Getting error with sscanf_s in Visual Studio 2012

Question

I am trying to parse following string- "DATA,97,103,100,97,84,69" . This string can have following varieties-

"DATA,100,10,1,9,82,60"
"DATA,57,27,59,30,11,64"
"DATA,12,86,100,97,103,23"
"DATA,1,10,78,38,45,52"
"DATA,99,43,85,28,84,26"

Note that the first "DATA," never changes. The remaining integer numbers varies from 0 to 200. Below is the code which uses sscanf_s function to parse these strings-

char ignore_data[5];
int x1,x2,x3,y1,y2,y3;
char str[]="DATA,97,103,100,97,84,69";

sscanf_s(str,"%5[^,],%d,%d,%d,%d,%d,%d", ignore_data, &x1, &x2, &x3, &y1, &y2, &y3);
printf("str=%s, x1=%d, x2=%d, x3=%d, x1=%d, y2=%d, y3=%d",str, x1, x2, x3, y1, y2,y3);

The code does not work and shows following error

Unhandled exception at 0x510A06E4 (msvcr110d.dll) in My Application.exe: 0xC0000005: Access violation writing location 0x00870000.

The above code works perfectly, when sscanf() is used with #define _CRT_SECURE_NO_WARNINGS preprocessor.

I want to get the values of x1 , x2 , x3 , y1 , y2 and y3 . What am I missing here?

Answer 1

I found that if you are able to implement the following you should be able to get the results you need:

#include <stdio.h>

int main()
{
    char ignore_data[5];
    int x1, x2, x3, y1, y2, y3;
    char str[] = "DATA,97,103,100,97,84,69";

    sscanf_s(str, "%5[^,],%d,%d,%d,%d,%d,%d", ignore_data, sizeof(ignore_data), &x1, &x2, &x3, &y1, &y2, &y3);
    printf("str=%s, x1=%d, x2=%d, x3=%d, x1=%d, y2=%d, y3=%d", str, x1, x2, x3, y1, y2, y3);

    return 0;
}

For sscanf_s when taking in a string, immediately after scanning the data you will need the size of the destination buffer. This helps in making a secure port between the input stream (whatever it may be) to the destination, reducing the possibility of buffer overflow when accepting strings.

Edit:

As Cremno pointed out it will need to be sscanf_s(str, "%5[^,],%d,%d,%d,%d,%d,%d", ignore_data, (unsigned)sizeof(ignore_data), &x1, &x2, &x3, &y1, &y2, &y3);

The attention given to the fact that the the sizeof(ignore data) should return an unsigned value.

Answer 2

Quoting the C11 standard, Annex K, Chapter §K.3.5.3.2, The fscanf_s() function , ( emphasis mine )

The fscanf_s() function is equivalent to fscanf() except that the c , s , and [ conversion specifiers apply to a pair of arguments (unless assignment suppression is indicated by a *) . The first of these arguments is the same as for fscanf() . That argument is immediately followed in the argument list by the second argument, which has type rsize_t and gives the number of elements in the array pointed to by the first argument of the pair. [...]

Now, coming to the scope of sscanf_s() , chapter §K.3.5.3.7

The sscanf_s function is equivalent to fscanf_s , except that input is obtained from a string (specified by the argument s ) rather than from a stream. [...]

Hope the above text is enough to point out the error, you're missing the second argument required for [ , present in the format string. You need to supply the size for ignore_data , as rsize_t type.