Jump to content United States-English
HP.com Home Products and Services Support and Drivers Solutions How to Buy
» Contact HP
More options
HP.com home
HP-UX Reference > P


HP-UX 11i Version 3: February 2007

Technical documentation

» Feedback
Content starts here

 » Table of Contents

 » Index


prealloc — preallocate fast disk storage


#include <unistd.h>

int prealloc(int fildes, off_t size);


prealloc() is used to preallocate space on a disk for faster storage operations.

fildes is a file descriptor obtained from a creat(), open(), dup(), or fcntl() system call for an ordinary file of zero length. It must be opened writable, because it will be written to by prealloc(). size is the size in bytes to be preallocated for the file specified by fildes. At least size bytes will be allocated. Space is allocated in an implementation-dependent fashion for fast sequential reads and writes. The EOF in an extended file is left at the end of the preallocated area. The current file pointer is left at zero. The file is zero-filled.

Using prealloc() on a file does not give the file an attribute that is inherited when copying or restoring the file using a program such as cp or tar (see cp(1) and tar(1)). It simply ensures that disk space has been preallocated for size bytes in a manner suited for sequential access. The file can be extended beyond these limits by write() operations past the original end of file. However, this space will not necessarily be allocated using any special strategy.


Upon successful completion, prealloc() returns 0; otherwise, it returns -1 and sets errno to indicate the error.


prealloc() fails and no disk space is allocated if any of the following conditions are encountered:


fildes is not a valid open file descriptor opened for writing.


User's disk quota block limit has been reached for this file system.


size exceeds the maximum file size or the process's file size limit. See ulimit(2).


Not enough space is left on the device to allocate the requested amount; no space was allocated.


fildes not associated with an ordinary file of zero length.


Assuming a process has opened a file for writing, the following call to prealloc() preallocates at least 50000 bytes on disk for the file represented by file descriptor outfd:

prealloc (outfd, 50000);


Allocation of the file space is highly dependent on current disk usage. A successful return does not tell you how fragmented the file actually might be if the disk is nearing its capacity.


prealloc() was developed by HP.

Printable version
Privacy statement Using this site means you accept its terms Feedback to webmaster
© 1983-2007 Hewlett-Packard Development Company, L.P.