For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

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

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


Storage location for data.

Size of the destination buffer in bytes.

Size of the item to read in bytes.

Maximum number of items to be read.

Pointer to FILE structure.

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 _doserrno, errno, _sys_errlist, and _sys_nerr.

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 is associated with stream (if there is one) is increased by the number of bytes actually read. If the given stream is opened in text mode, carriage return–linefeed pairs are replaced with single linefeed 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 cannot be determined.

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

FunctionRequired header

For additional compatibility information, see Compatibility.

// 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  
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:   
Wrote 22 items  
Number of 11-byte elements read = 2  
Contents of buffer after write/read:   

Stream I/O