_read

Artikel
07/22/2011

Reads data from a file.

int _read(
   int fd,
   void *buffer,
   unsigned int count 
);

Parameters

fd
File descriptor referring to the open file.
buffer
Storage location for data.
count
Maximum number of bytes.

Return Value

_read returns the number of bytes read, which might be less than count if there are fewer than count bytes left in the file or if the file was opened in text mode, in which case each carriage return–line feed (CR-LF) pair is replaced with a single linefeed character. Only the single linefeed character is counted in the return value. The replacement does not affect the file pointer.

If the function tries to read at end of file, it returns 0. If fd is invalid, the file is not open for reading, or the file is locked, the invalid parameter handler is invoked, as described in Parameter Validation. If execution is allowed to continue, the function returns –1 and sets errno to EBADF.

If buffer is NULL, the invalid parameter handler is invoked. If execution is allowed to continue, the function returns -1 and errno is set to EINVAL.

For more information about this and other return codes, see _doserrno, errno, _sys_errlist, and _sys_nerr.

Remarks

The _read function reads a maximum of count bytes into buffer from the file associated with fd. The read operation begins at the current position of the file pointer associated with the given file. After the read operation, the file pointer points to the next unread character.

If the file was opened in text mode, the read terminates when _read encounters a CTRL+Z character, which is treated as an end-of-file indicator. Use _lseek to clear the end-of-file indicator.

Requirements

Routine	Required header
_read	<io.h>

For more compatibility information, see Compatibility in the Introduction.

Libraries

All versions of the C run-time libraries.

Example

// crt_read.c
/* This program opens a file named crt_read.txt
 * and tries to read 60,000 bytes from
 * that file using _read. It then displays the
 * actual number of bytes read.
 */

#include <fcntl.h>      /* Needed only for _O_RDWR definition */
#include <io.h>
#include <stdlib.h>
#include <stdio.h>
#include <share.h>

char buffer[60000];

int main( void )
{
   int fh;
   unsigned int nbytes = 60000, bytesread;

   /* Open file for input: */
   if( _sopen_s( &fh, "crt_read.txt", _O_RDONLY, _SH_DENYNO, 0 ) )
   {
      perror( "open failed on input file" );
      exit( 1 );
   }

   /* Read in input: */
   if( ( bytesread = _read( fh, buffer, nbytes ) ) <= 0 )
      perror( "Problem reading file" );
   else
      printf( "Read %u bytes from file\n", bytesread );

   _close( fh );
}

Input: crt_read.txt

Line one.
Line two.

Output

Read 19 bytes from file

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