Thread: cheking the validity of data from stdin

Originally Posted by tindala

i am trying to test what happens in the sscanf converted types (command and string) if they are smaller, i got surprised that the printf worked, so i dont know why.

You are probably running into undefined behaviour. The space you set aside for command and string in the above example is not large enough to hold the strings that you are giving it. When you use %s in scanf, you have to make sure there is enough room in the destination

Originally Posted by tindala

what i really want to do is for example: i have this string from the stdin which must have the following format:
connect name.surname

In that case make use of %[...] in the scanf format and give the allowed characters. Then you must make sure each token corresponds to your specs (length and so on). Here is an example:

Code:

int main()
{
    char secret[LINE_MAX] = "command john.smith";
    char token[3][TOKEN_MAX];
    sscanf(secret, "%[a-zA-Z] %[a-zA-Z].%[a-zA-Z]", token[0], token[1], token[2]);
    for (int i=0; i < 3; i++) {
        printf("token%d: %s\n", i, token[i]);
    }
    return 0;
}

Originally Posted by c99tutorial

You are probably running into undefined behaviour.

That's a certainty. One of the ostensible joys of code with undefined behaviour is that it can seem to work in a well defined manner. That doesn't make it correct. It simply means the error has not been detected.

Unfortunately, programmers are often inclined to believe it is correct on the basis of it running, correctly, once ....