toolchain/gcc/newlib/libc/unix/pread.c

   1 /*
   2 FUNCTION
   3 <<pread>>---read a file from specified position
   4
   5 INDEX
   6         pread
   7 INDEX
   8         _pread_r
   9
  10 ANSI_SYNOPSIS
  11         #include <unistd.h>
  12         ssize_t pread(int <[fd]>, void *<[buf]>, size_t <[n]>, off_t <[off]>);
  13         ssize_t _pread_r(struct _reent *<[rptr]>, int <[fd]>,
  14                          void *<[buf]>, size_t <[n]>, off_t <[off]>);
  15
  16 TRAD_SYNOPSIS
  17         #include <unistd.h>
  18         ssize_t pread(<[fd]>, <[buf]>, <[n]>, <[off]>)
  19         int <[fd]>;
  20         void *<[buf]>;
  21         size_t <[n]>;
  22         off_t <[off]>;
  23
  24         ssize_t _pread_r(<[rptr]>, <[fd]>, <[buf]>, <[n]>, <[off]>)
  25         struct _reent *<[rptr]>;
  26         int <[fd]>;
  27         void *<[buf]>;
  28         size_t <[n]>;
  29         off_t <[off]>;
  30
  31 DESCRIPTION
  32 The <<pread>> function is similar to <<read>>.  One difference is that
  33 <<pread>> has an additional parameter <[off]> which is the offset to
  34 position in the file before reading.  The function also differs in that
  35 the file position is unchanged by the function (i.e. the file position
  36 is the same before and after a call to <<pread>>).
  37
  38 The <<_pread_r>> function is the same as <<pread>>, only a reentrant
  39 struct pointer <[rptr]> is provided to preserve reentrancy.
  40
  41 RETURNS
  42 <<pread>> returns the number of bytes read or <<-1>> if failure occurred.
  43
  44 PORTABILITY
  45 <<pread>> is non-ANSI and is specified by the Single Unix Specification.
  46
  47 Supporting OS subroutine required: <<read>>, <<lseek>>.
  48 */
  49
  50 #include <_ansi.h>
  51 #include <unistd.h>
  52 #include <reent.h>
  53
  54 ssize_t
  55 _DEFUN (_pread_r, (rptr, fd, buf, n, off),
  56      struct _reent *rptr _AND
  57      int fd _AND
  58      _PTR buf _AND
  59      size_t n _AND
  60      off_t off)
  61 {
  62   off_t cur_pos;
  63   _READ_WRITE_RETURN_TYPE num_read;
  64
  65   if ((cur_pos = _lseek_r (rptr, fd, 0, SEEK_CUR)) == (off_t)-1)
  66     return -1;
  67
  68   if (_lseek_r (rptr, fd, off, SEEK_SET) == (off_t)-1)
  69     return -1;
  70
  71   num_read = _read_r (rptr, fd, buf, n);
  72
  73   if (_lseek_r (rptr, fd, cur_pos, SEEK_SET) == (off_t)-1)
  74     return -1;
  75
  76   return (ssize_t)num_read;
  77 }
  78
  79 #ifndef _REENT_ONLY
  80
  81 ssize_t
  82 _DEFUN (pread, (fd, buf, n, off),
  83      int fd _AND
  84      _PTR buf _AND
  85      size_t n _AND
  86      off_t off)
  87 {
  88   return _pread_r (_REENT, fd, buf, n, off);
  89 }
  90
  91 #endif