In ‘C’ language the group of characters, digits and symbols enclosed with in double quotation marks are called String. The string is always declared as character arrays. In other words character arrays are called Strings. To manipulate text such as words and sentences normally strings are used. Every string is terminated with ‘\0’ (NULL) character. The NULL character is a byte with all bits at logic 0.Hence its decimal value is Zero.

For Example:
char ch[]={ ‘W’, ‘E’ , ‘L’ , ‘C’ , ‘O’ , ‘M’ , ‘E’ , ‘\0’ };
Note: It is not compulsory to write ‘\0’ in string.
The compiler automatically places ‘\0’ at the end of the character array (or) string.

‘C’ library supports a large number of String-handling functions that can be used to carry out many of the string manipulations.Following are the most commonly used string handling functions.

FUNCTION         ACTION         
strlen ()        Finds the length of a string.
strcpy ()        Copies one string over another.
strcmp ()        Compares two strings.
strcat ()        Concatenates two strings

strlen (): This function counts the number of characters in a given string.
Syntax: strlen(string);

void main()
	char ch[20];
	int i;
	printf("Enter the string");
	printf("Length of given string is   %d", i);

OUTPUT: Enter the string: Hello
Length of given string is 5.

strcpy (): This function copies the contents of one string to another.
Syntax: strcpy (string2, string1);
Where string1-<source string
string2-<Destination string
string1 is copied to string2.

void main()
	char str1[20],str2[20];
	printf("Enter the string1");
	printf("First string is   %s", str1);
	printf("second string is   %s", str2);

OUTPUT: Enter the string1 : hello
First string is hello.
Second string is hello.

strcmp(): This function compares two strings identified by the arguments and has a value 0 if they are equal.If they are not it has the numeric difference between the first non matching character in the strings.
Syntax: strcmp(string1 ,string2);
String1 and string2 may be string variables or string constants.

Example: strcmp(name1,name2);
Strcmp(name1, "abc");
Strcmp("Rom", "Ram");
--< Our major concern is to determine whether strings are equal;if not which is alphabetically above.
The value of mismatch is rarely important.

Example: strcmp("their", "there");
The above example will return -9 which is numric difference between ASCII "i" and ASCII "r".
i.e., "i" minus "r" in ASCII code is -9.
If the value is negative ,string1 is alphabetically above string2.
Example program:

void main()
	char str1[20],str2[20];
	int k;
		printf("Enter the string1");
		printf("Enter the string2");
			printf("Two Strings are equal");
		else  if(k<0)
			printf("string1< string2");
			printf("string1 > string2");

strcat():This function joins two strings together.
Syntax: strcat(string1,string2);
string1 and string2 are character arrays.

When the function "strcat" is executed, string2 is appended to string1.It does so by removing the null character at the end of the string1 and placing string2 from there. The string at string2 remains unchanged. Example:

void main()
	char str1[20]= "WELCOME TO";
	char str2[20]= "C LANG";
		printf("string1  is   %s", str1);
		printf("string2  is    %s", str2);

String2 is C LANG.

Some more string handling function:

1. strncpy (): This function performs the same task as strcpy().The only difference between them is that the former function copies specified length of characters from source to destination.
Syntax: strncpy (destination, source, n);

2. stricmp (): This function compares two strings. The characters of the strings may be in lower case or upper case the function doesn’t discriminates between them.i.e., This function compares two strings without case. If the strings are same it returns Zero otherwise non-zero value.
Syntax: stricmp(string1,string2);

3. strlwr (): This function is used to convert any string to a lower case. When you are passing any upper case string to this function it converts into lower case.
Syntax: strlwr (string);

4. strupr (): This function converts lowercase strings to upper case.
Syntax: strupr (string);

5. strdup (): This function is used for duplicating a given string at the allocated memory which is pointed by a pointer variable.
Syntax: str2=strdup (str1);

6. strrev (): This function simply reverses the given string.
Syntax: strrev (string);

7. strset (): This function replaces every character of a string with the symbol given by the programmer,i.e., the elements of strings are replaced with arguments given by the programmer.
Syntax: strset(string, symbol);