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.TH PREAD 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

pread, pwrite \- read from or write to a file descriptor at a given offset

.SH SYNOPSIS

.B #include <unistd.h>

.PP

.BI "ssize_t pread(int " fd ", void *" buf ", size_t " count \

", off_t " offset );

.PP

.BI "ssize_t pwrite(int " fd ", const void *" buf ", size_t " count \

", off_t " offset );

.PP

.in -4n

Feature Test Macro Requirements for glibc (see

.BR feature_test_macros (7)):

.in

.PP

.PD 0

.ad l

.BR pread (),

.BR pwrite ():

.RS 4

_XOPEN_SOURCE\ >=\ 500

.br

|| /* Since glibc 2.12: */ _POSIX_C_SOURCE\ >=\ 200809L

.RE

.ad

.PD

.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 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 cause of 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)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.