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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | CAVEATS | SEE ALSO | COLOPHON |
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posix_fallocate(3) Library Functions Manual posix_fallocate(3)
posix_fallocate - allocate file space
Standard C library (libc, -lc)
#include <fcntl.h>
int posix_fallocate(int fd, off_t offset, off_t size);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
posix_fallocate():
_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 size
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+size, then the file is
increased to this size; otherwise the file size is left unchanged.
posix_fallocate() returns zero on success, or an error number on
failure. Note that errno is not set.
EBADF fd is not a valid file descriptor, or is not opened for
writing.
EFBIG offset+size exceeds the maximum file size.
EINTR A signal was caught during execution.
EINVAL offset was less than 0, or size was less than or equal to
0, or the underlying filesystem does not support the
operation.
ENODEV fd does not refer to a regular file.
ENOSPC There is not enough space left on the device containing the
file referred to by fd.
EOPNOTSUPP
The filesystem containing the file referred to by fd does
not support this operation. This error code can be
returned by C libraries that don't perform the emulation
shown in CAVEATS, such as musl libc.
ESPIPE fd refers to a pipe.
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────────┬───────────────┬────────────────────────────┐
│ Interface │ Attribute │ Value │
├───────────────────┼───────────────┼────────────────────────────┤
│ posix_fallocate() │ Thread safety │ MT-Safe (but see CAVEATS) │
└───────────────────┴───────────────┴────────────────────────────┘
POSIX.1-2008.
glibc 2.1.94. POSIX.1-2001
POSIX.1-2008 says that an implementation shall give the EINVAL
error if size was 0, or offset was less than 0. POSIX.1-2001 says
that an implementation shall give the EINVAL error if size is less
than 0, or offset was less than 0, and may give the error if size
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:
• The emulation is inefficient.
• There is a race condition where concurrent writes from another
thread or process could be overwritten with null bytes.
• There is a race condition where concurrent file size increases
by another thread or process could result in a file whose size
is smaller than expected.
• If fd has been opened with the O_APPEND or O_WRONLY flags, the
function fails with the error EBADF.
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.
fallocate(1), fallocate(2), lseek(2), posix_fadvise(2)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 posix_fallocate(3)
Pages that refer to this page: fallocate(1), rsync(1), fallocate(2), lseek(2), posix_fadvise(2), off_t(3type)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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