What Is a String in C?

In many modern languages (like Python, JavaScript, Java), a string is a built-in type. In C, there is no separate “string type”. Instead, a string is:

Example: the string "Hello" in memory actually looks like:

Index:   0    1    2    3    4    5
Value:  'H'  'e'  'l'  'l'  'o'  '\0'

That last '\0' character tells C where the string ends. Without it, functions like printf("%s") and strlen() will not know where to stop.

char Array Basics – Declaring and Initializing Strings

We use char arrays to store strings.

Declaration

char name[20];    // can hold a string up to 19 characters + '\0'

Initialization with String Literals

char str1[6] = {'H', 'e', 'l', 'l', 'o', '\0'};
char str2[6] = "Hello";    // same as above
char str3[]  = "Hello";    // size is automatically 6

Using "Hello" is a string literal which automatically includes '\0'.

Assigning One Character at a Time

char word[6];
word[0] = 'H';
word[1] = 'e';
word[2] = 'l';
word[3] = 'l';
word[4] = 'o';
word[5] = '\0';

This is not convenient for long strings, which is why string functions and initialization with literals are commonly used.

String vs Single char

Very important difference:

• char c = 'A'; → single character using single quotes.
• char s[] = "A"; → string of length 1 + '\0', using double quotes.

char c = 'A';    // 'A' only
char s[] = "A";  // 'A' and '\0'

Input & Output of Strings – gets(), puts(), scanf(), printf()

C provides several ways to read and print strings. Let’s see the common ones and their pros/cons.

Printing Strings

Use printf with %s format specifier:

char name[30] = "Tamil Technicians";
printf("Name: %s\\n", name);

puts() – Simple Output Function

puts() prints a string and automatically adds a newline at the end.

#include <stdio.h>

int main() {
    char msg[] = "Hello from Tamil Technicians!";
    puts(msg);        // prints the string + newline
    return 0;
}

Reading Strings with scanf()

The simplest way to read a single word (no spaces) is:

char name[30];
scanf("%s", name);  // reads until first whitespace (space, tab, newline)

Example: if the user types Raj Kumar, name will only get "Raj". The part after the space goes into the input buffer for later.

gets() – Legacy Function (Unsafe)

Historically, gets() was used to read an entire line (including spaces):

char line[50];
gets(line);   // reads a line, but DANGEROUS

You may still see gets() in old textbooks, but in modern code, you should use fgets() instead.

Safer Alternative – fgets()

char line[50];
fgets(line, sizeof(line), stdin);

• line – buffer to store the string.
• sizeof(line) – maximum number of characters to read (including '\0').
• stdin – input stream (keyboard).

fgets() reads at most size - 1 characters and adds '\0'. It may keep the newline '\n' if there is enough space.

strlen() – Finding Length of a String

The function strlen() (from <string.h>) returns the length of a string (number of characters before '\0').

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

int main() {
    char s[] = "Hello";
    printf("Length = %zu\\n", strlen(s));
    return 0;
}

Output: Length = 5

strcpy(), strcat() – Copying & Concatenating Strings

Copying one string into another or joining strings is very common. For this, C provides:

• strcpy() – copy entire string.
• strncpy() – copy up to N characters.
• strcat() – concatenate (append) one string to another.
• strncat() – concatenate up to N characters.

strcpy() – Copy String

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

int main() {
    char source[] = "Tamil Technicians";
    char dest[50];

    strcpy(dest, source);   // copy source into dest

    printf("Source: %s\\n", source);
    printf("Dest:   %s\\n", dest);

    return 0;
}

strncpy() – Safer Copy (with Limit)

strncpy(dest, source, sizeof(dest) - 1);
dest[sizeof(dest) - 1] = '\0';   // ensure null terminator

strncpy() copies at most n characters and does not always add '\0' if the source is too long, so we manually set it.

strcat() – Concatenate Strings

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

int main() {
    char first[50] = "Tamil";
    char second[] = " Technicians";

    strcat(first, second);    // append second to first

    printf("Result: %s\\n", first);
    return 0;
}

After strcat, first becomes "Tamil Technicians".

strcmp(), strchr(), strstr() – Comparing & Searching Strings

C strings are compared lexicographically (dictionary order) using strcmp() and strncmp().

strcmp() – Compare Two Strings

int result = strcmp(s1, s2);

• Returns 0 if s1 and s2 are equal.
• Returns < 0 if s1 is “less than” s2.
• Returns > 0 if s1 is “greater than” s2.

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

int main() {
    char a[] = "apple";
    char b[] = "banana";

    int r = strcmp(a, b);
    if (r == 0) {
        printf("Equal\\n");
    } else if (r < 0) {
        printf("%s comes before %s\\n", a, b);
    } else {
        printf("%s comes after %s\\n", a, b);
    }
    return 0;
}

strncmp() – Compare Only First N Characters

strncmp(s1, s2, n);

Useful when you only care about a prefix (for example, checking if a command starts with "SET").

strchr() – Find a Character in a String

char *p = strchr(s, 'a');

• Returns pointer to the first occurrence of character.
• Returns NULL if the character is not found.

strstr() – Find a Substring

char *p = strstr(haystack, needle);

• Returns pointer to the first occurrence of substring.
• Returns NULL if not found.

These functions are very useful for parsing commands, reading simple protocols or searching in logs and messages.

Mini-Projects with Strings (Technician Style)

1. Simple Name List

Store and print a list of names using a 2D char array.

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

#define MAX_NAMES 5
#define MAX_LEN   30

int main() {
    char names[MAX_NAMES][MAX_LEN];
    int i;

    printf("Enter %d names:\\n", MAX_NAMES);
    for (i = 0; i < MAX_NAMES; i++) {
        printf("Name %d: ", i + 1);
        fgets(names[i], MAX_LEN, stdin);

        // Remove newline if present
        size_t len = strlen(names[i]);
        if (len > 0 && names[i][len - 1] == '\n') {
            names[i][len - 1] = '\0';
        }
    }

    printf("\\nNames entered:\\n");
    for (i = 0; i < MAX_NAMES; i++) {
        printf("%d. %s\\n", i + 1, names[i]);
    }
    return 0;
}

2. Simple Login Check (Username)

Compare typed username with a stored one.

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

int main() {
    char correctUser[] = "technician";
    char input[30];

    printf("Enter username: ");
    scanf("%29s", input);     // prevent overflow

    if (strcmp(correctUser, input) == 0) {
        printf("Login success\\n");
    } else {
        printf("Invalid username\\n");
    }
    return 0;
}

3. Count Vowels in a Message

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

int main() {
    char text[100];
    int i, vowels = 0;

    printf("Enter a line: ");
    fgets(text, sizeof(text), stdin);

    for (i = 0; text[i] != '\0'; i++) {
        char ch = tolower((unsigned char)text[i]);
        if (ch == 'a' || ch == 'e' || ch == 'i' ||
            ch == 'o' || ch == 'u') {
            vowels++;
        }
    }

    printf("Vowel count = %d\\n", vowels);
    return 0;
}

String Safety Checklist (Very Important)

• Always ensure the char array is large enough for the string + '\0'.
• Prefer fgets() over gets() for user input.
• When using scanf("%s"), limit input length (e.g., "%29s" for a 30-byte buffer).
• After using strncpy(), manually add '\0' at the end.
• Never use uninitialized strings; make sure they contain '\0' before using string functions.
• Be careful with strcpy() and strcat(); check buffer sizes before calling.

Quick Summary – Strings in C

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