posix_fallocate(3) Library Functions Manual posix_fallocate(3)
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 success-
ful call to posix_fallocate(), subsequent writes to bytes in the speci-
fied 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 in-
creased to this size; otherwise the file size is left unchanged.
RETURN VALUE
posix_fallocate() returns zero on success, or an error number on fail-
ure. 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 li-
braries that don't perform the emulation shown in CAVEATS, 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 CAVEATS) │
└───────────────────┴───────────────┴──────────────────────────────────┘
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 imple-
mentation 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 filesys-
tem 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 an-
other 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 func-
tion 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 gen-
eral, 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 emu-
lation provided by glibc.
SEE ALSO
fallocate(1), fallocate(2), lseek(2), posix_fadvise(2)
Linux man-pages 6.9.1 2024-06-16 posix_fallocate(3)
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