.\" Copyright (C) 1999 Joseph Samuel Myers.
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.TH PREAD 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
pread, pwrite \- read from or write to a file descriptor at a given offset
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "ssize_t pread(int " fd ", void *" buf ", size_t " count \
", off_t " offset );
.BI "ssize_t pwrite(int " fd ", const void *" buf ", size_t " count \
", off_t " offset );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR pread (),
.BR pwrite ():
.nf
    _XOPEN_SOURCE >= 500
        || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
.fi
.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,
.BR pread ()
returns the number of bytes read
(a return of zero indicates end of file)
and
.BR pwrite ()
returns the number of bytes written.
.PP
Note that it is not an error for a successful call to transfer fewer bytes
than requested (see
.BR read (2)
and
.BR write (2)).
.PP
On error, \-1 is returned and
.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.
C library 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, POSIX.1-2008.
.SH NOTES
The
.BR pread ()
and
.BR pwrite ()
system calls are especially useful in multithreaded applications.
They allow multiple threads to perform I/O on the same file descriptor
without being affected by changes to the file offset by other threads.
.\"
.SS C library/kernel differences
On Linux, the underlying system calls were renamed
in kernel 2.6:
.BR pread ()
became
.BR pread64 (),
and
.BR pwrite ()
became
.BR pwrite64 ().
The system call numbers remained the same.
The glibc
.BR pread ()
and
.BR pwrite ()
wrapper functions transparently deal with the change.
.PP
On some 32-bit architectures,
the calling signature for these system calls differ,
for the reasons described in
.BR syscall (2).
.SH BUGS
POSIX requires that opening a file with the
.BR O_APPEND
flag should have no effect on the location at which
.BR pwrite ()
writes data.
However, on Linux, if a file is opened with
.\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=43178
.BR O_APPEND ,
.BR pwrite ()
appends data to the end of the file, regardless of the value of
.IR offset .
.SH SEE ALSO
.BR lseek (2),
.BR read (2),
.BR readv (2),
.BR write (2)