C Tutorial – strings and string Library Functions

Important: Before you start this tutorial, did you follow the pointers and more on pointers tutorials? Strings and pointers are intertwined to a large extent.

A string in the C language is simply an array of characters. Strings must have a NULL or \0 character after the last character to show where the string ends. A string can be declared as a character array or with a string pointer. First we take a look at a character array example:

     char mystr[20];

As you can see the character array is declared in the same way as a normal array. This array can hold only 19 characters, because we must leave room for the NULL character.
Take a look at this example:

	#include<stdio.h>

	int main()
	{
		char mystring[20];

		mystring[0] = 'H';
		mystring[1] = 'E';
		mystring[2] = 'L';
		mystring[3] = 'L';
		mystring[4] = 'O';
		mystring[5] = '\n';
		mystring[6] = '\0';

		printf("%s", mystring);

		return 0;
	}

Note: %s is used to print a string. (The 0 without the ” will in most cases also work).

String pointers are declared as a pointer to a char. When there is a value assigned to the string pointer the NULL is put at the end automatically. Take a look at this example:

	#include<stdio.h>

	int main()
	{
		char *ptr_mystring;

		ptr_mystring = "HELLO";
		printf("%s\n", ptr_mystring);

		return 0;
	}

It is not possible to read, with scanf, a string with a string pointer. You have to use a character array and a pointer. See this example:

	#include<stdio.h>

	int main()
	{
        	char my_array[10];
        	char *ptr_section2;

        	printf("Type hello and enter\n");
       	        scanf("%s", my_array);
        	ptr_section2 = my_array;
        	printf("%s\n", ptr_section2);

        	return 0;
	}

string.h or strings.h

The C language provides no explicit support for strings in the language itself. The string-handling functions are implemented in libraries. String I/O operations are implemented in <stdio.h> (puts , gets, etc). A set of simple string manipulation functions are implemented in <string.h>, or on some systems in <strings.h>.

The string library (string.h or strings.h) has some useful functions for working with strings, like strcpy, strcat, strcmp, strlen, strcoll, etc. We will take a look at some of these string operations.

Important: Don’t forget to include the library string.h (or on some systems strings.h) if you want to use one of these library functions.

strcpy

This library function is used to copy a string and can be used like this: strcpy(destination, source). (It is not possible in C to do this: string1 = string2). Take a look at the following example:

	str_one = "abc";
	str_two = "def";
	strcpy(str_one , str_two); // str_one becomes "def"

Note: strcpy() will not perform any boundary checking, and thus there is a risk of overrunning the strings.

strcmp

This library function is used to compare two strings and can be used like this: strcmp(str1, str2).

• If the first string is greater than the second string a number greater than null is returned.
• If the first string is less than the second string a number less than null is returned.
• If the first and the second string are equal a null is returned.

Take look at an example:

	printf("Enter you name: ");
	scanf("%s", name);
	if( strcmp( name, "jane" ) == 0 )
		printf("Hello, jane!\n");

Note: strcmp() will not perform any boundary checking, and thus there is a risk of overrunning the strings.

strcat

This library function concatenates a string onto the end of the other string. The result is returned. Take a look at the example:

	printf("Enter you age: ");
	scanf("%s", age);
	result = strcat( age, " years old." ) == 0 )
	printf("You are %s\n", result);

Note: strcat() will not perform any boundary checking, and thus there is a risk of overrunning the strings.

strlen

This library function returns the length of a string. (All characters before the null termination.) Take a look at the example:

	name = "jane";
	result = strlen(name); //Will return size of four.

memcmp

This library function compares the first count characters of buffer1 and buffer2. The function is used like this: memcmp(buffer1,buffer2). The return values are as follows:

• If buffer1 is greater than buffer2 a number greater than null is returned.
• If buffer1 is less than buffer2 a number less than null is returned.
• If buffer1 and buffer2 are equal a null is returned.

Note: There are also library functions: memcpy, memset and memchr.

That is all for this tutorial.