Why am I getting a segfault, sprintf?

Question

In my small program, I am sigfaulting, why?

int main(int argc, char *argv[])
{
char *string;
int n = 5;

sprintf(string, "this is %d", n + 1);
printf(string);

return 0;
}

Answer 1

*string is a pointer. As it is not assigned to point to a location with sufficient space for your sprintf expression there could be and is a segmenation fault.

char str[32];

char *string = str;

int n = 5;

sprintf(string, "this is %d", n + 1);

printf(string)



would suffice.

Answer 2

Short answer, you don't currently own the variable:



char *string;



It's address in memory is currently unknown and potentially volatile. So, sprintf is doing exactly what it's supposed to do, and writing to that address. You would need to allocate space for it using malloc, and then free it when you're finished.



Also, your call to printf should look more like this:



printf("%s",string);



I hope this helps.

Answer 3

You need to allocate a size to string. Instead of

char *string;



use



char string[50];



(or replace 50 with the maximum size of a string you expect)