Question

I want to parse an int from a string (char*) in C. I’m writing a linux kernel module for embedded linux. I’m trying to use simple_strtol found here.

My tokens I’m parsing in an ideal situation will be numbers. The problem I have is "0" is a valid input. strtol returns 0 if a value could not be parsed (error). So that means that the following code:

char* token = "should_fail";

char **endptr;
char value = simple_strtol(token, endptr, 10); // 10 is the base to convert to
if (!**endptr){
    printf("failed!");
} else {
    printf(value);
}

char* token = "0"; // should pass
char **endptr;
char value = simple_strtol(token, endptr, 10); // 10 is the base to convert to
if (!**endptr){
    printf("failed!");
} else {
    printf(value);
}

prints out 0 in both cases, when it should fail in the first case. I am trying to use that *endptr argument to check to see if the conversion is successful, but its not working. According to what I’ve researched, *endptr is “A pointer to the end of the parsed string will be placed here”. And I believe if a conversion failed the pointer will point to nothing so I can identify that failure.

Does anybody know how I can parse "0" properly and get a return value of 0 while still identifying a failed parse, and not having it return 0?

Answer 1

You should not create a pointer-to-pointer variable for endptr. Instead declare a single pointer, and pass the address of that with the unary & operator:

char *endptr;

simple_strtol(token, &endptr, 10);

if (endptr == NULL) ...

---

I didn’t read the manual page but basically copied what the OP had used. Froom the manual page:

If the subject sequence is empty or does not have the expected form, no conversion is performed; the value of str is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

This means that if strtol (or your alternative if it’s working as specified) returns 0 and endptr == token then the string was invalid.

So the check should be

if (value == 0 && endptr == token)
{
    /* String is not a valid number */
}