man2/pread.2

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  24 .\"
  25 .TH PREAD 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 pread, pwrite \- read from or write to a file descriptor at a given offset
  28 .SH SYNOPSIS
  29 .B #include <unistd.h>
  30 .PP
  31 .BI "ssize_t pread(int " fd ", void *" buf ", size_t " count \
  32 ", off_t " offset );
  33 .PP
  34 .BI "ssize_t pwrite(int " fd ", const void *" buf ", size_t " count \
  35 ", off_t " offset );
  36 .PP
  37 .in -4n
  38 Feature Test Macro Requirements for glibc (see
  39 .BR feature_test_macros (7)):
  40 .in
  41 .PP
  42 .PD 0
  43 .ad l
  44 .BR pread (),
  45 .BR pwrite ():
  46 .RS 4
  47 _XOPEN_SOURCE\ >=\ 500
  48 .br
  49 || /* Since glibc 2.12: */ _POSIX_C_SOURCE\ >=\ 200809L
  50 .RE
  51 .ad
  52 .PD
  53 .SH DESCRIPTION
  54 .BR pread ()
  55 reads up to
  56 .I count
  57 bytes from file descriptor
  58 .I fd
  59 at offset
  60 .I offset
  61 (from the start of the file) into the buffer starting at
  62 .IR buf .
  63 The file offset is not changed.
  64 .PP
  65 .BR pwrite ()
  66 writes up to
  67 .I count
  68 bytes from the buffer starting at
  69 .I buf
  70 to the file descriptor
  71 .I fd
  72 at offset
  73 .IR offset .
  74 The file offset is not changed.
  75 .PP
  76 The file referenced by
  77 .I fd
  78 must be capable of seeking.
  79 .SH RETURN VALUE
  80 On success,
  81 .BR pread ()
  82 returns the number of bytes read
  83 (a return of zero indicates end of file)
  84 and
  85 .BR pwrite ()
  86 returns the number of bytes written.
  87 .PP
  88 Note that it is not an error for a successful call to transfer fewer bytes
  89 than requested (see
  90 .BR read (2)
  91 and
  92 .BR write (2)).
  93 .PP
  94 On error, \-1 is returned and
  95 .I errno
  96 is set to indicate the cause of the error.
  97 .SH ERRORS
  98 .BR pread ()
  99 can fail and set
 100 .I errno
 101 to any error specified for
 102 .BR read (2)
 103 or
 104 .BR lseek (2).
 105 .BR pwrite ()
 106 can fail and set
 107 .I errno
 108 to any error specified for
 109 .BR write (2)
 110 or
 111 .BR lseek (2).
 112 .SH VERSIONS
 113 The
 114 .BR pread ()
 115 and
 116 .BR pwrite ()
 117 system calls were added to Linux in
 118 version 2.1.60; the entries in the i386 system call table were added
 119 in 2.1.69.
 120 C library support (including emulation using
 121 .BR lseek (2)
 122 on older kernels without the system calls) was added in glibc 2.1.
 123 .SH CONFORMING TO
 124 POSIX.1-2001, POSIX.1-2008.
 125 .SH NOTES
 126 The
 127 .BR pread ()
 128 and
 129 .BR pwrite ()
 130 system calls are especially useful in multithreaded applications.
 131 They allow multiple threads to perform I/O on the same file descriptor
 132 without being affected by changes to the file offset by other threads.
 133 .\"
 134 .SS C library/kernel differences
 135 On Linux, the underlying system calls were renamed
 136 in kernel 2.6:
 137 .BR pread ()
 138 became
 139 .BR pread64 (),
 140 and
 141 .BR pwrite ()
 142 became
 143 .BR pwrite64 ().
 144 The system call numbers remained the same.
 145 The glibc
 146 .BR pread ()
 147 and
 148 .BR pwrite ()
 149 wrapper functions transparently deal with the change.
 150 .PP
 151 On some 32-bit architectures,
 152 the calling signature for these system calls differ,
 153 for the reasons described in
 154 .BR syscall (2).
 155 .SH BUGS
 156 POSIX requires that opening a file with the
 157 .BR O_APPEND
 158 flag should have no effect on the location at which
 159 .BR pwrite ()
 160 writes data.
 161 However, on Linux, if a file is opened with
 162 .\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=43178
 163 .BR O_APPEND ,
 164 .BR pwrite ()
 165 appends data to the end of the file, regardless of the value of
 166 .IR offset .
 167 .SH SEE ALSO
 168 .BR lseek (2),
 169 .BR read (2),
 170 .BR readv (2),
 171 .BR write (2)