CODEWITHSUNDEEP
cfunctionpointersarguments
Barak
Barak

Reputation: 43

C: How to make "pointer return function"'s argument NULL safe?

So, I am coding uppercase function with using pointers. I am trying to make a ERROR message when there is input but unfortunately in terminal I am seeing nothing like s==NULL is not working very well I cant see any ERROR messages.

Probably problem about argument types and pointer but I don't real seeing it!

If my Program Arguments like this:

"Hello 123dsd" "teSting"

Output is:

HELLO 123DSD

TESTING

If my program arguments is blank:

output is nothing:

I want to show ERROR message if arguments are empty like:

Please insert a string!

Here is my code:

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

#define MAGIC_NUMBER 32

char *uppercase(char *s)
{
    if (s == NULL)
    {
        printf("Please insert a string!");
        return 0; //Not working!!
    }
    else
    {
        for(char *p = s; *p!=0; ++p)
        {
            if ('a' <= *p && *p <= 'z')
                *p = *p - MAGIC_NUMBER;
        }

        return s;
    }
}


int main(int argc, char *argv[])
{
    for(int i=1; i < argc; i++)
    {
        printf("%s \n", uppercase(argv[i]));
    }


    return 0;
}

Upvotes: 0

Views: 264

Answers (2)

Bodo
Bodo

Reputation: 9855

If you call your program without arguments, it doesn't print anything because the loop will not call printf or uppercase since argc will have the value 1.

You have to add code to main to handle this case.

int main(int argc, char *argv[])
{

    if(argc < 2)
    {
        fprintf(stderr, "Please insert a string!\n")
        return 1;
    }

    for(int i=1; i < argc; i++)
    {
        printf("%s \n", uppercase(argv[i]));
    }


    return 0;
}

Note that I use fprintf(stderr, ...) for the error message because error messages should not be mixed with normal output.

Upvotes: 1

dbush
dbush

Reputation: 223699

If you don't enter any parameters to the program, argc will be 1 so the for loop in the main function is never entered. This also means that a NULL pointer will never be passed to uppercase.

You need to move the check for missing arguments to main:

int main(int argc, char *argv[])
{
    if (argc == 1) {
        printf("Please insert a string!");
        return 1;
    }
    for(int i=1; i < argc; i++)
    {
        printf("%s \n", uppercase(argv[i]));
    }

    return 0;
}

Upvotes: 1

Related Questions