Question

I’m having trouble with this…It compiles, but segfaults immediately when run…Using GDB, I determined that it segfaults while trying to initiate the long doubles. I feel like atoi might be the wrong function to use, but I tried other similar functions and still got a segfault.

int main(int argc, char *argv[]) {
  long double x = atoi(argv[1]);
  char oper = argv[2][0];
  long double y = atoi(argv[3]);

Answer 1

Use strtold:

#include <stdlib.h>
#include <errno.h>

char * e;
errno = 0;
long double d = strtold(argv[1], &e);

if (*e != 0 || errno != 0) { /* Error! Do not consume the result. */ }

// result now in "d"

You can use the value of *e to figure out whether any part of the string was consumed; see the manual for details. My example just checks whether the entire string could be parsed.

I’m actually not sure if there’s a thread-safe solution… (Oh, I think it is, because errno is thread-local.)