`fread_s`

Article
12/02/2022

Reads data from a stream. This version of fread has security enhancements, as described in Security features in the CRT.

Syntax

size_t fread_s(
   void *buffer,
   size_t bufferSize,
   size_t elementSize,
   size_t count,
   FILE *stream
);

Parameters

buffer
Storage location for data.

bufferSize
Size of the destination buffer in bytes.

elementSize
Size of the item to read in bytes.

count
Maximum number of items to be read.

stream
Pointer to FILE structure.

Return value

fread_s returns the number of (whole) items that were read into the buffer, which may be less than count if a read error or the end of the file is encountered before count is reached. Use the feof or ferror function to distinguish an error from an end-of-file condition. If size or count is 0, fread_s returns 0 and the buffer contents are unchanged. If stream or buffer is a null pointer, fread_s invokes the invalid parameter handler, as described in Parameter validation. If execution is allowed to continue, this function sets errno to EINVAL and returns 0.

For more information about error codes, see errno, _doserrno, _sys_errlist, and _sys_nerr.

Remarks

The fread_s function reads up to count items of elementSize bytes from the input stream and stores them in buffer. The file pointer that's associated with stream (if there's one) is advanced by the number of bytes fread_s read. If the given stream is opened in text mode, carriage return-line feed pairs are replaced with single line feed characters. The replacement has no effect on the file pointer or the return value. The file-pointer position is indeterminate if an error occurs. The value of a partially read item can't be determined.

This function locks out other threads. If you require a non-locking version, use _fread_nolock.

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

Requirements

Function	Required header
`fread_s`	`<stdio.h>`

For more compatibility information, see Compatibility.

Example

// crt_fread_s.c
// Command line: cl /EHsc /nologo /W4 crt_fread_s.c
//
// This program opens a file that's named FREAD.OUT and
// writes characters to the file. It then tries to open
// FREAD.OUT and read in characters by using fread_s. If the attempt succeeds,
// the program displays the number of actual items read.

#include <stdio.h>

#define BUFFERSIZE 30
#define DATASIZE 22
#define ELEMENTCOUNT 2
#define ELEMENTSIZE (DATASIZE/ELEMENTCOUNT)
#define FILENAME "FREAD.OUT"

int main( void )
{
   FILE *stream;
   char list[30];
   int  i, numread, numwritten;

   for ( i = 0; i < DATASIZE; i++ )
      list[i] = (char)('z' - i);
   list[DATASIZE] = '\0'; // terminal null so we can print it

   // Open file in text mode:
   if( fopen_s( &stream, FILENAME, "w+t" ) == 0 )
   {
      // Write DATASIZE characters to stream
      printf( "Contents of buffer before write/read:\n\t%s\n\n", list );
      numwritten = fwrite( list, sizeof( char ), DATASIZE, stream );
      printf( "Wrote %d items\n\n", numwritten );
      fclose( stream );
   } else {
      printf( "Problem opening the file\n" );
      return -1;
   }

   if( fopen_s( &stream, FILENAME, "r+t" ) == 0 )   {
      // Attempt to read in characters in 2 blocks of 11
      numread = fread_s( list, BUFFERSIZE, ELEMENTSIZE, ELEMENTCOUNT, stream );
      printf( "Number of %d-byte elements read = %d\n\n", ELEMENTSIZE, numread );
      printf( "Contents of buffer after write/read:\n\t%s\n", list );
      fclose( stream );
   } else {
      printf( "File could not be opened\n" );
      return -1;
   }
}

Contents of buffer before write/read:
        zyxwvutsrqponmlkjihgfe

Wrote 22 items

Number of 11-byte elements read = 2

Contents of buffer after write/read:
        zyxwvutsrqponmlkjihgfe

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