posix_fallocate man page


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

posix_fallocate — file space control (ADVANCED REALTIME)


#include <fcntl.h>

int posix_fallocate(int fd, off_t offset, off_t len);


The posix_fallocate() function shall ensure that any required storage for regular file data starting at offset and continuing for len bytes is allocated on the file system storage media. If posix_fallocate() returns successfully, subsequent writes to the specified file data shall not fail due to the lack of free space on the file system storage media.

If the offset+len is beyond the current file size, then posix_fallocate() shall adjust the file size to offset+len. Otherwise, the file size shall not be changed.

It is implementation-defined whether a previous posix_fadvise() call influences allocation strategy.

Space allocated via posix_fallocate() shall be freed by a successful call to creat() or open() that truncates the size of the file. Space allocated via posix_fallocate() may be freed by a successful call to ftruncate() that reduces the file size to a size smaller than offset+len.

Return Value

Upon successful completion, posix_fallocate() shall return zero; otherwise, an error number shall be returned to indicate the error.


The posix_fallocate() function shall fail if:

The fd argument is not a valid file descriptor.
The fd argument references a file that was opened without write permission.
The value of offset+len is greater than the maximum file size.
A signal was caught during execution.
The len argument is less than zero, or the offset argument is less than zero, or the underlying file system does not support this operation.
An I/O error occurred while reading from or writing to a file system.
The fd argument does not refer to a regular file.
There is insufficient free space remaining on the file system storage media.
The fd argument is associated with a pipe or FIFO.

The posix_fallocate() function may fail if:

The len argument is zero.

The following sections are informative.

Application Usage

The posix_fallocate() function is part of the Advisory Information option and need not be provided on all implementations.

See Also

creat(), ftruncate(), open(), unlink()

The Base Definitions volume of POSIX.1‐2008, <fcntl.h>