fgets doesn't work after scanf [duplicate]

Answer 1

Vayn,

Geekoaur has answered your question well, I'm just pointing out another "problem" with your code.

The line s1[strlen(s1)] = '\0'; is a no-op if s1 is allready properly null terminated BEFORE it executes.

But if s1 is NOT allready properly null terminated BEFORE this line executes (and you're unlucky) it will cause:

• a SIGSEGV on a POSIX (*nix) system.
• a GPF on Windows.

This is because strlen basicaly finds the index of the existing null-terminator and returns it! Here's a valid, unoptimized implementation of strlen:

int strlen(char *string) {
    int i = 0;
    while(string[i] != '\0') {
        ++i;
    }
    return i;
}

So... If you're REALLY worried about strings NOT being null-terminated then you'd do something like:

• string[sizeof(string)]='\0'; on local automatic strings (where the compiler "knows" the size of the string);
• or string[SIZE_OF_STRING] for all other strings, where SIZE_OF_STRING is (most commonly) a #define'd constant, or a variable which you maintain specifically to store the current-SIZE (not length) of a dynamically allocated string.

And if you're REALLY, REALLY, REALLY worried about strings not being null-terminated (like you're dealing with "dirty" libary methods (like Tuxedo's ATMI for example) you ALSO "clear" your "return strings" before passing them to the suspect library methods with:

• before:memset(string, NULL, SIZE_OF_STRING);
• invoke: DirtyFunction(/*out*/string);
• after: string[SIZE_OF_STRING]='\0'

SIG11's are a complete biatch to find because (unless you "hook" them with a signal-processor and say otherwise, they cause unix to hard-terminate your program, so you can't log anything (after the fact) to help figure out where-in-the-hell-did-that-come-from... especially considering that in many cases the line of code which throws the SIG11 is no-where-near the actual cause of the string loosing it's null-terminator.

Does that make sense to you?

Cheers mate. Keith.

PS: WARNING: strncpy does NOT allways nullterminate... you probably meant strlcpy instead. I learned this the hard way... when a 60 million dollar billing run crashed.

---

EDIT:

FYI: Here's a "safe" (unoptimized) version of strlen which I'll call strnlen (I reckon this should be in stdlib. Sigh.).

// retuns the length of the string (capped at size-1)
int strnlen(char *string, int size) {
    int i = 0;
    while( i