GCC 13.2.0 (C17 Standard)Isolated Sandbox

Online C Compiler — Write, Compile & Run C Code Online

Veyrai's Online C Compiler is powered by GCC 13.2.0 under the modern C17 standard. It provides an ultra-low-latency, zero-configuration compiler sandbox for learning C fundamentals, testing pointer arithmetic, debugging memory allocations with malloc and free, and solving low-level systems programming interview problems.

Example:

Execution Output

Click "Run (Ctrl+Enter)" to compile and execute your C program in the isolated sandbox.

How to Use the Online C Compiler

Include Standard Headers

Always start your code with standard library includes such as '#include <stdio.h>' for printing and '#include <stdlib.h>' for dynamic memory.

Define main() Returning an Integer

C programs require 'int main()'. Returning 0 signals successful execution to the operating system; non-zero values signal errors.

Standard Input with scanf()

When reading inputs with scanf(), pass the memory address of the target variable using the address-of operator '&' (e.g., scanf("%d", &val)).

Inspect Compiler Output & Warnings

Click 'Run Code'. GCC compiles with strict diagnostic output. Any warnings or syntax errors are printed with line numbers in the output console.

C Code Examples & Algorithms

Each example is verified to run in our production GCC 13.2.0 (C17 Standard) runtime. Click "Load in Editor" to test and modify any example above.

1. Formatted Standard Input & Arithmetic

Prompt and read multiple integers from stdin, computing sum and average.

Time: O(1)

#include <stdio.h>

int main() {
    int num1, num2;
    printf("Reading two integers from stdin...\n");
    if (scanf("%d %d", &num1, &num2) == 2) {
        int sum = num1 + num2;
        float avg = (float)sum / 2.0;
        printf("Number 1: %d\n", num1);
        printf("Number 2: %d\n", num2);
        printf("Sum:      %d\n", sum);
        printf("Average:  %.2f\n", avg);
    } else {
        printf("Error: Could not read 2 integers. Provide input in the stdin drawer.\n");
    }
    return 0;
}

2. In-Place Array Reversal Using Pointers

Reverse an integer array in-place using two pointer offsets without allocating secondary memory.

Time: O(N)

#include <stdio.h>

void reverseArray(int *arr, int size) {
    int *left = arr;
    int *right = arr + size - 1;
    while (left < right) {
        int temp = *left;
        *left = *right;
        *right = temp;
        left++;
        right--;
    }
}

int main() {
    int arr[] = { 10, 20, 30, 40, 50, 60 };
    int size = sizeof(arr) / sizeof(arr[0]);

    printf("Original: ");
    for (int i = 0; i < size; i++) printf("%d ", arr[i]);
    printf("\n");

    reverseArray(arr, size);

    printf("Reversed: ");
    for (int i = 0; i < size; i++) printf("%d ", arr[i]);
    printf("\n");
    return 0;
}

3. Dynamic Memory Allocation (malloc & free)

Allocate heap memory dynamically with malloc, check for NULL allocation, populate, and safely free.

Time: O(N)

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

int main() {
    int n = 5;
    int *heapArr = (int *)malloc(n * sizeof(int));

    if (heapArr == NULL) {
        fprintf(stderr, "Memory allocation failed!\n");
        return 1;
    }

    for (int i = 0; i < n; i++) {
        heapArr[i] = (i + 1) * 100;
    }

    printf("Allocated Heap Array: ");
    for (int i = 0; i < n; i++) {
        printf("%d ", heapArr[i]);
    }
    printf("\n");

    // Clean up heap allocation to prevent memory leaks
    free(heapArr);
    heapArr = NULL;
    printf("Memory successfully freed.\n");
    return 0;
}

Common C Beginner Mistakes & Fixes

Missing '&' in scanf() Arguments

Passing a value instead of a pointer address to scanf() writes input data to arbitrary memory addresses, causing a segmentation fault.

Fix: Write 'scanf("%d", &variable)' for primitives. Arrays and strings already decay to pointers.

Buffer Overflow with gets() or unbounded strings

Reading strings without bounds checks (e.g. gets or '%s' in scanf) overwrites adjacent stack memory, creating severe security vulnerabilities.

Fix: Use 'fgets(buffer, sizeof(buffer), stdin)' or '%19s' specifiers with scanf.

Dangling Pointers and Forgetting free()

Failing to free heap memory causes leaks; referencing memory after free() causes undefined behavior.

Fix: Always call free(ptr) when done and immediately set ptr = NULL.

Environment Specifications

Language: C
Compiler / Engine: GCC 13.2.0 (C17 Standard)
Standard Input: Supported (stdin)
Isolation Layer: Dockerized Sandbox
Execution Timeout: 8s to 25s max
Guest Access: Allowed (Rate limited)

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