C: using strtol endptr is never NULL, cannot check if value is integer only?

Question

So here is the problem. I have a set of data that is supposed to be :

int int int int ....

however, I want that if I have 1asdas 2 , I want to be able to catch the "asdas" part. However, at this moment, if I only have 1 2, then endptr is not NULL and so I cannot check if the value was only a digit or a digit and letters. Here is my code:

            else if(token != NULL && token2 != NULL && token3 == NULL){
                    //we POSSIBLY encountered row and column values for the matrix
                    //convert the two numbers to longs base 10 number
                    int row = strtol(token, &result, 10);
                    int col = strtol(token2, &result2, 10);
                    printf("Result is %s and result2 is %s\n", result, result2);
                    //check to see if both numbers are valid
                    //this will be true if there were only 2 digits on the line
                    if(!result && !result2){
                            //SUCCESSFULL parsing of row and column
                            printf("SUCCESSFUL PARSING\n");
                    }
            }

Thanks!

Answer 1

Assuming that earlier code has already split up the line into individual numbers, then the check you want is

errno = 0;
long row = strtol(token, &endtoken, 10);
if (*endtoken != '\0')
    fprintf(stderr, "invalid number '%s' (syntax error)\n", token);
else if (endtoken == token)
    fprintf(stderr, "invalid number '' (empty string)\n");
else if (errno)
    fprintf(stderr, "invalid number '%s' (%s)\n", token, strerror(errno));
else
    /* number is valid, proceed */;

strtol will never set endtoken to a null pointer; it will set it to point to the first character that is not a digit . If that character is the NUL string terminator (note the slightly different spelling), then the entire string was a valid number, unless endtoken == token , which means that you gave strtol the empty string, which probably doesn't count as a valid number. The errno manipulation is necessary to catch numbers which were syntactically correct but outside the range of long .

You might be able to simplify your code by pulling numbers directly out of the line buffer rather than splitting it up first: assuming there are supposed to be exactly two numbers on any given line,

char *p = linebuf;
char *endp;
errno = 0;
long row = strtol(p, &endp, 10);
if (endp == p || !isspace(p) || errno)
  /* error, abandon parsing */;
p = endp;
long col = strtol(p, &endp, 10);
if (endp == p || p != '\0' || errno)
  /* error, abandon parsing */;