using scanf to read a string and an int separated by /

You only need to run the following program:

#include <stdio.h>

int main (void) {
    char str[20] = {'\0'};
    int count, num = 42;

    count = sscanf ("hello/17", "%s/%d", str, &num);

    printf ("String was '%s'\n", str);
    printf ("Number was %d\n", num);
    printf ("Count  was %d\n", count);

    return 0;
}

to see why this is happening. The output is:

String was 'hello/17'
Number was 42
Count  was 1

The reason has to do with the %s format specifier. From C99 7.19.6.2 The fscanf function (largely unchanged in C11, and the italics are mine):

s: matches a sequence of non-white-space characters.

Since / is not white space, it gets included in the string bit, as does the 17 for the same reason. That's also indicated by the fact that sscanf returns 1, meaning that only one item was scanned.

What you'll then be looking for is something that scans any characters other than / into the string (including white space). The same section of the standard helps out there as well:

[: matches a nonempty sequence of characters from a set of expected characters (the scanset). The conversion specifier includes all subsequent characters in the format string, up to and including the matching right bracket (]). The characters between the brackets (the scanlist) compose the scanset, unless the character after the left bracket is a circumflex (^), in which case the scanset contains all characters that do not appear in the scanlist between the circumflex and the right bracket.

In other words, something like:

#include <stdio.h>
int main (void) {
    char str[20] = {'\0'};
    int count, num = 42;

    count = sscanf ("hello/17", "%[^/]/%d", str, &num);

    printf ("String was '%s'\n", str);
    printf ("Number was %d\n", num);
    printf ("Count  was %d\n", count);

    return 0;
}

which gives you:

String was 'hello'
Number was 17
Count  was 2

---

One other piece of advice: never ever use scanf with an unbounded %s or %[; you're asking for a buffer overflow attack. If you want a robust user input function, see this answer.

Once you have it in as a string, you can sscanf it to your heart's content without worrying about buffer overflow (since you've limited the size on input).

scanf awaits a whitespace terminated string when it tries to read %s.

Try to specify the forbidden character set directly:

  scanf("%19[^/]/%d", str, &num);

You can read more about the formating codes here