How to use library functions

The manual page for each function describes how you should use the function in your program. As an example, we'll look at the strcmp routine, which compares character strings. See string(S) for more information. Related functions are described there as well, but only the sections relevant to strcmp are shown in ``Excerpt from string(S) Manual Page''.

   

   NAME
   string: strcat, strdup, strncat, strcmp, strncmp, strcpy, strncpy, strlen,
   strchr, strrchr, strpbrk, strspn, strcspn, strtok, strtok_r, strstr,
   strlist - string operations.
   

   SYNTAX
   

   #include <string.h>
   

   ...
   

   int strcmp(const char s1, const char s2);
   

   ...
   DESCRIPTION
   

   ...
   

   strcmp compares its arguments and returns an integer less than, equal to, or
   greater than 0, based upon whether s1 is lexicographically less than,
   equal to, or greater than s2.
   ...
   

Excerpt from string(S) Manual Page

The DESCRIPTION section tells you what the function or macro does. The SYNTAX section contains the critical information about how you use the function or macro in your program.

Note that the first line in the SYNTAX is

   #include <string.h>

extern int strcmp(const char , const char );

that describes the kinds of arguments expected and returned by strcmp. This line is called a function prototype. Function prototypes afford a greater degree of argument type checking than old-style function declarations, so you lessen your chance of using the function incorrectly. By including string.h, you assure that the compiler checks calls to strcmp against the official interface. You can, of course, examine string.h in the standard place for header files on your system, usually the /usr/include directory.

The next thing in the SYNTAX section is the formal declaration of the function. The formal declaration tells you:

• the type of value returned by the function;

• the arguments the function expects to receive when called, if any;

• the argument types.

``How strcmp() Is Used in a Program'' shows a program fragment that will find the bird of your choice in an array of birds.

   #include <string.h>
   

   / birds must be in alphabetical order /
   char birds[] = { "albatross",  "canary", "cardinal", "ostrich", "penguin" };
   

   / Return the index of the bird in the array. /
   / If the bird is not in the array, return -1 /
   

   int is_bird(const char string)
   {
   	int low, high, midpoint;
   	int cmp_value;
   

   	/ use a binary search to find the bird /
   	low = 0;
   	high = sizeof(birds)/sizeof(char ) - 1;
   	while(low <= high)
   	{
   		midpoint = (low + high)/2;
   		cmp_value = strcmp(string, birds[midpoint]);
   		if (cmp_value < 0)
   			high = midpoint - 1;
   		else if (cmp_value > 0)
   			low = midpoint + 1;
   		else / found a match /
   			return midpoint;
   	}
   	return -1;
   }

How strcmp() Is Used in a Program