posix_fallocate

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



NAME
       posix_fallocate - allocate file space

SYNOPSIS
       #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_fallocate():
           _POSIX_C_SOURCE >= 200112L

DESCRIPTION
       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.

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

       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 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.

       ESPIPE fd refers to a pipe.

VERSIONS
       posix_fallocate() is available since glibc 2.1.94.

ATTRIBUTES
       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.1-2001.

       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
       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.

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

COLOPHON
       This page is part of release 5.08 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/.



GNU                                2017-09-15                 POSIX_FALLOCATE(3)