C String strncmp() Function

What You’ll Learn

The strncmp() function compares at most n characters of two C-strings. It works like strcmp() with a length cap—ideal for prefix checks, protocol headers, and bounded comparisons where you do not need to scan the entire string.

Definition and Usage

strncmp stands for string n compare. It walks both strings byte by byte, comparing up to n characters. It stops early if a mismatch is found or if a null terminator is reached in either string before n comparisons are done.

A return value of 0 means the compared portion is equal—not necessarily that the entire strings match. For example, strncmp("apple", "application", 5) == 0 even though strcmp would report a difference.

📝 Syntax

Standard C declaration:

int strncmp(const char *s1, const char *s2, size_t n);

Parameters

• s1 — first null-terminated string.
• s2 — second null-terminated string.
• n — maximum number of bytes to compare.

Return Value

• < 0 if the compared portion of s1 is less than s2.
• 0 if the compared portions are equal.
• > 0 if the compared portion of s1 is greater than s2.
• Only the sign is guaranteed for ordering—not the exact integer magnitude.

Header

• #include <string.h>

Examples Gallery

Compile with gcc strncmp.c -std=c11 -o strncmp. Each example shows a different way to interpret the return value.

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

int main(void) {
    const char *str1 = "Hello, World!";
    const char *str2 = "Hello, C!";

    int result = strncmp(str1, str2, 7);

    if (result == 0) {
        printf("The first 7 characters are equal.\n");
    } else {
        printf("The first 7 characters are not equal.\n");
    }

    return 0;
}

The first 7 characters are equal.

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

int main(void) {
    const char *word = "apple";
    const char *longer = "application";

    printf("strncmp(5): %d\n", strncmp(word, longer, 5));
    printf("strcmp:     %d\n", strcmp(word, longer));

    return 0;
}

strncmp(5): 0
strcmp:     -1

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

int main(void) {
    const char *request = "GET /index.html HTTP/1.1";

    if (strncmp(request, "GET ", 4) == 0) {
        printf("GET request detected\n");
    } else if (strncmp(request, "POST", 4) == 0) {
        printf("POST request detected\n");
    } else {
        printf("Other method\n");
    }

    return 0;
}

GET request detected

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

int main(void) {
    const char *a = "cat";
    const char *b = "dog";

    int r = strncmp(a, b, 3);

    if (r < 0) {
        printf("%s comes before %s\n", a, b);
    } else if (r > 0) {
        printf("%s comes after %s\n", a, b);
    } else {
        printf("Equal within 3 chars\n");
    }

    return 0;
}

cat comes before dog

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

int main(void) {
    const char *x = "completely different";
    const char *y = "nothing alike";

    printf("strncmp with n=0: %d\n", strncmp(x, y, 0));

    return 0;
}

strncmp with n=0: 0

🚀 Common Use Cases

• Prefix checks — verify filenames, commands, or tags start a certain way.
• Protocol parsing — classify HTTP methods, MIME types, or magic bytes in headers.
• Version strings — compare major version digits without the full semver tail.
• Bounded sorting — order fixed-width keys where only n bytes matter.
• Safer compares — limit work when strings may be very long.

📝 Notes

• 0 from strncmp means equal within the limit—not always full-string equality.
• Comparison is case-sensitive and uses unsigned char values.
• Not locale-aware; use strcoll for collation rules.
• Do not pass NULL pointers—behavior is undefined.
• Large n is safe; the function stops at null terminators anyway.

⚡ Optimization

Passing a small n can avoid scanning very long strings when you only need a prefix. For fixed-size binary blobs without null bytes, consider memcmp(s1, s2, n) instead—it always compares exactly n bytes and does not stop at '\0'.

💡 Best Practices