#include <fcntl.h> int posix_fallocate(int fd, off_t offset, off_t len);
The posix_fallocate() function ensures 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 will 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() adjusts the file size to offset+len. Otherwise, the file size is not changed.
Space allocated with posix_fallocate() is freed by a successful call to creat(2) or open(2) that truncates the size of the file. Space allocated with posix_fallocate() may be freed by a successful call to ftruncate(3C) that reduces the file size to a size smaller than offset+len.
Upon successful completion, posix_fallocate() returns zero. Otherwise, an error number is returned to indicate the error.
The posix_fallocate() function will fail if:
The fd argument is not a valid file descriptor or 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 or equal to zero, or the offset argument is less than zero, or the underlying file system does not support this operation. See NOTES.
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 has a transitional interface for 64-bit file offsets. See lf64(5).
See attributes(5) for descriptions of the following attributes:
The posix_fallocate() function is supported only for regular files residing on UFS filesystems. Attempts to use it with files on any other filesystem type results in an EINVAL error.