c - printf doesn't work, even with newline and fflush()

Question

I'm compiling the below C code with gcc. No errors are thrown during compilation or at runtime. I ran through the code with gdb, and the answer given in sum is correct at the end, yet the printf() does not display anything on the screen. I've tried all sorts of combinations of fprintf(), printf(), and fflush(), but nothing works.

What do I need to change so the program will print the result to stdout?

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

int main()
{
        int num = 9;
        int i, sum; i = 1, sum = 0;
        while (i < 2 * num) {
                sum = sum + i * i;
                ++i;
        }
        printf("sum: %d\n", sum);
        fflush(stdout);
        return 0;
}

Answer 1

The code is correct, and should print sum: 1785 for any conforming implementation.

This is a guess (update: which turns out to be correct), but ...

You've named the source file test.c, and you compile it with:

$ gcc test.c -o test

(or something similar) and execute it with:

$ test

which produces no output.

The problem is that test is a standard Unix command (and also a built-in command in some shells). When you type a command name to the shell, it first looks for built-in commands, then for executables in directories specified in your $PATH environment variable.

To execute a command in the current directory, prepend ./ to the name:

$ ./test
sum: 1785
$ 

This applies to any command in the current directory. There are so many built-in commands that you can't reasonably avoid colliding with them. Cultivating the habit of running executables in the current directory by typing ./whatever means that name collisions don't matter.

(Don't be tempted to add . to the front of your $PATH; that's dangerous. Think about what could happen if you cd into a directory and type ls, if there happens to be a malicious ls command there.)

Answer 2

There is nothing wrong with your program. It has to work. Try running it with redirection:

./a.out > myout

..and see if you get any output. If not, I'd suspect there is a problem with some kind of standard library mismatch.

Another option to check would be to build using SUN C compiler as opposed to gcc and see if that works. If it does, gcc is the culprit.