C String strncat() Function

What You’ll Learn

The strncat() function appends at most n characters from one C-string to another. It is the bounded cousin of strcat(): you control how much of the source is copied, and the result is always null-terminated.

Definition and Usage

strncat stands for string n concatenate. It finds the null terminator of dest, then copies up to n characters from src, and writes a new '\0' after them.

If strlen(src) < n, the entire source string is appended. If strlen(src) ≥ n, only the first n characters are copied—the '!' in "world!" is dropped when n is 5.

📝 Syntax

Standard C declaration:

char *strncat(char *dest, const char *src, size_t n);

Parameters

• dest — modifiable buffer that already holds a null-terminated string.
• src — string to append from (not modified).
• n — maximum number of characters to copy from src (excluding the null byte strncat adds).

Return Value

• Returns dest (pointer to the destination buffer).

Header

• #include <string.h>

Examples Gallery

Compile with gcc strncat.c -std=c11 -o strncat. Each example uses a buffer sized for the final string.

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

int main(void) {
    char dest[20] = "Hello, ";
    const char *src = "world!";

    strncat(dest, src, 5);

    printf("Concatenated string: %s\n", dest);

    return 0;
}

Concatenated string: Hello, world

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

int main(void) {
    char dest[32] = "Code";
    const char *src = "ToFun";

    strncat(dest, src, 10);

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

    return 0;
}

CodeToFun

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

int main(void) {
    char dest[12] = "ID:";
    const char *src = "1234567890";
    size_t cap = sizeof(dest) - strlen(dest) - 1;

    strncat(dest, src, cap);

    printf("%s (len=%zu)\n", dest, strlen(dest));

    return 0;
}

ID:12345678 (len=11)

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

int main(void) {
    char dest[16] = "unchanged";
    const char *src = "EXTRA";

    strncat(dest, src, 0);

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

    return 0;
}

unchanged

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

int main(void) {
    char a[20] = "Hi ";
    char b[20] = "Hi ";
    const char *tail = "there!";

    strcat(a, tail);
    strncat(b, tail, 4);

    printf("strcat:   [%s]\n", a);
    printf("strncat:  [%s]\n", b);

    return 0;
}

strcat:   [Hi there!]
strncat:  [Hi ther]

🚀 Common Use Cases

• Fixed-size buffers — append user input without exceeding array bounds.
• Truncated labels — add a suffix when only a few characters may fit on screen.
• Log messages — cap how much of a long error string is appended to a line.
• Protocol fields — concatenate fixed-width token fragments in embedded code.
• Teaching strcat safety — introduce bounded concatenation before snprintf.

📝 Notes

• n limits source characters, not destination capacity—track buffer size yourself.
• dest must already be null-terminated before the call.
• Unlike strncpy, strncat always adds a final '\0'.
• If dest and src overlap, behavior is undefined.
• Modern code often prefers snprintf(dest + len, room, "%s", src) for clearer bounds.

⚡ Optimization

strncat must scan dest to find its end on every call, so repeated appends to the same buffer cost O(n²) total. Keep a running length or use snprintf with an offset when building long strings in a loop.

💡 Best Practices