C strncmp function

last modified April 8, 2025

String comparison is fundamental in C programming, and strncmp is a key function for comparing strings safely. This tutorial covers strncmp in depth, including its syntax, usage, and advantages over strcmp. We'll explore practical examples and discuss security considerations. Understanding strncmp helps write safer code by preventing buffer overflows during string comparisons.

What Is strncmp?

The strncmp function compares two strings up to a specified number of characters. It's declared in string.h and takes three parameters: two string pointers and a maximum comparison length. Unlike strcmp, it won't read beyond the specified limit, making it safer for untrusted input. It returns zero for equal strings, negative if the first string is less, and positive if greater in lexicographical order.

Basic strncmp Usage

This example demonstrates basic string comparison using strncmp.

basic_compare.c

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

int main() {
    char str1[] = "Hello";
    char str2[] = "Hello, World!";
    
    // Compare first 5 characters
    int result = strncmp(str1, str2, 5);

    if (result == 0) {
        printf("First 5 characters are equal\n");
    } else {
        printf("First 5 characters are different\n");
    }

    return 0;
}

Here, strncmp compares only the first 5 characters of both strings. The comparison is case-sensitive and stops at the first differing character or when the specified length is reached. This example shows how to safely compare string prefixes without risking buffer overflows. The result is zero because both strings start with "Hello".

Comparing Fixed-Length Strings

strncmp is ideal for comparing fixed-length strings or buffers, as shown here.

fixed_length_compare.c

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

int main() {
    char password[8] = "secret";
    char input[8] = "secrets";
    
    // Compare exactly 8 bytes (buffer size)
    int match = strncmp(password, input, sizeof(password));

    if (match == 0) {
        printf("Access granted\n");
    } else {
        printf("Access denied\n");
    }

    return 0;
}

This example compares two fixed-size buffers completely, including any padding bytes. The comparison uses sizeof to get the exact buffer size. This approach prevents timing attacks that might occur with null-terminated string comparisons. Note that for security-critical code, specialized comparison functions might be better.

Case-Insensitive Comparison

This example demonstrates case-insensitive comparison using strncasecmp.

case_insensitive.c

#include <stdio.h>
#include <strings.h> // For strncasecmp

int main() {
    char str1[] = "HELLO";
    char str2[] = "hello";
    
    // Case-insensitive compare first 5 chars
    int result = strncasecmp(str1, str2, 5);

    if (result == 0) {
        printf("Strings are equal ignoring case\n");
    } else {
        printf("Strings are different\n");
    }

    return 0;
}

strncasecmp (or _strnicmp on Windows) performs case-insensitive comparison up to the specified length. This is useful for case-insensitive matching while maintaining length safety. The function is declared in strings.h on Unix-like systems. Note that locale settings may affect case conversion rules.

Comparing Substrings

This example shows how to compare substrings using pointer arithmetic with strncmp.

substring_compare.c

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

int main() {
    char text[] = "The quick brown fox";
    char search[] = "quick";
    
    // Compare substring starting at position 4
    int found = strncmp(text + 4, search, strlen(search));

    if (found == 0) {
        printf("Substring found\n");
    } else {
        printf("Substring not found\n");
    }

    return 0;
}

Here, we compare a substring of text starting at position 4 with the search string. Pointer arithmetic (text + 4) moves to the desired starting position. This technique is useful for parsing and searching within strings. Always ensure the substring length doesn't exceed the remaining string length to avoid undefined behavior.

Safe String Comparison with Length Check

This example demonstrates a safer comparison that checks string lengths first.

safe_compare.c

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

int safe_str_compare(const char *s1, const char *s2, size_t max_len) {
    size_t len1 = strnlen(s1, max_len);
    size_t len2 = strnlen(s2, max_len);
    
    if (len1 != len2) return 1; // Different lengths
    return strncmp(s1, s2, max_len);
}

int main() {
    char input1[20] = "password";
    char input2[20] = "password123";
    
    if (safe_str_compare(input1, input2, sizeof(input1)) {
        printf("Strings are different\n");
    } else {
        printf("Strings are equal\n");
    }

    return 0;
}

This example combines strnlen and strncmp for a more robust comparison. The helper function first checks if strings have equal lengths within the limit. This prevents partial matches from being considered equal when lengths differ. The strnlen function ensures we don't read beyond the specified maximum length, adding another safety layer.

Best Practices for Using strncmp

Specify maximum length: Always set a reasonable limit to prevent overflows.
Check lengths first: Consider comparing lengths before content for efficiency.
Handle null terminators: Remember strncmp may compare non-string data.
Use case-insensitive variants: When case doesn't matter, use strncasecmp.
Validate input: Ensure pointers are valid before comparison.

Source

C strncmp Documentation

This tutorial has explored the strncmp function, from basic usage to advanced safety considerations. While simple, proper string comparison is crucial for security and correctness in C programs. Always prefer length-limited functions like strncmp over their unlimited counterparts.

Author

My name is Jan Bodnar, and I'm a dedicated programmer with a deep passion for coding. Since 2007, I've been sharing my expertise through over 1,400 articles and 8 e-books. With more than a decade of teaching experience, I strive to make programming accessible and engaging.

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