Share via

scanf_s, _scanf_s_l, wscanf_s, _wscanf_s_l

Reads formatted data from the standard input stream. These versions of scanf, _scanf_l, wscanf, _wscanf_l have 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 control string.

Optional arguments.

The locale to use.

Return value

Returns the number of fields successfully converted and assigned. The return value doesn't include fields that were read but not assigned. A return value of 0 indicates 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 found 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 about these and other error codes, see errno, _doserrno, _sys_errlist, and _sys_nerr.


The scanf_s function reads data from the standard input stream, stdin, and writes it into argument. Each argument must be a pointer to a variable type that corresponds to the type specifier in format. If copying occurs 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 if the stream is opened in ANSI mode. scanf_s doesn't currently support input from a UNICODE stream.

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

Unlike scanf and wscanf, scanf_s and wscanf_s require you to specify buffer sizes for some parameters. Specify the sizes for all c, C, s, S, or string control set [] parameters. The buffer size in characters is passed as another parameter. It immediately follows the pointer to the buffer or variable. For example, if you're reading a string, the buffer size for that string is passed as follows:

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

The buffer size includes the terminal null. You can use a width specification field to ensure the token that's read in fits into the buffer. When a token is too large to fit, nothing is written to the buffer unless there's a width specification.


The size parameter is of type unsigned, not size_t. Use a static cast to convert a size_t value to unsigned for 64-bit build configurations.

The buffer size parameter describes the maximum number of characters, not bytes. In this example, the width of the buffer type doesn't match the width of the format specifier.

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

The S format specifier means use the character width that's "opposite" the default width 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 nine 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.

This example reads a single character:

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

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

char c[4];
scanf_s("%4c", c, (unsigned)_countof(c)); // not null terminated

For more information, see scanf Width Specification.

Generic-text routine mappings

TCHAR.H routine _UNICODE and _MBCS not defined _MBCS defined _UNICODE defined
_tscanf_s scanf_s scanf_s wscanf_s
_tscanf_s_l _scanf_s_l _scanf_s_l _wscanf_s_l

For more information, see Format specification fields: scanf and wscanf functions.


Routine Required header
scanf_s, _scanf_s_l <stdio.h>
wscanf_s, _wscanf_s_l <stdio.h> or <wchar.h>

The console isn't supported in Universal Windows Platform (UWP) apps. The standard stream handles stdin, stdout, and stderr must be redirected before C run-time functions can use them in UWP apps. For more compatibility information, see Compatibility.


// 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, (unsigned)_countof(s), ws, (unsigned)_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, (unsigned)_countof(s), ws, (unsigned)_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);

This program produces the following output when given this input:

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

See also

Math and 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