NAME

prealloc − preallocate fast disk storage

SYNOPSIS

int prealloc (fildes, size)
int fildes;
unsigned size;

DESCRIPTION

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, since 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 be allocated in an implementation-dependent fashion for fast sequential reads and writes.  The EOF in an extended file will be 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(1) or 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, a value of 0 is returned.  Otherwise, a value of −1 is returned and errno is set to indicate the error. 

ERRORS

Prealloc will fail and no disk space will be allocated if one or more of the following are true:

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

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

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

[EFBIG] Size exceeds the maximum file size or the process’s file size limit.  See ulimit(2). Upon successful completion, a value of 0 is returned. Otherwise, a value of −1 is returned and errno is set to indicate the error. 

AUTHOR

Prealloc was developed by the Hewlett-Packard Company. 

SEE ALSO

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

WARNINGS

The 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 7.0: Sept 1989