pread.2: Note that glibc emulation for these calls uses lseek(2)

[thirdparty/man-pages.git] / man2 / pread.2

.\" Copyright (C) 1999 Joseph Samuel Myers.
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.TH PREAD 2 2008-120-3 "Linux" "Linux Programmer's Manual"
.SH NAME
pread, pwrite \- read from or write to a file descriptor at a given offset
.SH SYNOPSIS
.B #define _XOPEN_SOURCE 500
.sp
.B #include <unistd.h>
.sp
.BI "ssize_t pread(int " fd ", void *" buf ", size_t " count \
", off_t " offset );
.sp
.BI "ssize_t pwrite(int " fd ", const void *" buf ", size_t " count \
", off_t " offset );
.SH DESCRIPTION
.BR pread ()
reads up to
.I count
bytes from file descriptor
.I fd
at offset
.I offset
(from the start of the file) into the buffer starting at
.IR buf .
The file offset is not changed.
.PP
.BR pwrite ()
writes up to
.I count
bytes from the buffer starting at
.I buf
to the file descriptor
.I fd
at offset
.IR offset .
The file offset is not changed.
.PP
The file referenced by
.I fd
must be capable of seeking.
.SH "RETURN VALUE"
On success, the number of bytes read or written is returned (zero
indicates that nothing was written, in the case of
.BR pwrite (),
or
end of file, in the case of
.BR pread (),
or \-1 on error, in which case
.I errno
is set to indicate the error.
.SH ERRORS
.BR pread ()
can fail and set
.I errno
to any error specified for
.BR read (2)
or
.BR lseek (2).
.BR pwrite ()
can fail and set
.I errno
to any error specified for
.BR write (2)
or
.BR lseek (2).
.SH VERSIONS
The
.BR pread ()
and
.BR pwrite ()
system calls were added to Linux in
version 2.1.60; the entries in the i386 system call table were added
in 2.1.69.
The libc support (including emulation using
.BR lseek (2)
on older kernels without the system calls) was added in glibc 2.1.
.SH "CONFORMING TO"
POSIX.1-2001.
.SH "SEE ALSO"
.BR lseek (2),
.BR read (2),
.BR write (2),
.BR feature_test_macros (7)