Supposedly there is a bug in this code, but it runs fine and with an output that I expect ("hello world"). Is there a problem with return str?
#include <string.h>
char* example(){
// your code goes here
char str[12];
strncpy(str, "hello world", 11);
str[11] = 0;
printf("%s\n",str);
return str;
}
int main() {
char * check = example();
return 0;
}
Output:
Success time: 0 memory: 2248 signal:0
hello world
Big problem!
return str;
str is a local variable. You must not pass its address to the caller because the contents of what it's pointing to are not guaranteed to be what you assigned to it in example. This wouldn't be a problem if you defined str to be static:
static char str[12];
Edit 1
You can also malloc memory for str:
char *str = malloc(12);
If you choose this method, you must remember to free the dynamically allocated memory to prevent a memory leak.
Another method is to make str global:
char str[12];
char *example(void)
{
...
}
It is generally best to keep the scope of variables as limited as possible, however.
Edit 2
Still another method is to have the caller allocate memory for the string. The following is an example:
void example(char *str, size_t length)
{
strncpy(str, "hello world", length);
str[length - 1] = '\0'; // Guarantee string '\0' terminated
}
int main(void)
{
char str[12];
example(str, sizeof(str));
printf("%s\n", str);
exit(EXIT_SUCCESS);
}
The problem with this method is that, generally, the caller does not know the length of the string to be returned.
mallocing it or making it global?
malloc() also works but places the onus on the calling code to release the allocated memory (but strdup() does precisely that.
The bug is that the example() function returns a pointer to the local array str. When a function returns, all its local variables are no longer valid, and using pointers to them is undefined behavior.
It's apparently working because the main() function never actually does anything with the returned pointer. If you put
printf("%s", check);
in main() you would probably get garbage output there.
null byte '\0'? In any case, you don't have to manually set the null byte with strncpy.