Parse string in C with sscanf

Question

The string I am trying to parse is called str1, and it contains PRETTY_NAME="Ubuntu 20.04.1 LTS"

The goal is to have one variable contain PRETTY_NAME, and the other contains Ubuntu 20.04.1 LTS. I have declared both variables as char *var1, *var2

This has to be done with the sscanf function in C.

My current code looks like: sscanf(str1, "%s[^=]^s", var1. var2); The output I am receiving is that both var1, and var2 return (null).

What am I doing wrong?

That line can not possibly compile. Please post real code, see How to create a Minimal, Reproducible Example. — dxiv
– dxiv, Commented Jan 25, 2021 at 0:19
char *catch_str; That's an uninitialized pointer, so sscanf(str1, "%s %s", catch_str, ...); is undefined behavior. Either change it to char catch_str[100]; or allocate it dynamically. Same goes for my_str, then after that you'll still have to fix the sscanf format string. — dxiv
– dxiv, Commented Jan 25, 2021 at 0:26
Thank you. Could you provide any hints as to what I am doing wrong with my sscanf format? — Solomander
– Solomander, Commented Jan 25, 2021 at 0:49
You probably want something like "%[^=]=\"%[^\"]\"". Don't forget to check the return value of sscanf to make sure that parsing succeeded. — dxiv
– dxiv, Commented Jan 25, 2021 at 0:56

thakee nathees · Accepted Answer · 2021-01-25 00:54:44Z

1

the %s matches the entire string terminates at white space or \0 including = in that case, you can't use that here, and you haven't initialized any of your char buffer with a size which leads to crash or undefined behavior. here is a simplified parser.

void parse(const char* str, char* pretty, char* release) {
    int pretty_end, start, end;
    sscanf(str, "%*[^=]%n=\"%n%*[^\"]%n\"", &pretty_end, &start, &end);
    
    strncpy(pretty, str, pretty_end);
    pretty[pretty_end] = '\0';
    
    strncpy(release, str+start, end-start);
    release[end - start] = '\0';
}

int main() {

    const char* str = "PRETTY_NAME=\"Ubuntu 20.04.1 LTS\"";
    char pretty[256];
    char release[256];
    parse(str, pretty, release);

    return 0;
}

answered Jan 25, 2021 at 0:54

thakee nathees

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1 Comment

chux Over a year ago

Nice use of "%n", yet better to first end = 0 and test after sscanf() that it is not zero before using pretty_end, start, end.

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