C Strings Made Easy: Declaration, Input, Output, and Library Functions

What is a String in C?

A string in C is a contiguous sequence of characters terminated by the null character (\0). Although the language does not have a dedicated string type, a string is usually represented as an array of char values. The compiler automatically appends the null terminator when you initialize a string literal.

"Welcome to the world of programming!"

The standard library header <string.h> offers a wealth of functions for manipulating strings efficiently.

Declaring and Initializing Strings

Because C treats a string as a character array, you must specify the array size. The size defines the maximum number of characters (excluding the null terminator) that can be stored.

char string_variable_name[array_size];

Typical declarations:

char first_name[15];    /* holds up to 14 characters + \0 */
char last_name[15];

When you provide an initializer, the compiler counts the characters and automatically adds the null terminator. The array size may be omitted if you give an initializer.

char first_name[15] = "ANTHONY";
char first_name[]   = "NATHAN";   /* size inferred */
char string3[6]     = {'h','e','l','l','o','\0'};  /* explicit terminator */

Reading Strings from the User

Input functions vary in safety and behaviour:

• scanf("%s", name) reads until whitespace but is vulnerable to buffer overflows.
• gets() reads an entire line but is unsafe and removed from C11.
• fgets(name, size, stdin) reads up to size-1 characters, leaving room for the null terminator.

Example using fgets:

#include <stdio.h>
int main() {
    char name[50];
    printf("Enter your full name: ");
    fgets(name, sizeof(name), stdin);
    printf("Your name is %s", name);
    return 0;
}

Printing Strings

The classic way to display a string is printf("%s", name);. Two convenient alternatives are:

• fputs(name, stdout) – writes directly to the output stream.
• puts(name) – writes the string followed by a newline.

Example with puts:

#include <stdio.h>
int main() {
    char greeting[] = "Hello, world!";
    puts(greeting);
    return 0;
}

The String Library (<string.h>)

Commonly used functions:

Illustrative program:

#include <stdio.h>
#include <string.h>
int main() {
    char str1[20] = "Hello";
    char str2[20] = " World!";
    char result[40];

    /* Comparison */
    if (strcmp(str1, str2) == 0)
        puts("Strings are equal");
    else
        puts("Strings are not equal");

    /* Concatenation */
    strcat(str1, str2);  // str1 now contains "Hello World!"
    printf("Concatenated: %s\n", str1);

    /* Length */
    printf("Length of str1: %zu\n", strlen(str1));

    /* Copy */
    strcpy(result, str1);
    printf("Copied string: %s\n", result);
    return 0;
}

Converting a String to a Number

Standard conversions:

• atoi – converts to int.
• atof – converts to double.
• atol – converts to long.

Example with atoi:

#include <stdio.h>
#include <stdlib.h>
int main() {
    char number_str[20];
    printf("Enter a number: ");
    fgets(number_str, sizeof(number_str), stdin);
    int num = atoi(number_str);
    printf("You entered %d\n", num);
    return 0;
}

Key Takeaways

• A string is a sequence of characters stored in a char array terminated by \0.
• Declare the array size before use, or let the compiler infer it when initializing.
• Use fgets for safe input; avoid gets and scanf("%s") without bounds checking.
• Leverage <string.h> for robust string operations.
• Convert string numbers with atoi, atof, or atol to avoid runtime errors.
• Remember that string literals are read‑only; modifying them leads to undefined behaviour.