prealloc - preallocate fast disk storage
int prealloc(int fildes, off_t size);
prealloc() is used to preallocate space on a disk for faster storage
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-
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:
[EBADF] fildes is not a valid open file descriptor opened
[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.
Hewlett-Packard Company - 1 - HP-UX Release 11i: November 2000
[ENOTEMPTY] fildes not associated with an ordinary file of
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.
prealloc(1), creat(2), dup(2), fcntl(2), open(2), prealloc64(2),
read(2), ulimit(2), write(2).
Hewlett-Packard Company - 2 - HP-UX Release 11i: November 2000