strtoull 是否总是返回有效数字或设置 errno？

Question

Is strtoull guaranteed to either return a valid number, or set errno (if no valid number could be parsed)?

In other words, is this

errno = 0;
n = strtoull(s, 0, 10);
if (errno) {
  perror(s);
  exit(1);
}
// do something with n

correct, or is some other check also required?

Answer 1

No. The C standard (§7.22.1.7 of at least drafts N1570 through N2454, so C11 through C18) only mentions setting errno in one place:

If the correct value is outside the range of representable values, LONG_MIN , LONG_MAX , LLONG_MIN , LLONG_MAX , ULONG_MAX , or ULLONG_MAX is returned (according to the return type and sign of the value, if any), and the value of the macro ERANGE is stored in errno .

There is another possible error condition, however. “If no conversion could be performed, zero is returned,” and “the value of nptr is stored in the object pointed to by endptr , provided that endptr is not a null pointer.” Otherwise, a pointer to “the final string” would be stored there, which would compare greater to nptr .

Therefore, a truly standard-compliant C program should check both conditions. This test program, compiled with clang -std=c18 on Windows 10:

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

int main (void) {
  const char s[] = "a1";

  errno = 0;
  char* unparsed = NULL;
  const unsigned long long int n =
    strtoull( s, &unparsed, 10 );
  
  if ( errno || (!n && s == unparsed) ) {
    fflush(stdout); // Don't cross the streams!
    perror(s);
    exit(EXIT_FAILURE);
  }
  
  printf( "%llu\n", n );
  return EXIT_SUCCESS;
}

produces the output a1: No error , so on this implementation, errno is not set.

As chqrlie brings up in the comments, it would have sufficed to check ( errno || s == unparsed ) . If neither of these conditions hold, there was a conversion performed on a non-empty subject sequence.