posix_fallocate — allocate file space


#include <fcntl.h>

int posix_fallocate(int fd, off_t offset, off_t len);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


_POSIX_C_SOURCE >= 200112L


The function posix_fallocate() ensures that disk space is allocated for the file referred to by the file descriptor fd for the bytes in the range starting at offset and continuing for len bytes. After a successful call to posix_fallocate(), subsequent writes to bytes in the specified range are guaranteed not to fail because of lack of disk space.

If the size of the file is less than offset+len, then the file is increased to this size; otherwise the file size is left unchanged.

Return Value

posix_fallocate() returns zero on success, or an error number on failure. Note that errno is not set.



fd is not a valid file descriptor, or is not opened for writing.


offset+len exceeds the maximum file size.


A signal was caught during execution.


offset was less than 0, or len was less than or equal to 0, or the underlying filesystem does not support the operation.


fd does not refer to a regular file.


There is not enough space left on the device containing the file referred to by fd.


fd refers to a pipe.


posix_fallocate() is available since glibc 2.1.94.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
posix_fallocate() Thread safety MT-Safe (but see Notes)

Conforming to


POSIX.1-2008 says that an implementation shall give the EINVAL error if len was 0, or offset was less than 0. POSIX.1-2001 says that an implementation shall give the EINVAL error if len is less than 0, or offset was less than 0, and may give the error if len equals zero.


In the glibc implementation, posix_fallocate() is implemented using the fallocate(2) system call, which is MT-safe. If the underlying filesystem does not support fallocate(2), then the operation is emulated with the following caveats:

In general, the emulation is not MT-safe. On Linux, applications may use fallocate(2) if they cannot tolerate the emulation caveats. In general, this is only recommended if the application plans to terminate the operation if EOPNOTSUPP is returned, otherwise the application itself will need to implement a fallback with all the same problems as the emulation provided by glibc.

See Also

fallocate(1), fallocate(2), lseek(2), posix_fadvise(2)


This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Referenced By

aria2c(1), fallocate(1), fallocate(2), fio(1), guestfs(3), guestfs-release-notes(1), lftp(1), lseek(2), pmemblk_create(3), pmem_is_pmem(3), pmemlog_create(3), pmemobj_open(3), posix_fadvise(2), rsync(1).

2017-09-15 GNU Linux Programmer's Manual