NAME

       posix_fallocate - allocate file space

LIBRARY

       Standard C library (libc, -lc)

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.

       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 NOTES, such as musl
              libc.

       ESPIPE fd refers to a pipe.

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)                                             │
       └──────────────────┴───────────────┴─────────────────────────────────────────────────────────────────────┘

STANDARDS

       POSIX.1-2008.

HISTORY

       glibc 2.1.94.  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.

CAVEATS

       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)