C String strcat() Function

What You’ll Learn

The strcat() function appends one C-string to another. It finds the null terminator of the destination, copies the source after it, and writes a new '\0'. It is one of the most common ways to build longer strings from smaller pieces.

Definition and Usage

strcat stands for string concatenate. The destination dest must already contain a valid null-terminated string—often initialized as an empty string with char buf[64] = ""; or char buf[64] = {0};.

The function walks dest until it finds '\0', then copies every character from src including the final null terminator. There is no length check: you are responsible for making sure the buffer is large enough.

📝 Syntax

Standard C declaration:

char *strcat(char *dest, const char *src);

Parameters

• dest — modifiable buffer that already holds a C-string.
• src — string to append (not modified).

Return Value

• Returns dest (pointer to the destination buffer).
• The concatenated string is stored in dest; no new string is allocated.

Header

• #include <string.h>

Examples Gallery

Compile with gcc strcat.c -std=c11 -o strcat. Every example uses a char array large enough for the result.

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

int main(void) {
    char destination[20] = "Hello, ";
    const char *source = "World!";

    strcat(destination, source);

    printf("Concatenated String: %s\n", destination);

    return 0;
}

Concatenated String: Hello, World!

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

int main(void) {
    char message[64] = "";

    strcat(message, "Learning ");
    strcat(message, "C ");
    strcat(message, "strings");

    printf("%s\n", message);

    return 0;
}

Learning C strings

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

int main(void) {
    char path[128] = "/home/user";
    const char *folder = "/projects";
    const char *file = "/main.c";

    strcat(path, folder);
    strcat(path, file);

    printf("Full path: %s\n", path);

    return 0;
}

Full path: /home/user/projects/main.c

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

int main(void) {
    char buf[32] = "Code";

    strcat(strcat(strcat(buf, "To"), "Fun"), "!");

    printf("%s\n", buf);

    return 0;
}

CodeToFun!

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

int main(void) {
    char line[48] = "Name: ";

    printf("%s\n", strcat(line, "Alex"));

    return 0;
}

Name: Alex

🚀 Common Use Cases

• Greeting messages — combine a prefix with a user name.
• File paths — join directory and file name segments.
• Log lines — append timestamps, levels, and text into one buffer.
• CSV or text output — add commas and fields to a row buffer.
• Protocol strings — build simple command lines in embedded systems.

📝 Notes

• dest must be writable—not a string literal like "hello".
• Leave room for strlen(dest) + strlen(src) + 1 bytes.
• dest and src must not overlap.
• No bounds checking—overflow corrupts memory and is a common bug.
• Prefer strncat, snprintf, or dynamic allocation for unknown lengths.

⚡ Optimization

Library strcat implementations scan dest from the start every time, so repeated appends to the same buffer cost O(n²) total if you keep re-walking the growing prefix. Track the write position or use stpcpy/snprintf when building long strings in a loop.

💡 Best Practices