fallocate - manipulate file space


#define _GNU_SOURCE #include <fcntl.h>

int fallocate(int fd, int mode, off_t offset ", off_t " len ");


This is a non-portable, Linux-specific system call. For the portable, POSIX.1-specified method of ensuring that space is allocated for a file, see posix_fallocate().

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 successful 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 offset+len.
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 difference 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 allocate 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 system.
ENODEV fd does not refer to a regular file or a directory. (If fd is a pipe or FIFO, a different error results.)
ENOSPC There is not enough space left on the device containing the file referred to by fd.
ENOSYS The file system containing the file referred to by fd does not support this operation.
 The mode is not supported by the file system containing the file referred to by fd.


fallocate() is available on Linux since kernel 2.6.23. Support is provided by glibc since version 2.10.


fallocate() is Linux-specific.


ftruncate(2), posix_fadvise(3), posix_fallocate(3)


This page is part of release 3.23 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at

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