How to portably convert a string into an uncommon integer type?

Question

Some background: If I wanted to use for, for instance, scanf() to convert a string into a standard integer type, like uint16_t, I’d use SCNu1

Answer 1

There is one robust and portable solution, which is to use strtoimax() and check for overflows.

That is, I parse an intmax_t, check for an error from strtoimax(), and then also see if it "fits" in a pid_t by casting it and comparing it to the original intmax_t value.

#include 
#include 
#include 
#include 
char *xs = "17";            /* The string to convert */
intmax_t xmax;
char *tmp;
pid_t x;                    /* Target variable */

errno = 0;
xmax = strtoimax(xs, &tmp, 10);
if(errno != 0 or tmp == xs or *tmp != '\0'
   or xmax != (pid_t)xmax){
  fprintf(stderr, "Bad PID!\n");
} else {
  x = (pid_t)xmax;
  ...
}

It is not possible to use scanf(), because, (as I said in a comment) scanf() will not detect overflows. But I was wrong in saying that none of the strtoll()-related functions takes an intmax_t; strtoimax() does!

It also will not work to use anything else than strtoimax() unless you know the size of your integer type (pid_t, in this case).