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/* $Id: pread.c 5049 2001-12-12 09:06:00Z rra $
**
** Replacement for a missing pread.
**
** Written by Russ Allbery <rra@stanford.edu>
** This work is hereby placed in the public domain by its author.
**
** Provides the same functionality as the standard library routine pread
** for those platforms that don't have it. Note that pread requires that
** the file pointer not move and without the library function, we can't
** copy that behavior; instead, we approximate it by moving the file
** pointer and then moving it back. This may break threaded programs.
*/
#include "config.h"
#include "clibrary.h"
#include <errno.h>
/* If we're running the test suite, rename pread to avoid conflicts with the
system version. #undef first because large file support may define a
macro pread (pointing to pread64) on some platforms (e.g. Solaris). */
#if TESTING
# undef pread
# define pread test_pread
ssize_t test_pread(int, void *, size_t, off_t);
#endif
ssize_t
pread(int fd, void *buf, size_t nbyte, off_t offset)
{
off_t current;
ssize_t nread;
int oerrno;
current = lseek(fd, 0, SEEK_CUR);
if (current == (off_t) -1 || lseek(fd, offset, SEEK_SET) == (off_t) -1)
return -1;
nread = read(fd, buf, nbyte);
/* Ignore errors in restoring the file position; this isn't ideal, but
reporting a failed read when the read succeeded is worse. Make sure
that errno, if set, is set by read and not lseek. */
oerrno = errno;
lseek(fd, current, SEEK_SET);
errno = oerrno;
return nread;
}
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