newlib/libc/unix/pread.c

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