November 11, 2018

Scanning unsigned types in C

printf and scanf are the C standard library functions for printing out and reading in data. They have string counterparts, prefixed with s, which work on strings (sprintf and sscanf). All these functions use a format string to match parts of the string with C variables. As an example, %lu in the format string corresponds to an unsigned long, a positive integer type in C.

Scanning unsigned types

What happens if you try to sscanf a negative number using %lu? printf will never print a negative number for %lu so sscanf should not scan one, right? Unfortunately, sscanf does match negative numbers for unsigned format strings. As an example, we can scan "-10" with format "%lu":

$ cat test.c
#include <stdio.h>

int main(void)
{
    unsigned long num = 0;
    int ret = sscanf("-10", "%lu", &num);
    printf("num=%lu\n", num);
    return ret;
}

The output shows that it matches, and converts the signed long to unsigned, like a type cast:

$ clang -o test test.c && ./test
num=18446744073709551606
$ echo $?
1

(The result is the same using gcc).

Introducing error checking

So, this can be annoying. You scan a small input string of 3 symbols, and get a 20 digit number as a result. In many cases, negative numbers should be invalid input.

One option is to check for the - sign before scanning, but this can be quite complicated for longer strings. The other option is to scan into a signed type, check the value, and then error if it’s incorrect. This means you lose 1 bit for the signed type, you cannot read as large numbers, but for most situations signed long is big enough.

A simple example:

$ cat test.c
#include <stdio.h>

int main(int argc, char **argv)
{
    if (argc != 2)
    {
        printf("Error: Please provide exactly 1 string argument.\n");
        return 1;
    }
    const char *const str = argv[1];
    long signed_num;

    int ret = sscanf(str, "%ld", &signed_num);
    if (ret <= 0)
    {
        printf("Error: Could not find a number in '%s'.\n", str);
        return 1;
    }
    if (signed_num < 0)
    {
        printf("Error: Negative numbers not allowed.\n");
        return 1;
    }

    unsigned long unsigned_num = signed_num;
    printf("Scanned num: %lu\n", unsigned_num);
    return 0;
}

This works as expected and has the necessary error checking:

$ gcc test.c -o test
$ ./test 1234
Scanned num: 1234
$ ./test -1
Error: Negative numbers not allowed.
$ ./test blah
Error: Could not find a number in 'blah'.
$ ./test
Error: Please provide exactly 1 string argument.
$

© Ole Herman Schumacher Elgesem 2018

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