How to Compile and Run C Code Online (No Install, With Input)

You don't need to install a compiler to write C. If you're learning the language, testing a snippet from a textbook or checking how a standard library function behaves, an online compiler runs your code in seconds. This guide shows how to compile and run C code online, how to feed it input, how to pick the language standard, and how to read the errors you'll meet first.

The examples below were compiled and run with GCC 11 using -std=c17 and also work in the online C compiler, which uses Clang. Paste any of them in and press Run.

Run your first C program

Open the C compiler, replace the code with this program and press Run:

#include <stdio.h>

int main(void) {
    printf("Hello, world!\n");
    return 0;
}

The output panel shows:

Hello, world!

Three parts matter in every C program:

• #include <stdio.h> pulls in the declarations for printf, scanf and the other standard input and output functions.
• int main(void) is the entry point. The program starts here and ends when main returns.
• return 0; tells the operating system the program succeeded. Any other value signals an error.

The \n at the end of the string is a newline. Without it, the output of the next printf runs onto the same line.

Read input from the keyboard

Programs that ask questions need stdin, the standard input stream. An online compiler can't pop up a terminal prompt mid-run, so you type the input into the stdin box before you press Run. Each line you type is what the program receives when it calls scanf, fgets or getchar.

This program reads two integers and prints their sum:

#include <stdio.h>

int main(void) {
    int a, b;
    if (scanf("%d %d", &a, &b) != 2) {
        fprintf(stderr, "expected two integers\n");
        return 1;
    }
    printf("sum = %d\n", a + b);
    return 0;
}

Type 3 4 into the stdin box and run it:

sum = 7

Type x instead and the program prints expected two integers to the error stream and exits with status 1. Always check the return value of scanf: it returns how many items it read, and ignoring that number is the most common cause of programs that print garbage when the input is wrong. For fgets, line-based input and the pitfalls of mixing scanf with getchar, see reading input in C and C++.

Choose the C standard

C has been revised several times, and some code only compiles in newer or older versions. The compiler lets you pick the standard, from C89 (also called ANSI C) up to C23:

If a loop such as for (int i = 0; i < n; i++) fails with an error about loop declarations (GCC says "'for' loop initial declarations are only allowed in C99 or C11 mode"), you've selected C89. Switch to C99 or later. This program uses a loop-scoped variable, so it needs C99 or newer:

#include <stdio.h>

int main(void) {
    int n = 5;
    for (int i = 1; i <= n; i++) {
        for (int j = 0; j < i; j++) putchar('*');
        putchar('\n');
    }
    return 0;
}

*
**
***
****
*****

Read the compiler errors

Compiler messages follow a pattern: file, line, column, severity, description. Take this program, which is missing a semicolon:

#include <stdio.h>

int main(void) {
    printf("Hello, world!\n")
    return 0;
}

GCC reports:

c.c: In function ‘main’:
c.c:4:30: error: expected ‘;’ before ‘return’

Two details to remember. First, the line number points at where the compiler noticed the problem, which is often the line after the real mistake: here the fix belongs at the end of line 4, but the compiler complains about the return that follows it. Second, one error can trigger a cascade of others, so fix the first message and recompile before chasing the rest. The full catalogue of messages you'll meet, such as undefined references and implicit declarations, is in C compiler errors explained.

Warnings are worth reading

A program can compile and still be wrong. Warnings catch many of those cases: an uninitialised variable, a format string that doesn't match its argument, a comparison between signed and unsigned values. Treat a warning as a bug report until you've proved it harmless. On your own machine you'd add -Wall -Wextra to the command; in the browser, read whatever the compiler prints alongside your output.

What an online compiler can and can't do

An online compiler is ideal for small programs, experiments and learning. Because the code runs in a sandbox, a few things behave differently from your desktop:

• No interactive prompts. All stdin must be supplied up front.
• No files from your disk. Programs can use the standard I/O streams, but they can't read your documents.
• No long-running or graphical programs. A program that waits for a key press or opens a window won't work.
• Limited resources. A program that runs forever or uses huge amounts of memory will be stopped by the browser.

On Devtoolry, the compiler runs entirely in your browser, so your code is not sent to a server. It uses Clang, which accepts the same C as GCC for everyday programs. If you're deciding which compiler to use locally, GCC vs Clang compares their error messages, warnings and speed.

Where to go next

Once you can run a program, the topics that trip up most beginners are strings and pointers. Both are covered with runnable examples: strings in C explains char arrays and the null terminator, and C pointers explained covers addresses, arrays and double pointers. Paste each example into the C compiler, change a line, and watch what breaks. That is the fastest way to learn the language.