FALLOCATE(2) Linux Programmer s Manual FALLOCATE(2)
NAME fallocate - manipulate file space
SYNOPSIS #define _GNU_SOURCE #include <fcntl.h>
int fallocate(int fd, int mode, off_t offset, off_t len);
DESCRIPTION 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. Details of the supported operations are given in the subsec- tions below.
Allocating disk space The default operation (i.e., mode is zero) of fallocate() allocates and initializes to zero the disk space within the range specified by offset and len. The file size (as reported by stat(2)) 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 func- tion, and is intended as a method of optimally implementing that func- tion.
After a successful call, subsequent writes into the range specified by offset and len are guaranteed not to fail because of lack of disk space.
If the FALLOC_FL_KEEP_SIZE flag is specified in mode, the behavior of the call is similar, but the file size will not be changed even if off- set+len is greater than the file size. Preallocating zeroed blocks beyond the end of the file in this manner is useful for optimizing append workloads.
Because allocation is done in block size chunks, fallocate() may allo- cate a larger range of disk space than was specified.
Deallocating file space Specifying the FALLOC_FL_PUNCH_HOLE flag (available since Linux 2.6.38) in mode deallocates space (i.e., creates a hole) in the byte range starting at offset and continuing for len bytes. Within the specified range, partial filesystem blocks are zeroed, and whole filesystem blocks are removed from the file. After a successful call, subsequent reads from this range will return zeroes.
The FALLOC_FL_PUNCH_HOLE flag must be ORed with FALLOC_FL_KEEP_SIZE in mode; in other words, even when punching off the end of the file, the file size (as reported by stat(2)) does not change.
Not all filesystems support FALLOC_FL_PUNCH_HOLE; if a filesystem doesnt support the operation, an error is returned.
RETURN VALUE fallocate() returns zero on success, and -1 on failure.
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.
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.
EOPNOTSUPP The mode is not supported by the file system containing the file referred to by fd.
VERSIONS fallocate() is available on Linux since kernel 2.6.23. Support is pro- vided by glibc since version 2.10.
CONFORMING TO fallocate() is Linux-specific.
SEE ALSO ftruncate(2), posix_fadvise(3), posix_fallocate(3)
COLOPHON This page is part of release 3.22 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
Linux 2009-03-13 FALLOCATE(2)