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 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 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 the data into the location that's 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 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 that they use the locale parameter that's 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 string control sets that are enclosed in []. The buffer size in characters is passed as an additional parameter immediately following the pointer to the buffer or variable. For example, if you are 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 terminating null. You can use a width specification field to ensure that the token that's read in will fit into the buffer. If no width specification field is used, and the token read in is too big to fit in the buffer, nothing is written to that buffer.


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 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(L"%9S", ws, (unsigned)_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, a single character may be read as follows:

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

When multiple characters for non-null terminated strings are read, integers are used as 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 & _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 is not supported in Universal Windows Platform (UWP) apps. The standard stream handles that are associated with the console, stdin, stdout, and stderr, must be redirected before C run-time functions can use them in UWP apps. For additional 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

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