What is strncmp()?

The strncmp() function in C compares at most the first n characters of two strings. It is part of the <string.h> library and provides a safer alternative to strcmp() by preventing comparison beyond specified length.

Function Syntax

int strncmp(const char *str1, const char *str2, size_t n);

Parameter	Description
`str1`	First string to compare
`str2`	Second string to compare
`n`	Maximum number of characters to compare
Return	Negative, zero, or positive integer

Return Values

Return Value	Meaning
0	Strings are equal for first n characters
< 0 (negative)	str1 is less than str2 (first differing char is smaller)
> 0 (positive)	str1 is greater than str2 (first differing char is larger)

Basic Usage

#include <stdio.h> #include <string.h> int main() { char str1[] = "Hello World"; char str2[] = "Hello C Programming"; // 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 differ\n"); } // Compare first 6 characters result = strncmp(str1, str2, 6); if(result == 0) { printf("First 6 characters are equal\n"); } else { printf("First 6 characters differ\n"); } return 0; }

Output:

First 5 characters are equal First 6 characters differ

Comparing Complete Strings

#include <stdio.h> #include <string.h> int main() { char str1[] = "Apple"; char str2[] = "Apple"; char str3[] = "Banana"; char str4[] = "Apricot"; // Compare full strings (use length or large n) if(strncmp(str1, str2, 10) == 0) { printf("str1 and str2 are equal\n"); } // Compare first 2 characters int result = strncmp(str1, str4, 2); printf("Comparing 'Ap' from Apple vs Apricot: %d\n", result); // 0 (equal) // Compare first 3 characters result = strncmp(str1, str4, 3); printf("Comparing 'App' vs 'Apr': %d\n", result); // Negative ('p' < 'r') // Compare strings with different lengths result = strncmp(str2, str3, 5); printf("Apple vs Banana: %d\n", result); // Negative ('A' < 'B') return 0; }

Practical Example 1: Case-Insensitive Comparison

#include <stdio.h> #include <string.h> #include <ctype.h> int strnicmp(const char *str1, const char *str2, size_t n) { for(size_t i = 0; i < n; i++) { char c1 = tolower(str1[i]); char c2 = tolower(str2[i]); if(c1 == '\0' && c2 == '\0') return 0; if(c1 != c2) return c1 - c2; } return 0; } int main() { char input[] = "Hello"; char command[] = "HELLO"; // Standard strncmp (case-sensitive) if(strncmp(input, command, 5) == 0) { printf("strncmp: Equal\n"); } else { printf("strncmp: Not equal (case matters)\n"); } // Case-insensitive comparison if(strnicmp(input, command, 5) == 0) { printf("strnicmp: Equal (ignoring case)\n"); } return 0; }

Practical Example 2: Input Validation

#include <stdio.h> #include <string.h> int main() { char userInput[100]; char validCommands[][10] = {"HELP", "EXIT", "LIST", "SAVE"}; printf("Enter command: "); fgets(userInput, sizeof(userInput), stdin); // Remove newline userInput[strcspn(userInput, "\n")] = '\0'; // Check each command int found = 0; for(int i = 0; i < 4; i++) { // Compare ignoring case by converting both to uppercase if(strncmp(validCommands[i], userInput, strlen(validCommands[i])) == 0) { printf("Command recognized: %s\n", validCommands[i]); found = 1; break; } } if(!found) { printf("Unknown command\n"); } return 0; }

Practical Example 3: File Extension Checker

#include <stdio.h> #include <string.h> int hasExtension(const char *filename, const char *extension) { int len = strlen(filename); int extLen = strlen(extension); if(len < extLen) return 0; // Compare last extLen characters const char *fileExt = filename + len - extLen; return (strncmp(fileExt, extension, extLen) == 0); } int main() { char files[][50] = { "document.txt", "image.jpg", "script.c", "data.csv", "notes.txt" }; printf("Text files (.txt):\n"); for(int i = 0; i < 5; i++) { if(hasExtension(files[i], ".txt")) { printf(" - %s\n", files[i]); } } printf("\nC files (.c):\n"); for(int i = 0; i < 5; i++) { if(hasExtension(files[i], ".c")) { printf(" - %s\n", files[i]); } } return 0; }

Output:

Text files (.txt): - document.txt - notes.txt C files (.c): - script.c

Practical Example 4: Prefix Checking

#include <stdio.h> #include <string.h> int startsWith(const char *str, const char *prefix) { return strncmp(str, prefix, strlen(prefix)) == 0; } int endsWith(const char *str, const char *suffix) { int strLen = strlen(str); int suffixLen = strlen(suffix); if(strLen < suffixLen) return 0; return strncmp(str + strLen - suffixLen, suffix, suffixLen) == 0; } int main() { char strings[][50] = { "https://google.com", "http://example.com", "ftp://files.com", "https://github.com" }; printf("URLs starting with 'https':\n"); for(int i = 0; i < 4; i++) { if(startsWith(strings[i], "https")) { printf(" - %s\n", strings[i]); } } printf("\nURLs ending with '.com':\n"); for(int i = 0; i < 4; i++) { if(endsWith(strings[i], ".com")) { printf(" - %s\n", strings[i]); } } return 0; }

Output:

URLs starting with 'https': - https://google.com - https://github.com URLs ending with '.com': - https://google.com - http://example.com

strncmp() vs strcmp()

Feature	strcmp()	strncmp()
Compares	Entire strings	At most n characters
Safety	Less safe (no limit)	Safer (bounds check)
Parameters	2 parameters	3 parameters
Null characters	Stops at '\0' in either string	Stops at '\0' or after n chars
Use case	Exact full string match	Prefix check, partial compare

#include <stdio.h> #include <string.h> int main() { char str1[] = "Hello A"; char str2[] = "Hello B"; char str3[] = "Hello World"; // strcmp compares full strings printf("strcmp(Hello A, Hello B): %d\n", strcmp(str1, str2)); printf("strcmp(Hello A, Hello World): %d\n", strcmp(str1, str3)); // strncmp compares only first 6 chars printf("strncmp(Hello A, Hello B, 6): %d\n", strncmp(str1, str2, 6)); printf("strncmp(Hello A, Hello World, 6): %d\n", strncmp(str1, str3, 6)); // Both functions find differences char str4[] = "Hello"; char str5[] = "Hello\0Extra"; printf("strcmp with null: %d\n", strcmp(str4, str5)); // 0 (stops at null) printf("strncmp with null: %d\n", strncmp(str4, str5, 10)); // 0 (stops at null) return 0; }

Manual Implementation of strncmp()

#include <stdio.h> int my_strncmp(const char *str1, const char *str2, size_t n) { for(size_t i = 0; i < n; i++) { // If both strings end at same position, they're equal if(str1[i] == '\0' && str2[i] == '\0') { return 0; } // If characters differ, return difference if(str1[i] != str2[i]) { return (unsigned char)str1[i] - (unsigned char)str2[i]; } // If only str1 ends, str1 is less if(str1[i] == '\0') { return -1; } // If only str2 ends, str1 is greater if(str2[i] == '\0') { return 1; } } // All n characters are equal return 0; } int main() { char test1[] = "Hello"; char test2[] = "Hello World"; char test3[] = "Help"; printf("My Implementation:\n"); printf("Compare 'Hello' vs 'Hello World' (5 chars): %d\n", my_strncmp(test1, test2, 5)); // 0 (equal) printf("Compare 'Hello' vs 'Help' (4 chars): %d\n", my_strncmp(test1, test3, 4)); // Positive ('o' vs 'p') printf("Compare 'Hello' vs 'Help' (3 chars): %d\n", my_strncmp(test1, test3, 3)); // 0 ('Hel' vs 'Hel') printf("\nStandard strncmp:\n"); printf("Compare 'Hello' vs 'Hello World' (5 chars): %d\n", strncmp(test1, test2, 5)); printf("Compare 'Hello' vs 'Help' (4 chars): %d\n", strncmp(test1, test3, 4)); printf("Compare 'Hello' vs 'Help' (3 chars): %d\n", strncmp(test1, test3, 3)); return 0; }

Checking String Prefixes

#include <stdio.h> #include <string.h> int main() { char strings[][30] = { "apple_fruit.txt", "apple_pie_recipe.txt", "banana_fruit.txt", "apple_sauce.txt" }; printf("All apple-related files:\n"); for(int i = 0; i < 4; i++) { if(strncmp(strings[i], "apple", 5) == 0) { printf(" - %s\n", strings[i]); } } // Check for multiple prefixes printf("\nFruit files:\n"); for(int i = 0; i < 4; i++) { if(strncmp(strings[i], "apple", 5) == 0 || strncmp(strings[i], "banana", 6) == 0) { printf(" - %s\n", strings[i]); } } return 0; }

Using strncmp() with User Input

#include <stdio.h> #include <string.h> int main() { char input[100]; char response[10]; printf("Do you want to continue? (yes/no): "); fgets(input, sizeof(input), stdin); // Remove newline input[strcspn(input, "\n")] = '\0'; // Check if user typed 'yes' (case-sensitive) if(strncmp(input, "yes", 3) == 0) { printf("Continuing...\n"); } // Check for 'no' else if(strncmp(input, "no", 2) == 0) { printf("Exiting...\n"); } // Check for 'y' or 'n' shortcuts else if(strncmp(input, "y", 1) == 0) { printf("Continuing (shortcut)...\n"); } else if(strncmp(input, "n", 1) == 0) { printf("Exiting (shortcut)...\n"); } else { printf("Invalid input\n"); } return 0; }

Common Mistakes

#include <stdio.h> #include <string.h> int main() { // MISTAKE 1: Mixing up parameter order char str1[] = "Apple"; char str2[] = "Banana"; // WRONG: Order matters int result = strncmp(str2, str1, 3); // Compares Banana vs Apple // CORRECT: strncmp(str1, str2, n) if(strncmp(str1, str2, 3) < 0) { printf("'Apple' < 'Banana'\n"); } // MISTAKE 2: Using negative n (size_t is unsigned) // strncmp(str1, str2, -1); // WRONG! Causes large positive number // MISTAKE 3: Assuming strncmp returns only -1, 0, 1 int cmp = strncmp("ABC", "ABD", 3); printf("Comparison result: %d\n", cmp); // Might be -1, -100, etc. // Fixed: Check sign, not specific value if(cmp < 0) { printf("'ABC' is less than 'ABD'\n"); } // MISTAKE 4: Not handling null terminators properly char text[] = "Hello"; char prefix[] = "HelloWorld"; // Compares only 'Hello' (stops at null in text) if(strncmp(text, prefix, 10) == 0) { printf("Equal within 10 chars (stops at null)\n"); } return 0; }

Performance Comparison

#include <stdio.h> #include <string.h> #include <time.h> int main() { char str1[] = "The quick brown fox jumps over the lazy dog"; char str2[] = "The quick brown fox jumps over the lazy cat"; clock_t start, end; double cpu_time; // Compare full string start = clock(); for(int i = 0; i < 1000000; i++) { strcmp(str1, str2); } end = clock(); printf("strcmp (full): %.3f seconds\n", (double)(end - start) / CLOCKS_PER_SEC); // Compare only first 20 chars start = clock(); for(int i = 0; i < 1000000; i++) { strncmp(str1, str2, 20); } end = clock(); printf("strncmp (20 chars): %.3f seconds\n", (double)(end - start) / CLOCKS_PER_SEC); // Compare only first 5 chars start = clock(); for(int i = 0; i < 1000000; i++) { strncmp(str1, str2, 5); } end = clock(); printf("strncmp (5 chars): %.3f seconds\n", (double)(end - start) / CLOCKS_PER_SEC); return 0; }

Quick Reference Table

Scenario	Code Example	Use Case
Exact full match	`strncmp(a, b, strlen(a) + 1)`	Full string comparison
Prefix check	`strncmp(str, prefix, strlen(prefix)) == 0`	Check if string starts with prefix
First n chars	`strncmp(a, b, n) == 0`	Compare only first n characters
Case-insensitive	Custom function using `tolower()`	Ignore case differences
File extension	`strncmp(filename + len - 3, ".txt", 3)`	Check file type
User command	`strncmp(input, "exit", 4) == 0`	Partial command matching

Summary

strncmp() compares at most n characters of two strings
Returns 0 if first n characters are equal
Returns negative if str1 < str2
Returns positive if str1 > str2
Safer than strcmp() for bounded comparison
Useful for prefix checking and input validation
Stops at null terminator even before n characters
n should be less than or equal to buffer sizes
Include <string.h> header file
Returns difference of ASCII values at first mismatch

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What is strncmp()?

Function Syntax

Return Values

Basic Usage

Comparing Complete Strings

Practical Example 1: Case-Insensitive Comparison

Practical Example 2: Input Validation

Practical Example 3: File Extension Checker

Practical Example 4: Prefix Checking

strncmp() vs strcmp()

Manual Implementation of strncmp()

Checking String Prefixes

Using strncmp() with User Input

Common Mistakes

Performance Comparison

Quick Reference Table

Summary

Complete C Programming Guide + Compilers Collection

1. C srand() Function – Understanding Seed Initialization

2. C rand() Function Mechanics and Limitations

3. C log() Function

4. Mastering Date and Time in C

5. Mastering time_t Type in C

6. C exp() Function

7. C log() Function (Alternate Guide)

8. C log10() Function

9. C tan() Function

10. Random Numbers in C (Secure vs Predictable)

11. Free Online C Compiler

C Functions, Arguments, Parameters & Flow

Mastering Functions in C – Complete Guide

Function Arguments in C

Function Parameters in C

Function Declarations in C

Function Calls in C

Void Functions in C

Return Values in C

Pass-by-Value in C

Pass-by-Reference in C

C strstr() Function

C Preprocessor & Macros

C Structures, Memory, Scope & Linkage

C Scope, Storage Classes & Typedef

Extra Articles

Online Compilers

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