Using atoi to check string

Question

i am trying to use atoi to convert a string into an int, and then check if the string was indeed all integers.

Here is my code:

printf("Please input the port: ");
scanf("%s",port_number);
int i = atoi(port_number);
if(i != port_number)
{
    errno = ERR_INVALID_PARAM;                  
    printf("Error invalid port number\n");
}

However, even when my string "port_number" is an integer, it still goes in the if statement? Any help?

Answer 1

I'm assuming that port_number is of type char *.

The if statement you have there is comparing the address of the port_number C string with value of i, and I don't think that's what you want to do.

If you want to use ctype.h, then you can use the function "isdigit()" to check each element of port_number. Otherwise, the next best thing is to cycle through port_number, and figure out if each element is between ascii char '0' and '9', to make sure that the port number is entered correctly.

Answer 2

You need to do some error-checking on the string coming in before you convert it to an int. You may want to try the strtol() function. It checks whether a string can be converted to a long int. It returns either the converted value, or zero if the string cannot be converted.

the second arg to strtol() is a char**. After the function finishes, it points to the first character in the string that is NOT a legal number character. So if it points anywhere before the end of the string, then the conversion failed.

http://www.tutorialspoint.com/c_standard_library/c_function_strtol.htm

Answer 3

Think about what you are comparing there:

if(i != port_number)
{
    //code
}

i is of type int and port_number is a char*.

Answer 4

char *check = strdup(port_number);
sprintf(check, "%i", i);
if(strcmp(check, port_number))