Question

I try to convert a unsigned long into a character string, appending a comma at the end of the it. When compiling and running the test code you can find below, I get the following output:

"1234," "1234"
"1234"

The test code is:

#include <cstdio>
#include <iostream>

int main () {

    unsigned long c = 1234;
    char ch[50];
    char ch1[50];

    sprintf(ch, "%lu,", c);
    std::cout << "\"" << ch << "\"" << " \"" << c << "\"" << std::endl;

    snprintf(ch1, 5, "%s", ch); 
    std::cout << "\"" << ch1 << "\"" << std::endl;

    return 1;
}

As far as I understand, ch should be of length 5, 4 digits plus 1 for the comma.

Do I miss an extra plus one for the termination character?

Cheers!

Answer 1

The size that is passed to snprintf includes the terminating null character. Although it is not printed, it still takes space in the buffer.

You should pass strlen(ch) + 1 instead. Or even better, just sizeof(ch1) will suffice since you want to accommodate the entire result before filling the buffer.

Also make sure that the destination buffer is always of a sufficient size, equal to or greater than the size you pass to snprintf. In your particular case it can hardly happen, but in general you should keep that in mind.