NAME

prealloc − preallocate fast disk storage

SYNOPSIS

#include <unistd.h>

int prealloc(int fildes, off_t size);

DESCRIPTION

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

fildes is a file descriptor obtained from a creat(), pen(), 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. 

EXAMPLES

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);

DEPENDENCIES

Since the exact effect and performance benefits obtainable by using this call vary with the implementation of the file system, performance related details are described in the system administrator manuals for each specific machine. 

RETURN VALUE

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

ERRORS

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

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

[EDQUOT] User’s disk quota block limit has been reached for this file system. 

[EFBIG] size exceeds the maximum file size or the process’s file size limit.  See ulimit(2).

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

[ENOTEMPTY] fildes not associated with an ordinary file of zero length. 

AUTHOR

prealloc() was developed by HP. 

SEE ALSO

prealloc(1), creat(2), dup(2), fcntl(2), open(2), read(2), ulimit(2), write(2). 

WARNINGS

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. 

Hewlett-Packard Company  —  HP-UX Release 9.0: August 1992