fscanf_s, _fscanf_s_l, fwscanf_s, _fwscanf_s_l

Reads formatted data from a stream. These versions of fscanf, _fscanf_l, fwscanf, _fwscanf_l have security enhancements, as described in Security Features in the CRT.

int fscanf_s( 
   FILE *stream,
   const char *format [,
   argument ]... 
int _fscanf_s_l( 
   FILE *stream,
   const char *format,
   locale_t locale [,
   argument ]... 
int fwscanf_s( 
   FILE *stream,
   const wchar_t *format [,
   argument ]... 
int _fwscanf_s_l( 
   FILE *stream,
   const wchar_t *format,
   locale_t locale [,
   argument ]... 


  • stream
    Pointer to FILE structure.

  • format
    Format-control string.

  • argument
    Optional arguments.

  • locale
    The locale to use.

Return Value

Each of these functions returns the number of fields that are successfully converted and assigned; the return value does not include fields that were read but not assigned. A return value of 0 indicates that no fields were assigned. If an error occurs, or if the end of the file stream is reached before the first conversion, the return value is EOF for fscanf_s and fwscanf_s.

These functions validate their parameters. If stream is an invalid file pointer, or format is a null pointer, these functions invoke the invalid parameter handler, as described in Parameter Validation. If execution is allowed to continue, these functions return EOF and set errno to EINVAL.


The fscanf_s function reads data from the current position of stream into the locations that are given by argument (if any). Each argument must be a pointer to a variable of a type that corresponds to a type specifier in format. format controls the interpretation of the input fields and has the same form and function as the format argument for scanf_s; see Format Specification Fields: scanf and wscanf Functions for a description of format. fwscanf_s is a wide-character version of fscanf_s; the format argument to fwscanf_s is a wide-character string. These functions behave identically if the stream is opened in ANSI mode. fscanf_s doesn't currently support input from a UNICODE stream.

The main difference between the more secure functions, which have the _s suffix, and the other versions is that the more secure functions require the size in characters of each c, C, s, S, and [ type field to be passed as an argument immediately following the variable. For more information, see scanf_s, _scanf_s_l, wscanf_s, _wscanf_s_l and scanf Width Specification.


The size parameter is of type unsigned, not size_t.

The versions of these functions that have the _l suffix are identical except that they use the locale parameter that's passed in instead of the current thread locale.

Generic-Text Routine Mappings

TCHAR.H routine

_UNICODE & _MBCS not defined

_MBCS defined

_UNICODE defined











Required header

fscanf_s, _fscanf_s_l


fwscanf_s, _fwscanf_s_l

<stdio.h> or <wchar.h>

For additional compatibility information, see Compatibility.


// crt_fscanf_s.c
// This program writes formatted
// data to a file. It then uses fscanf to
// read the various data back from the file.
#include <stdio.h>
#include <stdlib.h>

FILE *stream;

int main( void )
   long l;
   float fp;
   char s[81];
   char c;

   errno_t err = fopen_s( &stream, "fscanf.out", "w+" );
   if( err )
      printf_s( "The file fscanf.out was not opened\n" );
      fprintf_s( stream, "%s %ld %f%c", "a-string", 
               65000, 3.14159, 'x' );
      // Set pointer to beginning of file:
      fseek( stream, 0L, SEEK_SET );

      // Read data back from file:
      fscanf_s( stream, "%s", s, _countof(s) );
      fscanf_s( stream, "%ld", &l );

      fscanf_s( stream, "%f", &fp );
      fscanf_s( stream, "%c", &c, 1 );

      // Output data read:
      printf( "%s\n", s );
      printf( "%ld\n", l );
      printf( "%f\n", fp );
      printf( "%c\n", c );

      fclose( stream );

.NET Framework Equivalent

System::IO::StreamReader::ReadLine. See also Parse methods, such as System::Double::Parse.

See Also


Stream I/O

_cscanf_s, _cscanf_s_l, _cwscanf_s, _cwscanf_s_l

fprintf_s, _fprintf_s_l, fwprintf_s, _fwprintf_s_l

scanf_s, _scanf_s_l, wscanf_s, _wscanf_s_l

sscanf_s, _sscanf_s_l, swscanf_s, _swscanf_s_l

fscanf, _fscanf_l, fwscanf, _fwscanf_l