man2/pread.2

   1 .\" Copyright (C) 1999 Joseph Samuel Myers.
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  23 .TH PREAD 2 1999-01-21 "Linux 2.2.0-pre9" "Linux Programmer's Manual"
  24 .SH NAME
  25 pread, pwrite \- read from or write to a file descriptor at a given offset
  26 .SH SYNOPSIS
  27 .B #define _XOPEN_SOURCE 500
  28 .sp
  29 .B #include <unistd.h>
  30 .sp
  31 .BI "ssize_t pread(int " fd ", void *" buf ", size_t " count ", off_t " offset );
  32 .sp
  33 .BI "ssize_t pwrite(int " fd ", const void *" buf ", size_t " count ", off_t " offset );
  34 .fi
  35 .SH DESCRIPTION
  36 .BR pread ()
  37 reads up to
  38 .I count
  39 bytes from file descriptor
  40 .I fd
  41 at offset
  42 .I offset
  43 (from the start of the file) into the buffer starting at
  44 .IR buf .
  45 The file offset is not changed.
  46 .PP
  47 .BR pwrite ()
  48 writes up to
  49 .I count
  50 bytes from the buffer starting at
  51 .I buf
  52 to the file descriptor
  53 .I fd
  54 at offset
  55 .IR offset .
  56 The file offset is not changed.
  57 .PP
  58 The file referenced by
  59 .I fd
  60 must be capable of seeking.
  61 .SH "RETURN VALUE"
  62 On success, the number of bytes read or written is returned (zero
  63 indicates that nothing was written, in the case of \fBpwrite\fR(), or
  64 end of file, in the case of \fBpread\fR), or \-1 on error, in which
  65 case
  66 .I errno
  67 is set to indicate the error.
  68 .SH ERRORS
  69 .BR pread ()
  70 can fail and set
  71 .I errno
  72 to any error specified for \fBread\fR(2) or \fBlseek\fR(2).
  73 .BR pwrite ()
  74 can fail and set
  75 .I errno
  76 to any error specified for \fBwrite\fR(2) or \fBlseek\fR(2).
  77 .SH "CONFORMING TO"
  78 POSIX.1-2001.
  79 .SH HISTORY
  80 The \fBpread\fR() and \fBpwrite\fR() system calls were added to Linux in
  81 version 2.1.60; the entries in the i386 system call table were added
  82 in 2.1.69.  The libc support (including emulation on older kernels
  83 without the system calls) was added in glibc 2.1.
  84 .SH "SEE ALSO"
  85 .BR lseek (2),
  86 .BR read (2),
  87 .BR write (2),
  88 .BR feature_test_macros (7)