C String Functions

String functions in C are built-in library functions used to perform common operations on strings, such as copying, comparing, and concatenating. These functions are declared in the <string.h> header file.

• String functions simplify common string operations without requiring manual implementation.
• They are used for tasks such as finding string length, copying, concatenating, comparing, and searching strings.

Example: Copying one string to another using strcpy(), comparing two strings using strcmp(), or joining two strings using strcat().

strlen()

The strlen() function is used to determine the length of a string. It returns the number of characters in the string, excluding the null terminator '\0'.

• It counts characters until it encounters the null character '\0'.
• It is declared in the <string.h> header file.

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

int main() {
    char s[] = "Gfg";
  
  	// Finding and printing length of string s
    printf("%lu", strlen(s));
    return 0;
}

3

strcpy()

The strcpy() function copies a string from the source to the destination. It copies all characters, including the null terminator '\0'.

• It is declared in the <string.h> header file.
• The destination array must have enough space to store the copied string.

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

int main() {
    char src[] = "Hello";
    char dest[20];
    
    // Copies "Hello" to dest
    strcpy(dest, src);  
    printf("%s", dest);
    return 0;
}

Hello

strncpy()

The strncpy() function copies at most n characters from the source string to the destination string. It is useful when you want to limit the number of characters copied.

• It is declared in the <string.h> header file.
• If the source string is shorter than n, the remaining characters in the destination are filled with '\0'.

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

int main() {
    char src[] = "Hello";
    char dest[20];
    
    // Copies "Hello" to dest
    strncpy(dest, src, 4);
    printf("%s", dest);
    return 0;
}

Hell

strcat()

The strcat() function appends one string to the end of another. It adds the source string after the destination string and places a null terminator at the end.

• It is declared in the <string.h> header file.
• The destination array must have enough space to store the concatenated string.

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

int main() {
    char s1[30] = "Hello, ";
    char s2[] = "Geeks!";
    
    // Appends "Geeks!" to "Hello, "
    strcat(s1, s2);  
    printf("%s", s1);
    return 0;
}

Hello, Geeks!

strncat()

The strncat() function appends at most n characters from the source string to the end of the destination string. It automatically adds a null terminator after the appended characters.

• It is declared in the <string.h> header file.
• The destination array must have enough space to store the resulting string.

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

int main() {
    char s1[30] = "Hello, ";
    char s2[] = "Geeks!";
    
    // Appends "Geeks!" to "Hello, "
    strncat(s1, s2, 4);  
    printf("%s", s1);
    return 0;
}

Hello, Geek

strcmp()

The strcmp() function compares two strings lexicographically and returns an integer indicating their relationship. It compares the strings character by character until a difference is found or both strings end.

• It is declared in the <string.h> header file.
• It returns 0 if the strings are equal, a negative value if the first string is smaller, and a positive value if the first string is greater.

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

int main() {
    char s1[] = "Apple";
    char s2[] = "Applet";
    
  	// Compare two strings 
  	// and print result
    int res = strcmp(s1, s2);
    if (res == 0) 
        printf("s1 and s2 are same");
  	else if (res < 0)
      	printf("s1 is lexicographically " 
      	        "smaller than s2");
  	else
      	printf("s1 is lexicographically " 
      	       "greater than s2");
    return 0;
}

s1 is lexicographically smaller than s2

strncmp()

The strncmp() function compares the first n characters of two strings lexicographically. It returns an integer value based on the comparison result.

• It is declared in the <string.h> header file.
• It returns 0 if the first n characters are equal, a negative value if the first string is smaller, and a positive value if the first string is greater.

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

int main() {
    char s1[] = "Apple";
    char s2[] = "Applet";
    
  	// Compare two strings upto 
  	// 4 characters and print result
    int res = strncmp(s1, s2, 4);
    if (res == 0) 
        printf("s1 and s2 are same");
  	else if (res < 0)
      	printf("s1 is lexicographically "
      	        "smaller than s2");
  	else
      	printf("s1 is lexicographically "
      	"greater than s2");
    return 0;
}

s1 and s2 are same

strchr()

The strchr() function searches for the first occurrence of a specified character in a string. It returns a pointer to the first matching character if found.

• It is declared in the <string.h> header file.
• If the character is not found, the function returns NULL.

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

int main() {
    char s[] = "Hello, World!";
  
  	// Finding the first occurence of 'o' in string s
    char *res = strchr(s, 'o');
    if (res != NULL)
        printf("Character found at: %ld index", res - s);
    else
        printf("Character not found");
    return 0;
}

Character found at: 4 index

strrchr()

The strrchr() function searches for the last occurrence of a specified character in a string. It returns a pointer to the last matching character if found.

• It is declared in the <string.h> header file.
• If the character is not found, the function returns NULL.

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

int main() {
    char s[] = "Hello, World!";
  
  	// Finding the last occurence of 'o' is string s
    char *res = strrchr(s, 'o');
    
    if (res != NULL)
        printf("Character found at: %ld index", res - s);
    else
        printf("Character not found\n");
    return 0;
}

Character found at: 8 index

strstr()

The strstr() function searches for the first occurrence of a substring within another string. It returns a pointer to the beginning of the matched substring if found.

• It is declared in the <string.h> header file.
• If the substring is not found, the function returns NULL.

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

int main() {
    char s[] = "Hello, Geeks!";
  
  	// Find the occurence of "Geeks" in string s
    char *pos = strstr(s, "Geeks");
    
    if (pos != NULL)
        printf("Found"); 
    else
        printf("Not Found");
    return 0;
}

Found

sprintf()

The sprintf() function formats data and stores the resulting string in a character buffer. It works like printf(), but writes the output to a string instead of displaying it on the screen.

• It is declared in the <stdio.h> header file.
• Ensure the destination buffer is large enough to hold the formatted string to avoid buffer overflow.

#include <stdio.h>

int main() {
    char s[50];
    int n = 10;
    
    // Output formatted string into string bugger s
    sprintf(s, "The value is %d", n);
    printf("%s", s);
    return 0;
}

The value is 10

strtok()

The strtok() function splits a string into tokens based on specified delimiter characters. It modifies the original string by replacing delimiters with the null character '\0'.

• It is declared in the <string.h> header file.
• Successive calls to strtok() return the next token until no more tokens are found, after which it returns NULL.

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

int main() {
    char s[] = "Hello, Geeks, C!";
  
  	// Initializing tokens
    char *t = strtok(s, ", ");

  	// Printing rest of the tokens
    while (t != NULL) {
        printf("%s\n", t);
        t = strtok(NULL, ", ");
    }
    return 0;
}

Hello
Geeks
C!

Most commonly used string functions in the C

The below table lists some of the most commonly used string functions in the C language: