g++ -shared -g -Wall -o libtest1-v0.so test1-v0.cc
$ g++ -shared -g -Wall -o libtest1-v1.so test1-v1.cc

Let’s ask abidiff which ABI differences it sees between libtest1-v0.so and libtest1-v1.so:

$ abidiff libtest1-v0.so libtest1-v1.so
Functions changes summary: 0 Removed, 1 Changed, 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

1 function with some indirect sub-type change:

  [C]'function void foo(opaque_type*)' has some indirect sub-type changes:
    parameter 0 of type 'opaque_type*' has sub-type changes:
      in pointed to type 'struct opaque_type':
        size changed from 64 to 96 bits
        1 data member insertion:
          'char opaque_type::added_member', at offset 0 (in bits)
        2 data member changes:
         'int opaque_type::member0' offset changed from 0 to 32
         'char opaque_type::member1' offset changed from 32 to 64

So abidiff reports that the opaque_type’s layout has changed in a significant way, as far as ABI implications are concerned, in theory.  After all, a sub-type (struct opaque_type) of an exported function (foo()) has seen its layout change.  This might have non negligible ABI implications.  But in practice here, the programmer of the litest1-v1.so library knows that the “soft” contract between the function foo() and the type struct opaque_type is to stay away from the data members of the type. So layout changes of struct opaque_type should not impact foo().

Now to teach abidiff about this soft contract and have it avoid emitting what amounts to false positives in this case, we write the suppression specification file below:

$ cat test1.suppr
[suppress_type]
  type_kind = struct
  name = opaque_type

Translated in plain English, this suppression specification would read: “Do not emit change reports about a struct which name is opaque_type”.

Let’s now invoke abidiff on the two versions of the library again, but this time with the suppression specification:

$ abidiff --suppressions test1.suppr libtest1-v0.so libtest1-v1.so
Functions changes summary: 0 Removed, 0 Changed (1 filtered out), 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

As you can see, abidiff does not report the change anymore; it tells us that it was filtered out instead.

Suppressing change reports about types with data member insertions

Suppose the first version of a library named libtest3-v0.so has this source code:

/* Compile this with:
     gcc -g -Wall -shared -o libtest3-v0.so test3-v0.c
 */

struct S
{
  char member0;
  int member1; /*
                  between member1 and member2, there is some padding,
                  at least on some popular platforms.  On
                  these platforms, adding a small enough data
                  member into that padding shouldn't change
                  the offset of member1.  Right?
                */
};

int
foo(struct S* s)
{
  return s->member0 + s->member1;
}

Now, suppose the second version of the library named libtest3-v1.so has this source code in which a data member has been added in the padding space of struct S and another data member has been added at its end:

/* Compile this with:
     gcc -g -Wall -shared -o libtest3-v1.so test3-v1.c
 */

struct S
{
  char member0;
  char inserted1; /* <---- A data member has been added here...  */
  int member1;
  char inserted2; /* <---- ... and another one has been added here.  */
};

int
foo(struct S* s)
{
  return s->member0 + s->member1;
}

In libtest3-v1.so, adding char data members S::inserted1 and S::inserted2 can be considered harmless (from an ABI compatibility perspective), at least on the x86 platform, because that doesn’t change the offsets of the data members S::member0 and S::member1.  But then running abidiff on these two versions of library yields:

$ abidiff libtest3-v0.so libtest3-v1.so
Functions changes summary: 0 Removed, 1 Changed, 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

1 function with some indirect sub-type change:

  [C]'function int foo(S*)' has some indirect sub-type changes:
    parameter 0 of type 'S*' has sub-type changes:
      in pointed to type 'struct S':
        type size changed from 64 to 96 bits
        2 data member insertions:
          'char S::inserted1', at offset 8 (in bits)
          'char S::inserted2', at offset 64 (in bits)
$

That is, abidiff shows us the two changes, even though we (the developers of that very involved library) know that these changes are harmless in this particular context.

Luckily, we can devise a suppression specification that essentially tells abidiff to filter out change reports about adding a data member between S::member0 and S::member1, and adding a data member at the end of struct S.  We have written such a suppression specification in a file called test3-1.suppr and it unsurprisingly looks like:

[suppress_type]
  name = S
  has_data_member_inserted_between = {offset_after(member0), offset_of(member1)}
  has_data_member_inserted_at = end

Now running abidiff with this suppression specification yields:

$ ../build/tools/abidiff --suppressions test3-1.suppr libtest3-v0.so libtest3-v1.so
Functions changes summary: 0 Removed, 0 Changed (1 filtered out), 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

$

Hooora! \o/ (I guess)

Suppressing change reports about types accessed either directly or through pointers

Suppose we have a first version of an object file which source code is the file widget-v0.cc below:

// Compile with: g++ -g -c widget-v0.cc

struct widget
{
  int x;
  int y;

  widget()
    :x(), y()
  {}
};

void
fun0(widget*)
{
  // .. do stuff here.
}

void
fun1(widget&)
{
  // .. do stuff here ..
}

void
fun2(widget w)
{
  // ... do other stuff here ...
}

Now suppose in the second version of that file, named widget-v1.cc, we have added some data members at the end of the type struct widget; here is what the content of that file would look like:

// Compile with: g++ -g -c widget-v1.cc

struct widget
{
  int x;
  int y;
  int w; // We have added these two new data members here ..
  int h; // ... and here.

  widget()
    : x(), y(), w(), h()
  {}
};

void
fun0(widget*)
{
  // .. do stuff here.
}

void
fun1(widget&)
{
  // .. do stuff here ..
}

void
fun2(widget w)
{
  // ... do other stuff here ...
}

When we invoke abidiff on the object files resulting from the compilation of the two file above, here is what we get:

 $ abidiff widget-v0.o widget-v1.o
 Functions changes summary: 0 Removed, 2 Changed (1 filtered out), 0 Added functions
 Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

 2 functions with some indirect sub-type change:

   [C]'function void fun0(widget*)' has some indirect sub-type changes:
     parameter 1 of type 'widget*' has sub-type changes:
       in pointed to type 'struct widget':
         type size changed from 64 to 128 bits
         2 data member insertions:
           'int widget::w', at offset 64 (in bits)
           'int widget::h', at offset 96 (in bits)

   [C]'function void fun2(widget)' has some indirect sub-type changes:
     parameter 1 of type 'struct widget' has sub-type changes:
       details were reported earlier
$

I guess a little bit of explaining is due here.  abidiff detects that two data member got added at the end of struct widget.  it also tells us that the type change impacts the exported function fun0() which uses the type struct widget through a pointer, in its signature.

Careful readers will notice that the change to struct widget also impacts the exported function fun1(), that uses type struct widget through a reference.  But then abidiff doesn’t tell us about the impact on that function fun1() because it has evaluated that change as being redundant with the change it reported on fun0().  It has thus filtered it out, to avoid cluttering the output with noise.

Redundancy detection and filtering is fine and helpful to avoid burying the important information in a sea of noise.  However, it must be treated with care, by fear of mistakenly filtering out relevant and important information.

That is why abidiff tells us about the impact that the change to struct widget has on function fun2().  In this case, that function uses the type struct widget directly (in its signature).  It does not use it via a pointer or a reference. In this case, the direct use of this type causes fun2() to be exposed to a potentially harmful ABI change.  Hence, the report about fun2() is not filtered out, even though it’s about that same change on struct widget.

To go further in suppressing reports about changes that are harmless and keeping only those that we know are harmful, we would like to go tell abidiff to suppress reports about this particular struct widget change when it impacts uses of struct widget through a pointer or reference.  In other words, suppress the change reports about fun0() and fun1().  We would then write this suppression specification, in file widget.suppr:

[suppress_type]
  name = widget
  type_kind = struct
  has_data_member_inserted_at = end
  accessed_through = reference-or-pointer

  # So this suppression specification says to suppress reports about
  # the type 'struct widget', if this type was added some data member
  # at its end, and if the change impacts uses of the type through a
  # reference or a pointer.

Invoking abidiff on widget-v0.o and widget-v1.o with this suppression specification yields:

$ abidiff --suppressions widget.suppr widget-v0.o widget-v1.o
Functions changes summary: 0 Removed, 1 Changed (2 filtered out), 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

1 function with some indirect sub-type change:

  [C]'function void fun2(widget)' has some indirect sub-type changes:
    parameter 1 of type 'struct widget' has sub-type changes:
      type size changed from 64 to 128 bits
      2 data member insertions:
        'int widget::w', at offset 64 (in bits)
        'int widget::h', at offset 96 (in bits)
$

As expected, I guess.

Suppressing change reports about functions.

Suppose we have a first version a library named libtest2-v0.so whose source code is:

 $ cat -n test2-v0.cc

  1     struct S1
  2     {
  3       int m0;
  4
  5       S1()
  6         : m0()
  7       {}
  8     };
  9
 10     struct S2
 11     {
 12       int m0;
 13
 14       S2()
 15         : m0()
 16       {}
 17     };
 18
 19     struct S3
 20     {
 21       int m0;
 22
 23       S3()
 24         : m0()
 25       {}
 26     };
 27
 28     int
 29     func(S1&)
 30     {
 31       // suppose the code does something with the argument.
 32       return 0;
 33
 34     }
 35
 36     char
 37     func(S2*)
 38     {
 39       // suppose the code does something with the argument.
 40       return 0;
 41     }
 42
 43     unsigned
 44     func(S3)
 45     {
 46       // suppose the code does something with the argument.
 47       return 0;
 48     }
$

And then we come up with a second version libtest2-v1.so of that library; the source code is modified by making the structures S1, S2, S3 inherit another struct:

$ cat -n test2-v1.cc
      1 struct base_type
      2 {
      3   int m_inserted;
      4 };
      5
      6 struct S1 : public base_type // <--- S1 now has base_type as its base
      7                              // type.
      8 {
      9   int m0;
     10
     11   S1()
     12     : m0()
     13   {}
     14 };
     15
     16 struct S2 : public base_type // <--- S2 now has base_type as its base
     17                              // type.
     18 {
     19   int m0;
     20
     21   S2()
     22     : m0()
     23   {}
     24 };
     25
     26 struct S3 : public base_type // <--- S3 now has base_type as its base
     27                              // type.
     28 {
     29   int m0;
     30
     31   S3()
     32     : m0()
     33   {}
     34 };
     35
     36 int
     37 func(S1&)
     38 {
     39   // suppose the code does something with the argument.
     40   return 0;
     41
     42 }
     43
     44 char
     45 func(S2*)
     46 {
     47   // suppose the code does something with the argument.
     48   return 0;
     49 }
     50
     51 unsigned
     52 func(S3)
     53 {
     54   // suppose the code does something with the argument.
     55   return 0;
     56 }
 $

Now let’s build the two libraries:

g++ -Wall -g -shared -o libtest2-v0.so test2-v0.cc
g++ -Wall -g -shared -o libtest2-v0.so test2-v0.cc

Let’s look at the output of abidiff:

$ abidiff libtest2-v0.so libtest2-v1.so
Functions changes summary: 0 Removed, 3 Changed, 0 Added functions
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

3 functions with some indirect sub-type change:

  [C]'function unsigned int func(S3)' has some indirect sub-type changes:
    parameter 0 of type 'struct S3' has sub-type changes:
      size changed from 32 to 64 bits
      1 base class insertion:
        struct base_type
      1 data member change:
       'int S3::m0' offset changed from 0 to 32

  [C]'function char func(S2*)' has some indirect sub-type changes:
    parameter 0 of type 'S2*' has sub-type changes:
      in pointed to type 'struct S2':
        size changed from 32 to 64 bits
        1 base class insertion:
          struct base_type
        1 data member change:
         'int S2::m0' offset changed from 0 to 32

  [C]'function int func(S1&)' has some indirect sub-type changes:
    parameter 0 of type 'S1&' has sub-type changes:
      in referenced type 'struct S1':
        size changed from 32 to 64 bits
        1 base class insertion:
          struct base_type
        1 data member change:
         'int S1::m0' offset changed from 0 to 32
$

Let’s tell abidiff to avoid showing us the differences on the overloads of func that takes either a pointer or a reference. For that, we author this simple suppression specification:

$ cat -n libtest2.suppr
     1 [suppress_function]
     2   name = func
     3   parameter = '0 S1&
     4
     5 [suppress_function]
     6   name = func
     7   parameter = '0 S2*
$

And then let’s invoke abidiff with the suppression specification:

$ ../build/tools/abidiff --suppressions libtest2.suppr libtest2-v0.so libtest2-v1.so
Functions changes summary: 0 Removed, 1 Changed (2 filtered out), 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

1 function with some indirect sub-type change:

       [C]'function unsigned int func(S3)' has some indirect sub-type changes:
         parameter 0 of type 'struct S3' has sub-type changes:
           size changed from 32 to 64 bits
           1 base class insertion:
             struct base_type
           1 data member change:
            'int S3::m0' offset changed from 0 to 32

The suppression specification could be reduced using regular expressions:

$ cat -n libtest2-1.suppr
          1   [suppress_function]
          2     name = func
          3     parameter = '0 /^S.(&|\\*)/
$

$ ../build/tools/abidiff --suppressions libtest2-1.suppr libtest2-v0.so libtest2-v1.so
Functions changes summary: 0 Removed, 1 Changed (2 filtered out), 0 Added function
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

1 function with some indirect sub-type change:

       [C]'function unsigned int func(S3)' has some indirect sub-type changes:
         parameter 0 of type 'struct S3' has sub-type changes:
           size changed from 32 to 64 bits
           1 base class insertion:
             struct base_type
           1 data member change:
            'int S3::m0' offset changed from 0 to 32

$