strcmp() / strncmp()

Since:		C89（1989）

Functions that compare two strings in lexicographic (dictionary) order. Use them for equality checks and sort comparison logic. Since strings cannot be compared with the == operator, you must always use these functions instead.

Syntax

// Compares s1 and s2 in lexicographic order.
// Returns: negative if s1 < s2, 0 if s1 == s2, positive if s1 > s2.
int strcmp(const char *s1, const char *s2);

// Compares only the first n bytes.
int strncmp(const char *s1, const char *s2, size_t n);

// Case-insensitive comparison (POSIX extension, not standard C).
int strcasecmp(const char *s1, const char *s2);   // Unix-like systems

// Case-insensitive comparison on Windows.
int _stricmp(const char *s1, const char *s2);     // Windows (MSVC)

Return Value

Return value	Meaning	Example usage
0	The two strings are equal.	Use `if (strcmp(a, b) == 0)` to check for equality.
Negative	s1 comes before s2 in lexicographic order.	Can be used as the return value of a sort comparison function.
Positive	s1 comes after s2 in lexicographic order.	Can be used as the return value of a sort comparison function.

Sample Code

sample_strcmp_strncmp.c

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

int main(void) {
    // Check for equality using strcmp.
    const char *s1 = "Kiryu Kazuma";
    const char *s2 = "Kiryu Kazuma";
    const char *s3 = "Majima Goro";

    if (strcmp(s1, s2) == 0) {
        printf("s1 and s2 are equal.\n"); // This line is printed.
    }

    // Check the ordering relationship.
    int cmp = strcmp(s1, s3);
    if (cmp < 0) {
        printf("%s comes before %s in lexicographic order.\n", s1, s3);
        // Prints: "Kiryu Kazuma comes before Majima Goro in lexicographic order."
    }

    // Use strncmp to compare only the first N characters (e.g., command prefix matching).
    const char *cmd = "set_value";
    if (strncmp(cmd, "set_", 4) == 0) {
        printf("The command starts with set_.\n"); // This line is printed.
    }

    // Example: sort a string array using strcmp.
    const char *members[] = {"Nishikiyama Akira", "Kiryu Kazuma", "Majima Goro"};
    int n = 3;
    // Sort using bubble sort.
    for (int i = 0; i < n - 1; i++) {
        for (int j = 0; j < n - 1 - i; j++) {
            if (strcmp(members[j], members[j + 1]) > 0) {
                const char *tmp = members[j];
                members[j] = members[j + 1];
                members[j + 1] = tmp;
            }
        }
    }
    printf("Sorted: ");
    for (int i = 0; i < n; i++) printf("%s ", members[i]);
    printf("\n"); // Prints: "Kiryu Kazuma Majima Goro Nishikiyama Akira"

    return 0;
}

gcc strcmp_strncmp.c -o strcmp_strncmp
./strcmp_strncmp
s1 and s2 are equal.
Kiryu Kazuma comes before Majima Goro in lexicographic order.
The command starts with set_.
Sorted: Kiryu Kazuma Majima Goro Nishikiyama Akira

Notes

Writing if (s1 == s2) compares pointer addresses, so two strings with the same content but stored at different addresses will evaluate to false. Always use strcmp() to compare string contents.

Comparison is performed on byte values (as unsigned char). This means the result may differ from alphabetical order, and in particular it cannot be used to sort multibyte characters such as Japanese text. Use strcoll() for locale-aware comparisons.

For applying these functions to general-purpose sorting, see also qsort() / bsearch().

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