C Program to Compare Two Strings – strcmp() and Manual Method

This C program compares two strings and reports whether they are equal, or which one comes first in dictionary (lexicographic) order. Comparison works character by character: walk both strings together until two characters differ or a string ends, then decide from that first difference.

This page shows a manual character-by-character implementation and the standard library strcmp() shortcut, plus strncmp() for length-limited comparison.

C Program to Compare Two Strings — Manual Method

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

int main(void)
{
    char str1[100], str2[100];
    int i = 0, result;

    printf("Enter the first string  : ");
    fgets(str1, sizeof str1, stdin);
    printf("Enter the second string : ");
    fgets(str2, sizeof str2, stdin);

    str1[strcspn(str1, "\n")] = '\0';
    str2[strcspn(str2, "\n")] = '\0';

    while (str1[i] != '\0' && str2[i] != '\0' && str1[i] == str2[i])
        i++;

    result = (unsigned char)str1[i] - (unsigned char)str2[i];

    if (result == 0)
        printf("The two strings are equal.\n");
    else if (result < 0)
        printf("\"%s\" comes before \"%s\".\n", str1, str2);
    else
        printf("\"%s\" comes after \"%s\".\n", str1, str2);

    return 0;
}

How to Compile and Run

gcc -Wall -o comparestr comparestr.c
./comparestr

Sample Output

Enter the first string  : apple
Enter the second string : apricot
"apple" comes before "apricot".

Enter the first string  : hello
Enter the second string : hello
The two strings are equal.

Enter the first string  : zebra
Enter the second string : ant
"zebra" comes after "ant".

How the Program Works

fgets + strcspn: fgets reads a whole line safely without buffer overflow. It keeps the trailing \n, so strcspn(s, "\n") finds it and replaces it with \0.
The loop: Advances index i while both strings have characters that match. It stops at the first difference, or when either string ends.
(unsigned char) cast: The subtraction str1[i] - str2[i] needs unsigned chars to avoid undefined behavior on platforms where char is signed and characters above ASCII 127 appear.
Result zero: Both strings matched all the way to their null terminators — they are equal.

This replaces the common bug seen in older textbook code: str1[i] != '' — an empty character constant '' is invalid C. The correct null terminator is '\0'.

Using strcmp() — The Standard Library Way

In real code, use strcmp() from <string.h>. It does the same comparison and returns a negative, zero, or positive value:

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

int main(void)
{
    char str1[100], str2[100];

    printf("Enter first string  : ");
    fgets(str1, sizeof str1, stdin);
    str1[strcspn(str1, "\n")] = '\0';

    printf("Enter second string : ");
    fgets(str2, sizeof str2, stdin);
    str2[strcspn(str2, "\n")] = '\0';

    int r = strcmp(str1, str2);

    if (r == 0)       printf("Equal\n");
    else if (r < 0)  printf("\"%s\" comes first\n", str1);
    else              printf("\"%s\" comes first\n", str2);

    return 0;
}

strncmp() — Compare Only the First N Characters

strncmp(s1, s2, n) compares at most n characters. Useful when you only care about a prefix:

/* Check if both strings start with "pre" */
if (strncmp(str1, "pre", 3) == 0)
    printf("str1 starts with 'pre'\n");

Key strcmp() Rules

Return value: negative if s1 < s2, zero if equal, positive if s1 > s2. The exact non-zero values are implementation-defined — only the sign matters.
Never use == to compare strings in C. str1 == str2 compares pointers (addresses), not the string content.
Case-sensitive: strcmp("Hello", "hello") returns non-zero. For case-insensitive comparison, use strcasecmp() (POSIX) or convert both strings to lowercase first.

What This Program Teaches

Why '\0' (null terminator) is the right sentinel for string loops — empty character literals '' are illegal in C
The (unsigned char) cast pattern when doing arithmetic on char values
Why pointer equality (==) does not compare string content — always use strcmp()
How fgets + strcspn replaces the dangerous gets() for safe line input

Related Programs

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Recommended Book

Strings, pointers, and the standard library are covered in chapters 5 and 6 of The C Programming Language by Kernighan & Ritchie. Also on Amazon.com.

C Program to Compare Two Strings – Manual and strcmp()