How to Concatenate Strings in C: Simple Guide with Examples

In C, you concatenate strings using the strcat function from string.h, which appends one string to another. You must ensure the destination string has enough space to hold the combined result to avoid errors.

Syntax

The basic syntax to concatenate strings in C is using the strcat function:

• strcat(destination, source);

Here, destination is the string to which source will be appended. The destination must have enough space to hold the combined string including the null terminator.

c

char *strcat(char *destination, const char *source);

Example

This example shows how to concatenate two strings safely using strcat. We allocate enough space in the destination array to hold both strings.

c

#include <stdio.h>
#include <string.h>

int main() {
    char greeting[50] = "Hello, ";
    char name[] = "Alice!";

    strcat(greeting, name);  // Append name to greeting

    printf("%s\n", greeting);  // Output the combined string
    return 0;
}

Output

Hello, Alice!

Common Pitfalls

Common mistakes when concatenating strings in C include:

• Not allocating enough space in the destination string, causing buffer overflow.
• Using uninitialized or non-null-terminated strings.
• Overwriting memory by concatenating to a string literal (which is read-only).

Always ensure the destination array is large enough and properly initialized.

c

#include <stdio.h>
#include <string.h>

int main() {
    // Wrong: destination too small
    char small[5] = "Hi";
    char add[] = " there!";
    // strcat(small, add); // This causes overflow - unsafe!

    // Right: allocate enough space
    char large[20] = "Hi";
    strcat(large, add);
    printf("%s\n", large);
    return 0;
}

Output

Hi there!

Quick Reference

Function	Description
strcat(dest, src)	Appends src string to dest string
strcpy(dest, src)	Copies src string to dest string
strlen(str)	Returns length of string str
Ensure dest has enough space	Prevent buffer overflow errors

Key Takeaways

Use strcat to append one string to another in C.

Always allocate enough space in the destination string to hold the result.

Never concatenate to string literals; use writable arrays instead.

Initialize strings properly with a null terminator before concatenation.

Check string lengths with strlen to avoid overflow.