C String strlen() Function

What You’ll Learn

The strlen() function counts how many characters appear in a C-string before the terminating '\0'. It is one of the most frequently used functions in C—essential for allocating buffers, copying strings, and validating input.

Definition and Usage

A C-string is a sequence of bytes ending with a null character. strlen walks from the start until it finds that null byte and returns how many non-null characters it passed.

The count does not include '\0'. When allocating memory for a copy, use strlen(s) + 1 to leave room for the terminator. When checking whether a fixed buffer can hold a string, compare length to available space carefully.

📝 Syntax

Standard C declaration:

size_t strlen(const char *s);

Parameter

• s — pointer to a null-terminated C-string.

Return Value

• Number of characters before '\0', as size_t.
• 0 for an empty string "".

Header

• #include <string.h>

Examples Gallery

Compile with gcc strlen.c -std=c11 -o strlen. Every example includes <string.h>.

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

int main(void) {
    const char *message = "Hello, C!";
    size_t length = strlen(message);

    printf("The length of the string is: %zu\n", length);

    return 0;
}

The length of the string is: 9

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

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

    printf("Length of empty string: %zu\n", strlen(empty));

    return 0;
}

Length of empty string: 0

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

int main(void) {
    const char *name = "CodeToFun";
    size_t chars = strlen(name);
    size_t bytes_needed = chars + 1;

    char dest[16];

    if (bytes_needed <= sizeof dest) {
        strcpy(dest, name);
        printf("Copied %zu chars into buffer of %zu bytes\n",
               chars, sizeof dest);
    }

    return 0;
}

Copied 9 chars into buffer of 16 bytes

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

int main(void) {
    char buf[20] = "Hello";

    printf("strlen(buf) = %zu\n", strlen(buf));
    printf("sizeof(buf) = %zu\n", sizeof buf);

    return 0;
}

strlen(buf) = 5
sizeof(buf) = 20

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

int main(void) {
    const char *word = "C-string";
    size_t i, n = strlen(word);

    printf("Characters: ");
    for (i = 0; i < n; i++) {
        printf("%c ", word[i]);
    }
    printf("\nTotal: %zu\n", n);

    return 0;
}

Characters: C - s t r i n g 
Total: 8

🚀 Common Use Cases

• Buffer allocation — malloc(strlen(s) + 1) before copy.
• Bounds checks — verify text fits before strcat or strcpy.
• Input validation — enforce maximum name or password lengths.
• Formatting — pad or align output based on text width.
• Teaching C-strings — connect length to null termination.

📝 Notes

• Requires a null-terminated string—binary blobs need explicit lengths.
• Time complexity is O(n)—scans the whole string each call.
• Do not call strlen twice in a row on the same unchanged string in tight loops—cache the result.
• sizeof(pointer) is pointer size, not string length.
• Unicode code points are not counted—UTF-8 multibyte sequences count bytes.

⚡ Optimization

Library strlen is highly optimized. If you already know the length (for example from a length-prefixed protocol), store it instead of rescanning. Avoid strlen in inner loops when the string has not changed.

💡 Best Practices