79 lines
1.8 KiB
C
79 lines
1.8 KiB
C
#ifndef _NO_PREAD
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/*
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FUNCTION
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<<pread>>---read a file from specified position
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INDEX
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pread
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INDEX
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_pread_r
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SYNOPSIS
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#include <unistd.h>
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ssize_t pread(int <[fd]>, void *<[buf]>, size_t <[n]>, off_t <[off]>);
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ssize_t _pread_r(struct _reent *<[rptr]>, int <[fd]>,
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void *<[buf]>, size_t <[n]>, off_t <[off]>);
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DESCRIPTION
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The <<pread>> function is similar to <<read>>. One difference is that
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<<pread>> has an additional parameter <[off]> which is the offset to
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position in the file before reading. The function also differs in that
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the file position is unchanged by the function (i.e. the file position
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is the same before and after a call to <<pread>>).
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The <<_pread_r>> function is the same as <<pread>>, only a reentrant
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struct pointer <[rptr]> is provided to preserve reentrancy.
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RETURNS
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<<pread>> returns the number of bytes read or <<-1>> if failure occurred.
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PORTABILITY
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<<pread>> is non-ANSI and is specified by the Single Unix Specification.
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Supporting OS subroutine required: <<read>>, <<lseek>>.
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*/
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#include <_ansi.h>
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#include <unistd.h>
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#include <reent.h>
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ssize_t
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_DEFUN (_pread_r, (rptr, fd, buf, n, off),
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struct _reent *rptr,
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int fd,
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void *buf,
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size_t n,
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off_t off)
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{
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off_t cur_pos;
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_READ_WRITE_RETURN_TYPE num_read;
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if ((cur_pos = _lseek_r (rptr, fd, 0, SEEK_CUR)) == (off_t)-1)
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return -1;
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if (_lseek_r (rptr, fd, off, SEEK_SET) == (off_t)-1)
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return -1;
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num_read = _read_r (rptr, fd, buf, n);
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if (_lseek_r (rptr, fd, cur_pos, SEEK_SET) == (off_t)-1)
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return -1;
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return (ssize_t)num_read;
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}
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#ifndef _REENT_ONLY
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ssize_t
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_DEFUN (pread, (fd, buf, n, off),
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int fd,
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void *buf,
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size_t n,
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off_t off)
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{
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return _pread_r (_REENT, fd, buf, n, off);
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}
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#endif /* !_REENT_ONLY */
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#endif /* !_NO_PREAD */
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