How to Find Length of String in C: Simple Guide

In C, you find the length of a string using the strlen() function from string.h. It counts characters until the null character '\0' that ends the string.

Syntax

The strlen() function is declared in the string.h header. It takes a const char * (a pointer to a string) and returns the length as size_t, which is an unsigned integer type.

• strlen(str): Returns the number of characters in str before the null terminator.

c

size_t strlen(const char *str);

Example

This example shows how to use strlen() to find and print the length of a string.

c

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

int main() {
    char greeting[] = "Hello, world!";
    size_t length = strlen(greeting);
    printf("Length of string: %zu\n", length);
    return 0;
}

Output

Length of string: 13

Common Pitfalls

Common mistakes when finding string length in C include:

• Not including string.h, causing strlen() to be undefined.
• Passing a pointer that is not null-terminated, which leads to undefined behavior or crashes.
• Using sizeof on a pointer instead of strlen(), which returns the pointer size, not string length.

c

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

int main() {
    char *str = "Hello";
    // Wrong: sizeof(str) returns size of pointer, not string length
    printf("Wrong length: %zu\n", sizeof(str));

    // Right: use strlen to get string length
    printf("Correct length: %zu\n", strlen(str));
    return 0;
}

Output

Wrong length: 8
Correct length: 5

Quick Reference

Remember these tips when working with string lengths in C:

• Always include <string.h> to use strlen().
• strlen() counts characters until the null terminator '\0'.
• Do not use sizeof on pointers to get string length.
• Strings must be properly null-terminated.

Key Takeaways

Use strlen() from string.h to find string length in C.

strlen() counts characters until the null terminator '\0'.

Never use sizeof on a pointer to get string length.

Always ensure strings are null-terminated to avoid errors.

Include string.h to avoid compilation errors with strlen().