C Language | Advanced Features | String Operations - strcat(), strlen(), strcpy(), strcmp(), and More

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The standard library provides functions for joining, measuring, copying, comparing, and formatting strings.

Joining and Copying Strings

String functions are declared in string.h.

char *strcat(char *destination, const char *source);

strcat() appends source to a null-terminated string in destination. The destination buffer must have enough space, and the two regions must not overlap.

Code 1

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

int main() {
 char text[256] = "Kitty on your lap";
 strcat(text , " and a book");
 printf("%s\n" , text);
 return 0;
}

Use strlen() to obtain a string length, excluding the terminating null character.

size_t strlen(const char *string);

Use strcpy() to copy a null-terminated string.

char *strcpy(char *destination, const char *source);

Code 2

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

int main() {
 const char *left = "Kitty on your lap";
 const char *right = " and a book";
 char *text = malloc(strlen(left) + strlen(right) + 1);
 if (text == NULL) return 1;

 strcpy(text , left);
 strcat(text , right);
 printf("%s\n" , text);
 free(text);
 return 0;
}

Comparing Strings

Use strcmp() to compare string contents.

int strcmp(const char *left, const char *right);

The function returns zero for equal strings, a negative value when left sorts before right, and a positive value when left sorts after right.

Code 3

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

int main() {
 char left[255] , right[255];
 printf("Enter two strings. >");
 scanf("%254s %254s" , left , right);

 if (strcmp(left , right) == 0)
   printf("The strings are equal.\n");
 else if (strcmp(left , right) < 0)
   printf("%s sorts before %s.\n" , left , right);
 else
   printf("%s sorts after %s.\n" , left , right);
 return 0;
}

Formatting Strings

sprintf() writes formatted output to a buffer, while sscanf() reads formatted values from a string.

int sprintf(char *buffer, const char *format, ...);
int sscanf(const char *buffer, const char *format, ...);

Code 4

#include <stdio.h>

int main() {
 char text[256];
 sprintf(text , "Today is %d-%02d-%02d." , 2017 , 11 , 26);
 printf("%s\n" , text);
 return 0;
}

In production code, prefer bounded alternatives such as snprintf() when available so that output cannot exceed the destination buffer.