Determine if a C string is a valid int in C

Question

I need to check if a C string is a valid integer.

I tried both

int num=atoi(str);

and

int res=sscanf(str, "%d", &num);

But sending the string "8 -9 10" in both of the lines returned simply 8, without indicating the invalidity of this string.

Can anyone suggest an alternative?

Answer 1

Have a look at strtol(), it can tell you about invalid parts of the string by pointer return.

And beware of enthusiastic example code.. see the man page for comprehensive error-handling.

Answer 2

Maybe I'll get flamed for not using strtol or similar libc functions, but reasoning about this problem is not that hard:

#include <stdbool.h>  // if using C99...  for C++ leave this out.
#include <ctype.h>

bool is_valid_int(const char *str)
{
   // Handle negative numbers.
   //
   if (*str == '-')
      ++str;

   // Handle empty string or just "-".
   //
   if (!*str)
      return false;

   // Check for non-digit chars in the rest of the stirng.
   //
   while (*str)
   {
      if (!isdigit(*str))
         return false;
      else
         ++str;
   }

   return true;
}

[NB: I might have otherwise done isdigit(*str++) instead of the else to keep it shorter but my recollection is that the standards say it's possible that isdigit is a macro.]

I guess one limitation is that this does not return false if the number in the string won't fit in an integer. That may or may not matter to you.