C String strcmp() Function

What You’ll Learn

The strcmp() function compares two C-strings lexicographically—character by character using byte values. It tells you whether strings are equal or which one sorts first. It is one of the most frequently used functions in C.

Definition and Usage

strcmp walks both strings in parallel. At the first position where bytes differ, it compares those bytes as unsigned char values and returns a negative or positive result. If all compared bytes match and both strings end together, the result is zero.

Use strcmp(s1, s2) == 0 to test equality. For sorting, the sign of the return value is what matters—do not rely on the exact integer (such as -1 or 1) being the same on every platform.

📝 Syntax

Standard C declaration:

int strcmp(const char *s1, const char *s2);

Parameters

• s1 — first null-terminated string.
• s2 — second null-terminated string.

Return Value

• 0 — strings are equal.
• < 0 — s1 is less than s2.
• > 0 — s1 is greater than s2.

Header

• #include <string.h>

Examples Gallery

Compile with gcc strcmp.c -std=c11 -o strcmp. Each program includes <string.h>.

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

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

    int result = strcmp(str1, str2);

    if (result == 0) {
        printf("The strings are equal.\n");
    } else if (result < 0) {
        printf("str1 is less than str2.\n");
    } else {
        printf("str1 is greater than str2.\n");
    }

    return 0;
}

str1 is greater than str2.

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

int main(void) {
    const char *expected = "admin";
    const char *entered = "admin";

    if (strcmp(entered, expected) == 0) {
        printf("Access granted.\n");
    } else {
        printf("Access denied.\n");
    }

    return 0;
}

Access granted.

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

int main(void) {
    const char *names[] = { "Charlie", "Alice", "Bob" };
    int i, j;

    for (i = 0; i < 3; i++) {
        for (j = i + 1; j < 3; j++) {
            if (strcmp(names[i], names[j]) > 0) {
                const char *tmp = names[i];
                names[i] = names[j];
                names[j] = tmp;
            }
        }
    }

    for (i = 0; i < 3; i++) {
        printf("%s\n", names[i]);
    }

    return 0;
}

Alice
Bob
Charlie

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

int main(void) {
    const char *a = "OpenAI";
    const char *b = "openai";

    printf("strcmp: %d\n", strcmp(a, b));

    if (strcmp(a, b) == 0) {
        printf("Equal\n");
    } else {
        printf("Not equal (case differs at first letter).\n");
    }

    return 0;
}

strcmp: -32
Not equal (case differs at first letter).

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

int main(void) {
    const char *input = "exit";

    if (strcmp(input, "exit") == 0) {
        printf("Goodbye!\n");
    } else if (strcmp(input, "help") == 0) {
        printf("Commands: help, exit\n");
    } else {
        printf("Unknown command: %s\n", input);
    }

    return 0;
}

Goodbye!

🚀 Common Use Cases

• Equality checks — verify tokens, usernames, or enum-like string values.
• Sorting — order names, words, or paths with qsort.
• Search structures — binary search on sorted string arrays.
• Branching logic — dispatch on command names or mode strings.
• Testing — assert expected output strings in unit tests.

📝 Notes

• Case-sensitive—use strcasecmp when case should not matter.
• Compares bytes as unsigned char—not locale-aware collation.
• Only the sign of the return value is guaranteed for ordering.
• Do not pass NULL pointers—behavior is undefined.
• Shorter strings can sort before longer prefixes: "file" vs "file2" differ at the end.

⚡ Optimization

Standard library strcmp is heavily optimized. For most programs, use it directly rather than hand-rolling loops. When comparing the same prefix repeatedly, consider strncmp with a fixed limit or cache string lengths from strlen.

💡 Best Practices