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

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