The C standard library contains several functions for searching through strings. We will look at two functions here. The first searches for a single character in a string; the second searches for multiple characters -- another string! -- inside a string.

Let's start with searching for an individual character in a string.

The strchr function takes two parameters: the first is the string to search; the second is the character to search for in the string. It searches for the character from left to right in the string.

You might expect that strchr would return the index in the string where the character was found. However, this is not what strchr does. Instead, strchr returns a pointer to the character that was found, or returns NULL if the character was not found in the string.

This code shows an example of calling strchr. Notice how the first argument to strchr is a string and the second is a character.

If strchr returns NULL, then the if-condition is true, and we print an appropriate message.

If we reach the else clause, then the character 'v' is found in s1. We can use pointer arithmetic to determine the index of 'v' in s1. Notice that both s1 and p are pointers into the same array, so pointer subtraction is well-defined.

Let's move on to a function that we can use to find a longer sequence of characters in a string. We call the string we're searching for the "substring".

strstr searches left to right in s1 for the first occurrence of the substring s2. If s2 is found in s1, strstr returns a pointer to the character of s1 that begins the match with s2.

Let's add to our program to include an example for strstr.

Notice that in the call to strstr both arguments are strings. The program prints that the substring starts at index 6, because index 6 in s1 is the start of 'sity' which matches our second parameter.