What is a String?

One extremely useful application of pointers is their use with strings. A string is simply a list of characters; in fact, in C, you can represent a string as a character array. But a string can also be thought of a pointer to characters. Indeed, the standard declaration of a string variable is char *. You've already seen string literals: anything inside double quotes. But now you can use string variables as well.

When you think of a string as a character array, there is one pitfall to watch out for. All strings in C must be terminated by the null character ('\0', or ASCII code 0). Thus, any string you put into an array is going to be one element longer than you'd expect. Also, remember that spaces and newlines are characters just like anything else, and so also count!

The String Library

The library string.h defines several useful functions for dealing with string data. I will present some of the more useful of these functions here.

int strlen(char *str)

strlen takes a string as its argument and returns the number of characters in the string, not including the terminating null character.

int strcmp(char *str1, char *str2)

This function compares the characters in two strings based on their numeric ASCII values. If the first string is determined to come "before" the second as according to this code, the function returns a negative number. If the opposite is true, it returns a positive number. If the two strings are equal, it returns zero. The function strncmp(char *str1, char *str2, int n) is similar, except that it compares at most n characters in the strings.

char *strcpy(char dest[], char *src)

This function copies the string src into the destination buffer dest. It also writes the null character at the end of the buffer for you. The return value is the address of the buffer. As with strcmp(), there is a corresponding strncpy() function that works the same way.

char *strcat(char dest[], char *src)

This function copies the characters from src onto the end of dest. You can also use the strncat() function, which works the same as the functions above.