.\" 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.
.\" 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.\" manual page may be incorrect or out-of-date.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
.\" 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 1999-01-21 "Linux 2.2.0-pre9" "Linux Programmer's Manual"
+.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 #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 );
-.fi
+.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
-.B pread()
+.BR pread ()
reads up to
.I count
bytes from file descriptor
.IR buf .
The file offset is not changed.
.PP
-.B pwrite()
+.BR pwrite ()
writes up to
.I count
bytes from the buffer starting at
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 \fBpwrite\fR(), or
-end of file, in the case of \fBpread\fR), or \-1 on error, in which
-case
+.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.
+is set to indicate the cause of the error.
.SH ERRORS
.BR pread ()
can fail and set
.I errno
-to any error specified for \fBread\fR(2) or \fBlseek\fR(2).
+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 \fBwrite\fR(2) or \fBlseek\fR(2).
-.SH "CONFORMING TO"
-Unix98
-.SH HISTORY
-The \fBpread\fR() and \fBpwrite\fR() system calls were added to Linux in
+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 on older kernels
-without the system calls) was added in glibc 2.1.
-.SH "SEE ALSO"
+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)
projects / thirdparty / man-pages.git / blobdiff