C Online Compiler | Free Online Compiler

Code

#include <stdio.h>

int main(int argc, char *argv[]) {
    printf("Hello, C!\\n");
    if (argc > 1) {
        printf("First arg: %s\\n", argv[1]);
    }
    return 0;
}

STDIN (optional)

Args (space separated)

Output

What Is the C Online Compiler?

This tool is a browser-based C compiler that lets you write, compile, and run C programs directly in your browser without installing anything locally. It is powered by the Piston API, which compiles and executes your C code on a remote sandboxed server and returns the output instantly.

The editor supports standard input via the STDIN field, which feeds data to functions like scanf() and fgets(). You can also pass command-line arguments through the Args field, making it possible to test programs that use argc and argv. Both compilation errors and runtime output appear in the Output panel, giving you immediate feedback on your code.

How It Works

Write your C code in the editor panel. The editor starts with a simple example that prints a greeting and reads command-line arguments, but you can replace it with any valid C program.
Provide input in the STDIN field if your program reads from standard input using scanf(), fgets(), or similar functions. Each line in the STDIN box corresponds to one line of input your program will receive. Leave this field empty if your program does not require any input.
Add command-line arguments in the Args field if your program uses argc and argv. Enter arguments separated by spaces, just as you would on a terminal.
Click the Run button to send your code to the Piston API execution server. The server compiles your C code with GCC and runs the resulting binary inside a sandboxed container.
View the output in the Output panel. You will see everything your program prints to stdout and stderr, including compilation warnings, errors, and runtime output.

Step-by-Step Example

Suppose you want to write a program that reads a name from standard input, greets the user, and also demonstrates command-line argument handling. Here is how you would do it using this compiler:

First, type the following code into the editor panel:

#include <stdio.h>

int main(int argc, char *argv[]) {
    char name[50];
    printf("Enter your name: ");
    scanf("%49s", name);
    printf("Hello, %s!\n", name);

    printf("You passed %d argument(s).\n", argc - 1);
    for (int i = 1; i < argc; i++) {
        printf("  argv[%d] = %s\n", i, argv[i]);
    }
    return 0;
}

Next, go to the STDIN field and type a name, for example Alice. This value will be read by the scanf() call when the program runs.

Then enter some arguments in the Args field, for example alpha beta.

Now click the Run button. The compiler sends your code and input to the execution server. After a moment, the Output panel displays the result:

Enter your name: Hello, Alice!
You passed 2 argument(s).
  argv[1] = alpha
  argv[2] = beta

The program reads the name from STDIN via scanf(), prints the greeting with printf(), and then iterates over the command-line arguments passed through the Args field. You can change the STDIN value or arguments and click Run again to test different inputs without modifying the code.

Use Cases

Learning C programming fundamentals. Beginners can experiment with variables, pointers, arrays, loops, structs, and other core language features in a zero-setup environment.
Testing algorithms and data structures. Quickly implement and verify sorting algorithms, linked lists, trees, and other data structures with sample inputs before integrating them into larger projects.
Practicing for coding interviews. Many technical interviews involve writing C functions. Use this compiler to practice problems, test edge cases, and refine your solutions with immediate feedback.
Completing homework and coursework assignments. Students can write and test C programs for class assignments directly in the browser without needing to install a compiler on their machine.
Prototyping embedded and systems programming concepts. While this environment does not provide hardware access, you can prototype algorithmic logic, bit manipulation routines, and memory management patterns before deploying to actual embedded targets.

Limitations and Notes

Sandboxed execution environment. Code runs inside an isolated container. The sandbox prevents access to the host system and restricts certain system calls for security.
No file I/O access. The execution environment does not allow reading or writing files on disk. Functions like fopen() and fwrite() will not work as expected.
Limited to the C standard library. Only headers and functions from the C standard library are available. Third-party libraries and system-specific headers are not supported.
Single-file compilation only. You cannot split your code across multiple source files or header files. Everything must be in a single main.c file.
Execution timeout enforced. Programs that take too long or enter infinite loops are terminated automatically to prevent resource abuse.
No interactive debugger. There is no step-through debugger available. Use printf() statements to inspect values and trace program flow.
Designed for testing and learning. This tool is intended for experimentation, practice, and quick verification. It should not be used for production workloads or processing sensitive data.

Frequently Asked Questions

What C standard is used?

The compiler typically supports C11 or later, depending on the GCC version.

Can I use scanf for input?

Yes, enter input in the STDIN field. scanf reads from it.

Can I include standard headers like stdio.h?

Yes, C standard library headers are fully available.

Are there external libraries?

No, only the C standard library. No third-party libs.

Can I compile multiple files?

No, single file (main.c) only.

Is there pointer and memory support?

Yes, malloc, free, pointer arithmetic all work.

What compiler is used?

GCC (GNU Compiler Collection) via Piston API.

Is there a time limit?

Yes, to prevent infinite loops.

C Compiler