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 SCNu16 from <inttypes.h>, like this:

#include <stdio.h> #include <inttypes.h> uint16_t x; char *xs = '17'; sscanf(xs, '%' SCNu16, &x); 

But a more uncommon integer type like pid_t does not have any such thing; only the normal integer types are supported by <inttypes.h>. To convert the other way, to portably printf() a pid_t, I can cast it to intmax_t and use PRIdMAX, like this:

#include <stdio.h> #include <inttypes.h> #include <sys/types.h> pid_t x = 17; printf('%' PRIdMAX, (intmax_t)x); 

However, there does not seem to be a way to portably scanf() into a pid_t. So this is my question: How to do this portably?

#include <stdio.h> #include <sys/types.h> pid_t x; char *xs = 17; sscanf(xs, '%u', &x);  /* Not portable! pid_t might not be int! /* 

I thought of scanf()ing to an intmax_t and then checking that the value is within pid_t’s limits before casting to pid_t, but there does not seem to be a way to get the maximum or minimum values for pid_t.

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 <inttypes.h> #include <stdio.h> #include <iso646.h> #include <sys/types.h> 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).