fread

Reads data from a stream.

Syntax

size_t fread(
   void *buffer,
   size_t size,
   size_t count,
   FILE *stream
);

Parameters

buffer
Storage location for data.

size
Item size in bytes.

count
Maximum number of items to be read.

stream
Pointer to FILE structure.

Return Value

fread returns the number of full items actually read, which may be less than count if an error occurs or if the end of the file is encountered before reaching count. Use the feof or ferror function to distinguish a read error from an end-of-file condition. If size or count is 0, fread returns 0 and the buffer contents are unchanged. If stream or buffer is a null pointer, fread 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.

See _doserrno, errno, _sys_errlist, and _sys_nerr for more information on these error codes.

Remarks

The fread function reads up to count items of size bytes from the input stream and stores them in buffer. The file pointer 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, Windows-style newlines are converted into Unix-style newlines. That is, carriage return-linefeed (CRLF) pairs are replaced by single linefeed (LF) 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.

When used on a text mode stream, if the amount of data requested (that is, size * count) is greater than or equal to the internal FILE * buffer size (by default this is 4096 bytes, configurable by using setvbuf), stream data is copied directly into the user-provided buffer, and newline conversion is done in that buffer. Since the converted data may be shorter than the stream data copied into the buffer, data past buffer[return_value * size] (where return_value is the return value from fread) may contain unconverted data from the file. For this reason, we recommend you null-terminate character data at buffer[return_value * size] if the intent of the buffer is to act as a C-style string. See fopen for details on the effects of text mode and binary mode.

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

Requirements

Function Required header
fread <stdio.h>

For additional compatibility information, see Compatibility.

Example

// crt_fread.c
// This program opens a file named FREAD.OUT and
// writes 25 characters to the file. It then tries to open
// FREAD.OUT and read in 25 characters. If the attempt succeeds,
// the program displays the number of actual items read.

#include <stdio.h>

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

   // Open file in text mode:
   if( fopen_s( &stream, "fread.out", "w+t" ) == 0 )
   {
      for ( i = 0; i < 25; i++ )
         list[i] = (char)('z' - i);
      // Write 25 characters to stream
      numwritten = fwrite( list, sizeof( char ), 25, stream );
      printf( "Wrote %d items\n", numwritten );
      fclose( stream );

   }
   else
      printf( "Problem opening the file\n" );

   if( fopen_s( &stream, "fread.out", "r+t" ) == 0 )
   {
      // Attempt to read in 25 characters
      numread = fread( list, sizeof( char ), 25, stream );
      printf( "Number of items read = %d\n", numread );
      printf( "Contents of buffer = %.25s\n", list );
      fclose( stream );
   }
   else
      printf( "File could not be opened\n" );
}
Wrote 25 items
Number of items read = 25
Contents of buffer = zyxwvutsrqponmlkjihgfedcb

See also

Stream I/O
Text and Binary File I/O
fopen
fwrite
_read