.\" Copyright (C) 1999 Joseph Samuel Myers.
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH pread 2 2024-05-02 "Linux man-pages 6.8"
.SH NAME
pread, pwrite \- read from or write to a file descriptor at a given offset
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.P
.BI "ssize_t pread(int " fd ", void " buf [. count "], size_t " count ,
.BI "              off_t " offset );
.BI "ssize_t pwrite(int " fd ", const void " buf [. count "], size_t " count ,
.BI "              off_t " offset );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.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.
.P
.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.
.P
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.
.P
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)).
.P
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 STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.P
Added in Linux 2.1.60;
the entries in the i386 system call table were added in Linux 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.
.SS C library/kernel differences
On Linux, the underlying system calls were renamed
in Linux 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.
.P
On some 32-bit architectures,
the calling signature for these system calls differ,
for the reasons described in
.BR syscall (2).
.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.
.SH BUGS
POSIX requires that opening a file with the
.B 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)