`_read`

Article
12/02/2022

Reads data from a file.

Syntax

int _read(
   int const fd,
   void * const buffer,
   unsigned const buffer_size
);

Parameters

fd
File descriptor referring to the open file.

buffer
Storage location for data.

buffer_size
Maximum number of bytes to read.

Return value

_read returns the number of bytes read, which might be less than buffer_size if there are fewer than buffer_size bytes left in the file, or if the file was opened in text mode. In text mode, each carriage return-line feed pair \r\n is replaced with a single line feed character \n. Only the single line feed character is counted in the return value. The replacement doesn't affect the file pointer.

If the function tries to read at end of file, it returns 0. If fd isn't valid, the file isn't 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, or if buffer_size > INT_MAX, 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 errno, _doserrno, _sys_errlist, and _sys_nerr.

Remarks

The _read function reads a maximum of buffer_size 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.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Requirements

Routine	Required header
`_read`	`<io.h>`

For more compatibility information, see Compatibility.

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, bytesread;
   unsigned int nbytes = 60000;

   /* 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