How to Test a Small C Snippet Quickly

Write a few lines of C, then build and run them with one gcc command.

Sometimes we only want to know whether a few lines of C work. Opening a full IDE for that is slow. Here are four quick ways, from no setup at all to a faster loop on our own machine.

1. Online compiler (no setup)

To check an algorithm or a syntax idea, an online sandbox is enough. We don’t need to install anything.

OnlineGDB: has a debugger with breakpoints.
Compiler Explorer (Godbolt): shows the assembly that GCC or Clang generate from our code.
Replit: good for saving and sharing snippets.

2. One command in the terminal

If GCC or Clang is installed, we save the snippet as test.c and build and run it in one line:

gcc -Wall -Wextra test.c -o test && ./test

&& runs ./test only if the compilation succeeded.
-Wall -Wextra turn on common warnings, so mistakes show up immediately. For example, int x; return x; gives warning: 'x' is used uninitialized.

We can even skip the file. gcc -x c - reads C code from standard input:

printf '#include <stdio.h>\nint main(void){printf("hi\\n");return 0;}\n' | gcc -x c - -o t && ./t

hi

3. Editor shortcuts

If we prefer an editor, a plugin can compile and run for us:

VS Code: install the Code Runner extension, then press Ctrl + Alt + N (Cmd + Option + N on Mac) with the .c file open.
Sublime Text: with a C build system, press Ctrl + B (Cmd + B on Mac).

4. Feed input from a file

If the program uses scanf, retyping the values on every run gets tedious. We put them in a file instead. Take this program:

#include <stdio.h>

int main(void) {
    int a, b;
    scanf("%d %d", &a, &b);
    printf("%d + %d = %d\n", a, b, a + b);
    return 0;
}

Create input.txt containing 3 4, then redirect it into the program with <:

./test < input.txt

3 + 4 = 7

Catch memory bugs too

Adding -fsanitize=address,undefined -g to the gcc command makes the program report out-of-bounds access and undefined behaviour while it runs. It is handy for short tests.

Summary

Quick idea, no compiler: use an online sandbox.
Compiler installed: gcc -Wall -Wextra test.c -o test && ./test.
Repeated scanf input: ./test < input.txt.

1. Online compiler (no setup)

2. One command in the terminal

3. Editor shortcuts

4. Feed input from a file

Summary

References