scanf_s, _scanf_s_l, wscanf_s, _wscanf_s_l

Read formatted data from the standard input stream. These are versions of scanf, _scanf_l, wscanf, _wscanf_l with security enhancements as described in Security Features in the CRT.

int scanf_s(
   const char *format [,
int _scanf_s_l(
   const char *format,
   locale_t locale [,
int wscanf_s(
   const wchar_t *format [,
int _wscanf_s_l(
   const wchar_t *format,
   locale_t locale [,


  • format
    Format control string.

  • argument
    Optional arguments.

  • locale
    The locale to use.

Return Value

Returns the number of fields 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. The return value is EOF for an error or if the end-of-file character or the end-of-string character is encountered in the first attempt to read a character. If format is a NULL pointer, the invalid parameter handler is invoked, as described in Parameter Validation. If execution is allowed to continue, scanf_s and wscanf_s return EOF and set errno to EINVAL.

For information on these and other error codes, see _doserrno, errno, _sys_errlist, and _sys_nerr.


The scanf_s function reads data from the standard input stream stdin and writes the data into the location given by argument. Each argument must be a pointer to a variable of a type that corresponds to a type specifier in format. If copying takes place between strings that overlap, the behavior is undefined.

wscanf_s is a wide-character version of scanf_s; the format argument to wscanf_s is a wide-character string. wscanf_s and scanf_s behave identically identically if the stream is opened in ANSI mode. scanf_s doesn't currently support input from a UNICODE stream.

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

Unlike scanf and wscanf, scanf_s and wscanf_s require the buffer size to be specified for all input parameters of type c, C, s, S, or [. The buffer size in characters is passed as an additional parameter immediately following the pointer to the buffer or variable. For example, if reading a string, the buffer size for that string is passed as follows:

char s[10];

scanf_s("%9s", s, _countof(s)); // buffer size is 10, width specification is 9

The buffer size includes the terminating null. A width specification field may be used to ensure that the token read in will fit into the buffer. If no width specification field is used, and the token read is too big to fit in the buffer, nothing will be written to that buffer.


The size parameter is of type unsigned, not size_t.

The following example shows that the buffer size parameter describes the maximum number of characters, not bytes. In the call to wscanf_s, the character width that is indicated by the buffer type does not match the character width that is indicated by the format specifier.

wchar_t ws[10];
wscanf_s("%9S", ws, _countof(ws));

The S format specifier indicates the use of the character width that is "opposite" the default width that is supported by the function. The character width is single-byte, but the function supports double-byte characters. This example reads in a string of up to 9 single-byte-wide characters and puts them in a double-byte-wide character buffer. The characters are treated as single-byte values; the first two characters are stored in ws[0], the second two are stored in ws[1], and so on.

In the case of characters, one may read a single character as follows:

char c;

scanf_s("%c", &c, 1);

When reading multiple characters for non-null terminated strings, integers are used as the width specification and the buffer size.

char c[4];

scanf_s("%4c", &c, _countof(c)); // not null terminated

For more information, see scanf Width Specification.

Generic-Text Routine Mappings

TCHAR.H routine

_UNICODE & _MBCS not defined

_MBCS defined

_UNICODE defined









For more information, see Format Specification Fields — scanf functions and wscanf Functions.



Required header

scanf_s, _scanf_s_l


wscanf_s, _wscanf_s_l

<stdio.h> or <wchar.h>

For additional compatibility information, see Compatibility in the Introduction.


// crt_scanf_s.c
// This program uses the scanf_s and wscanf_s functions
// to read formatted input.
#include <stdio.h>
#include <stdlib.h>

int main( void )
   int      i,
   float    fp;
   char     c,
   wchar_t  wc,

   result = scanf_s( "%d %f %c %C %s %S", &i, &fp, &c, 1,
                     &wc, 1, s, _countof(s), ws, _countof(ws) );
   printf( "The number of fields input is %d\n", result );
   printf( "The contents are: %d %f %c %C %s %S\n", i, fp, c,
           wc, s, ws);
   result = wscanf_s( L"%d %f %hc %lc %S %ls", &i, &fp, &c, 2,
                      &wc, 1, s, _countof(s), ws, _countof(ws) );
   wprintf( L"The number of fields input is %d\n", result );
   wprintf( L"The contents are: %d %f %C %c %hs %s\n", i, fp,
            c, wc, s, ws);
  71 98.6 h z Byte characters
36 92.3 y n Wide characters
The number of fields input is 6
The contents are: 71 98.599998 h z Byte characters
The number of fields input is 6
The contents are: 36 92.300003 y n Wide characters

.NET Framework Equivalent

See Also


Floating-Point Support

Stream I/O


fscanf, _fscanf_l, fwscanf, _fwscanf_l

printf, _printf_l, wprintf, _wprintf_l

sprintf, _sprintf_l, swprintf, _swprintf_l, __swprintf_l

sscanf, _sscanf_l, swscanf, _swscanf_l