C String strcasecmp() Function

What You’ll Learn

The strcasecmp() function compares two C-strings without regard to letter case. "Hello" and "hello" match, which is ideal for user commands, yes/no prompts, and filename checks where capitalization should not matter.

Definition and Usage

On POSIX systems (Linux, macOS, BSD), strcasecmp() is declared in <strings.h>. It walks both strings in parallel, comparing corresponding bytes while treating uppercase ASCII letters as equal to their lowercase forms.

The return value follows the same convention as strcmp(): zero means equal (ignoring case), a negative value means s1 sorts before s2, and a positive value means s1 sorts after s2. It is not part of ISO C—you will not find it in every embedded toolchain without an extension library.

📝 Syntax

POSIX declaration:

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

Headers and Portability

• Linux / macOS / BSD — #include <strings.h>
• Windows (MSVC) — #include <string.h> and use _stricmp(s1, s2)
• Related — strncasecmp(s1, s2, n) compares at most n bytes

Return Value

• 0 — strings equal ignoring case.
• < 0 — s1 is less than s2 (case-insensitive order).
• > 0 — s1 is greater than s2.

Examples Gallery

Compile on Linux or macOS with gcc strcasecmp.c -std=c11 -o strcasecmp. Examples use POSIX <strings.h>; see the portability notes for Windows.

#include <stdio.h>
#include <strings.h>

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

    int result = strcasecmp(str1, str2);

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

    return 0;
}

The strings are equal (ignoring case).

#include <stdio.h>
#include <strings.h>

int main(void) {
    const char *a = "Hello";
    const char *b = "Help";

    if (strcasecmp(a, b) == 0) {
        printf("Match\n");
    } else {
        printf("No match (characters differ at 'l' vs 'p').\n");
    }

    return 0;
}

No match (characters differ at 'l' vs 'p').

#include <stdio.h>
#include <strings.h>

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

    if (strcasecmp(input, "quit") == 0) {
        printf("Goodbye!\n");
    } else if (strcasecmp(input, "help") == 0) {
        printf("Showing help...\n");
    } else {
        printf("Unknown command: %s\n", input);
    }

    return 0;
}

Goodbye!

#include <stdio.h>
#include <strings.h>

int main(void) {
    const char *words[] = { "apple", "Banana", "cherry" };
    int i, j;

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

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

    return 0;
}

apple
Banana
cherry

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

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

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

    return 0;
}

strcmp:     -32
strcasecmp: 0

🚀 Common Use Cases

• CLI commands — accept help, HELP, or Help.
• Yes/No prompts — match y, Y, yes, YES.
• Filename checks — compare names on case-insensitive file systems.
• HTTP headers — header names are case-insensitive (often handled at a higher layer).
• Sorting names — build case-insensitive indexes for display lists.

📝 Notes

• Include <strings.h> on POSIX—not the usual <string.h> alone.
• Not ISO C standard; verify availability on your platform or provide a fallback.
• ASCII-centric case rules—not full Unicode or locale-aware collation.
• Do not pass NULL pointers—behavior is undefined.
• For password comparison, prefer exact strcmp and constant-time helpers where security matters.

⚡ Optimization

Library implementations of strcasecmp() are tuned for typical string lengths. For hot paths comparing known short tokens, a hand-written loop may inline well, but for most programs calling the library function is clear and fast enough.

💡 Best Practices