#define _GNU_SOURCE /* See feature_test_macros(7) */
int fallocate(int fd, int mode, off_t offset, off_t len);
This is a nonportable, Linux-specific system call. For the portable,
POSIX.1-specified method of ensuring that space is allocated for a
file, see posix_fallocate(3).
fallocate() allows the caller to directly manipulate the allocated disk
space for the file referred to by fd for the byte range starting at
offset and continuing for len bytes.
The mode argument determines the operation to be performed on the given
range. Currently only one flag is supported for mode:
This flag allocates and initializes to zero the disk space
within the range specified by offset and len. After a success-
ful call, subsequent writes into this range are guaranteed not
to fail because of lack of disk space. Preallocating zeroed
blocks beyond the end of the file is useful for optimizing
append workloads. Preallocating blocks does not change the file
size (as reported by stat(2)) even if it is less than off-
If FALLOC_FL_KEEP_SIZE flag is not specified in mode, the default
behavior is almost same as when this flag is specified. The only dif-
ference is that on success, the file size will be changed if offset +
len is greater than the file size. This default behavior closely
resembles the behavior of the posix_fallocate(3) library function, and
is intended as a method of optimally implementing that function.
Because allocation is done in block size chunks, fallocate() may allo-
cate a larger range than that which was specified.
fallocate() returns zero on success, and -1 on failure.
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.
EIO An I/O error occurred while reading from or writing to a file
EPERM The file referred to by fd is marked immutable (see chattr(1)).
ESPIPE fd refers to a pipe or FIFO.
fallocate() is available on Linux since kernel 2.6.23. Support is pro-
vided by glibc since version 2.10.
fallocate() is Linux-specific.
ftruncate(2), posix_fadvise(3), posix_fallocate(3)
This page is part of release 3.35 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://man7.org/linux/man-pages/.
Linux 2011-09-19 FALLOCATE(2)
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