.\" Copyright (C) 1999 Joseph Samuel Myers.
.\"
-.\" %%%LICENSE_START(verbatim)
+.\" %%%LICENSE_START(VERBATIM)
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.\" manual provided the copyright notice and this permission notice are
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-.TH PREAD 2 2012-04-30 "Linux" "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 #include <unistd.h>
-.sp
+.PP
.BI "ssize_t pread(int " fd ", void *" buf ", size_t " count \
", off_t " offset );
-.sp
+.PP
.BI "ssize_t pwrite(int " fd ", const void *" buf ", size_t " count \
", off_t " offset );
-.sp
+.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
+.PP
.PD 0
.ad l
-.sp
.BR pread (),
.BR pwrite ():
.RS 4
.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
+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
.BR lseek (2)
on older kernels without the system calls) was added in glibc 2.1.
.SH CONFORMING TO
-POSIX.1-2001.
+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 ()
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 affect on the location at which
+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
+.\" 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