ltrace — A library call tracer
Print (trace) library calls of a program binary:
Count library calls. Print a handy summary at the bottom:
ltrace -c /path/to/program
Trace calls to malloc and free, omit those done by libc:
ltrace -e email@example.com* /path/to/program
Write to file instead of terminal:
ltrace -o file /path/to/program
ltrace [-e filter|-L] [-l|--library=library_pattern] [-x filter] [-S] [-b|--no-signals] [-i] [-w|--where=nr] [-r|-t|-tt|-ttt] [-T] [-F pathlist] [-A maxelts] [-s strsize] [-C|--demangle] [-a|--align column] [-n|--indent nr] [-o|--output filename] [-D|--debug mask] [-u username] [-f] [-p pid] [[--] command [arg ...]]
ltrace -c [-e filter|-L] [-l|--library=library_pattern] [-x filter] [-S] [-o|--output filename] [-f] [-p pid] [[--] command [arg ...]]
ltrace is a program that simply runs the specified command until it exits. It intercepts and records the dynamic library calls which are called by the executed process and the signals which are received by that process. It can also intercept and print the system calls executed by the program.
Its use is very similar to strace(1).
ltrace shows parameters of invoked functions and system calls. To determine what arguments each function has, it needs external declaration of function prototypes. Those are stored in files called prototype libraries--see ltrace.conf(5) for details on the syntax of these files. See the section Prototype Library Discovery to learn how ltrace finds prototype libraries.
- -a, --align column
Align return values in a specific column (default column is 5/8 of screen width).
- -A maxelts
Maximum number of array elements to print before suppressing the rest with an ellipsis ("..."). This also limits number of recursive structure expansions.
- -b, --no-signals
Disable printing of signals recieved by the traced process.
Count time and calls for each library call and report a summary on program exit.
- -C, --demangle
Decode (demangle) low-level symbol names into user-level names. Besides removing any initial underscore prefix used by the system, this makes C++ function names readable.
- -D, --debug mask
Show debugging output of ltrace itself. mask is a number describing which debug messages should be displayed. Use the option -Dh to see what can be used, but note that currently the only reliable debugmask is 77, which shows all debug messages.
- -e filter
A qualifying expression which modifies which library calls to trace. The format of the filter expression is described in the section Filter Expressions. If more than one -e option appears on the command line, the library calls that match any of them are traced. If no -e is given, @MAIN is assumed as a default.
Trace child processes as they are created by currently traced processes as a result of the fork(2) or clone(2) system calls. The new process is attached immediately.
- -F pathlist
Contains a colon-separated list of paths. If a path refers to a directory, that directory is considered when prototype libraries are searched (see the section Prototype Library Discovery). If it refers to a file, that file is imported implicitly to all loaded prototype libraries.
- -h, --help
Show a summary of the options to ltrace and exit.
Print the instruction pointer at the time of the library call.
- -l, --library library_pattern
Display only calls to functions implemented by libraries that match library_pattern. Multiple library patters can be specified with several instances of this option. Syntax of library_pattern is described in section Filter Expressions.
Note that while this option selects calls that might be directed to the selected libraries, there's no actual guarantee that the call won't be directed elsewhere due to e.g. LD_PRELOAD or simply dependency ordering. If you want to make sure that symbols in given library are actually called, use -x @library_pattern instead.
When no -e option is given, don't assume the default action of @MAIN. In practice this means that library calls will not be traced.
- -n, --indent nr
Indent trace output by nr spaces for each level of call nesting. Using this option makes the program flow visualization easy to follow. This indents uselessly also functions that never return, such as service functions for throwing exceptions in the C++ runtime.
- -o, --output filename
Write the trace output to the file filename rather than to stderr.
- -p pid
Attach to the process with the process ID pid and begin tracing. This option can be used together with passing a command to execute. It is possible to attach to several processes by passing more than one option -p.
Print a relative timestamp with each line of the trace. This records the time difference between the beginning of successive lines.
- -s strsize
Specify the maximum string size to print (the default is 32).
Display system calls as well as library calls
Prefix each line of the trace with the time of day.
If given twice, the time printed will include the microseconds.
If given thrice, the time printed will include the microseconds and the leading portion will be printed as the number of seconds since the epoch.
Show the time spent inside each call. This records the time difference between the beginning and the end of each call.
- -u username
Run command with the userid, groupid and supplementary groups of username. This option is only useful when running as root and enables the correct execution of setuid and/or setgid binaries.
- -w, --where nr
Show backtrace of nr stack frames for each traced function. This option enabled only if elfutils or libunwind support was enabled at compile time.
- -x filter
A qualifying expression which modifies which symbol table entry points to trace. The format of the filter expression is described in the section Filter Expressions. If more than one -x option appears on the command line, the symbols that match any of them are traced. No entry points are traced if no -x is given.
- -V, --version
Show the version number of ltrace and exit.
Filter expression is a chain of glob- or regexp-based rules that are used to pick symbols for tracing from libraries that the process uses. Most of it is intuitive, so as an example, the following would trace calls to malloc and free, except those done by libc:
This reads: trace malloc and free, but don't trace anything that comes from libc. Semi-formally, the syntax of the above example looks approximately like this:
Symbol_pattern is used to match symbol names, library_pattern to match library SONAMEs. Both are implicitly globs, but can be regular expressions as well (see below). The glob syntax supports meta-characters * and ? and character classes, similarly to what basic bash globs support. ^ and $ are recognized to mean, respectively, start and end of given name.
Both symbol_pattern and library_pattern have to match the whole name. If you want to match only part of the name, surround it with one or two *'s as appropriate. The exception is if the pattern is not mentioned at all, in which case it's as if the corresponding pattern were *. (So malloc is really malloc@* and @libc.* is really *@libc.*.)
In libraries that don't have an explicit SONAME, basename is taken for SONAME. That holds for main binary as well: /bin/echo has an implicit SONAME of echo. In addition to that, special library pattern MAIN always matches symbols in the main binary and never a library with actual SONAME MAIN (use e.g. ^MAIN or [M]AIN for that).
If the symbol or library pattern is surrounded in slashes (/like this/), then it is considered a regular expression instead. As a shorthand, instead of writing /x/@/y/, you can write /x@y/.
If the library pattern starts with a slash, it is not a SONAME expression, but a path expression, and is matched against the library path name.
The first rule may lack a sign, in which case + is assumed. If, on the other hand, the first rule has a - sign, it is as if there was another rule @ in front of it, which has the effect of tracing complement of given rule.
The above rules are used to construct the set of traced symbols. Each candidate symbol is passed through the chain of above rules. Initially, the symbol is unmarked. If it matches a + rule, it becomes marked, if it matches a - rule, it becomes unmarked again. If, after applying all rules, the symbol is marked, it will be traced.
Prototype Library Discovery
When a library is mapped into the address space of a traced process, ltrace needs to know what the prototypes are of functions that this library implements. For purposes of ltrace, prototype really is a bit more than just type signature: it's also formatting of individual parameters and of return value. These prototypes are stored in files called prototype libraries.
After a library is mapped, ltrace finds out what its SONAME is. It then looks for a file named SONAME.conf--e.g. protolib for libc.so.6 would be in a file called libc.so.6.conf. When such file is found (more about where ltrace looks for these files is below), ltrace reads all prototypes stored therein. When a symbol table entry point (such as those traced by -x) is hit, the prototype is looked up in a prototype library corresponding to the library where the hit occured. When a library call (such as those traced by -e and -l) is hit, the prototype is looked up in all prototype libraries loaded for given process. That is necessary, because a library call is traced in a PLT table of a caller library, but the prototype is described at callee library.
If a library has no SONAME, basename of library file is considered instead. For the main program binary, basename is considered as well (e.g. protolib for /bin/echo would be called echo.conf). If a name corresponding to soname (e.g. libc.so.6.conf) is not found, and the module under consideration is a shared library, ltrace also tries partial matches. Ltrace snips one period after another, retrying the search, until either a protolib is found, or X.so is all that's left. Thus libc.so.conf would be considered, but libc.conf not.
When looking for a prototype library, ltrace potentially looks into several directories. On Linux, those are $XDG_CONFIG_HOME/ltrace, $HOME/.ltrace, X/ltrace for each X in $XDG_CONFIG_DIRS and /usr/share/ltrace. If the environment variable XDG_CONFIG_HOME is not defined, ltrace looks into $HOME/.config/ltrace instead.
There's also a mechanism for loading legacy config files. If $HOME/.ltrace.conf exists it is imported to every loaded prototype library. Similarly for /etc/ltrace.conf. If both exist, both are imported, and $HOME/.ltrace.conf is consulted before /etc/ltrace.conf.
If -F contains any directories, those are searched in precedence to the above system directories, in the same order in which they are mentioned in -F. Any files passed in -F are imported similarly to above legacy config files, before them.
See ltrace.conf(5) for details on the syntax of ltrace prototype library files.
It has most of the bugs stated in strace(1).
It only works on Linux and in a small subset of architectures.
If you would like to report a bug, send a message to the mailing list (firstname.lastname@example.org), or use the reportbug(1) program if you are under the Debian GNU/Linux distribution.
System configuration file
Personal config file, overrides /etc/ltrace.conf
ltrace.conf(5), strace(1), ptrace(2)
guestfish(1), guestfs(3), libzzuf(3), ltrace.conf(5), ptrace(2), strace(1).