posix_fallocate(3) — Linux manual page


POSIX_FALLOCATE(3)      Linux Programmer's Manual     POSIX_FALLOCATE(3)

NAME         top

       posix_fallocate - allocate file space

SYNOPSIS         top

       #include <fcntl.h>

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

   Feature Test Macro Requirements for glibc (see

           _POSIX_C_SOURCE >= 200112L

DESCRIPTION         top

       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

RETURN VALUE         top

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

ERRORS         top

       EBADF  fd is not a valid file descriptor, or is not opened for

       EFBIG  offset+len exceeds the maximum file size.

       EINTR  A signal was caught during execution.

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

       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.

              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 NOTES, such as musl libc.

       ESPIPE fd refers to a pipe.

VERSIONS         top

       posix_fallocate() is available since glibc 2.1.94.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

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

CONFORMING TO         top


       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.

NOTES         top

       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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.13 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

GNU                            2021-03-22             POSIX_FALLOCATE(3)

Pages that refer to this page: fallocate(1)rsync(1)fallocate(2)lseek(2)posix_fadvise(2)system_data_types(7)