Standard Library Functions
--------------------------

C programs have access to a standard, preprogrammed set of functions for
handling input and output of data, computing mathematical quantities, and
manipulating strings of characters.  These preprogrammed functions are
stored in a system library that contains the collection of standard functions 
available on your system.

Before using a function available in the system library, you must
know:

	1.	The name of the function.
	2.	The arguments required by the function.
	3.	The data type of the result (if any) returned by the
		function.
	4.	What the function does.

For example, if the following function appears in a program:

	double sqrt(double sum);

then we known the following about the function:

	1.	its name is "sqrt"
	2.	arguments are "sum" of type double
	3.	the return data type is double
	4.	no description is given, need to consult man pages

Most library functions require bringing in a standard header file with
the #include< > statement.  This has been seen before.  For example,
OpenGL requires the GL/gl.h header file.  


Input/Output Library Functions
------------------------------

We have already talked about a set of these.  These include:

	printf()
	scanf()
	getchar()
	putchar()

The header for getchar() is:

	int getchar(void);

The value returned is an integer (ascii value converted to char if assigned to
char variable).  There are no input parameters (the meaning of the void type).

The header for putchar() would be something like:

	void putchar(char);

This means it returns no value and accepts a char value as input.


Mathematical Library Functions
------------------------------

Standard arithmetic operators such as +, -, * and / exist in C, but raising
numbers to a power, finding the square root of a number and determining 
absolute values are not directly available.  To access these routines, the 
math library is needed.  Two additional actions are needed:

	1.	add #include <math.h> to include the math library header file
	2.	use the -lm compile option on the compilation line like:

		cc program.c -lm -o program

The functions available are:

	int abs(int num)		Returns the absolute value of an
					integer.

					Ex: abs(-10) = 10
					Ex: abs( 10) = 10

	long labs(long num)		Returns the absolute value of a
					long integer.

					Ex: labs(-100000) = 100000

	double fabs(double num)		Returns the absolute value of a
					double precision number.

					Ex: fabs(-1.23456) = 1.23456

	double pow(double x, double y)	Returns x raised to the power of y.

					Ex: pow(2.0, 3.0) = 8.0

	int rand(void)			Returns a pseudorandom number.

	double sin(double angle)	Returns the sine of an angle.  The
					angle must be in radians.

					Ex: sin(3.14159) = 0

	double cos(double angle)	Returns the cosine of an angle.  The
					angle must be in radians.

					Ex: cos(3.14159) = -1;

	double sqrt(double num)		Returns the square root of a number.

					Ex: sqrt(81.0) = 9.0

The arguments for an function may be expressions, such as:  x - 10 * 3 / 4 * t .  
The expression will be evaluated first and then the resulting value will be 
the effective argument for the function.  For example:

	sqrt(4.0 + 7 * 3) the same as
	sqrt(4.0 + 21)    the same as
	sqrt(25.0) which equals 5.0

See page 221 for more examples.


String Library Functions
------------------------

This library is valuable for performing string manipulation.  If you wanted
to determine whether two strings, such as 

	char str1[] = "abc";
	char str2[] = "abcd";

were equivalent, then you would have to check each element of the string with 
standard comparisons:

	str1[0] == str2[0] && str1[1] == str2[1] && ...

This could get tedious if the strings were 1000 characters long.  There are some 
built-in routines in the string library to assist you.  To access the string library, 
all that is needed is add the line:

	#include <string.h>

Here are some sample functions:

	char *strcat(char *s1, char *s2)	Concatenates string s2 to s1
						and returns that string.

						Ex: char s[20],
							 s1[]="hi",
							 s2[]="there";

						    s = strcat(s1, s2);
						    s is "hithere"

	char *strchr(char *s1, char c)		Locates the first occurrence
						of the character in c within
						the string s1.  It returns the
						string at that location.

	int strcmp(char *s1, char *s2)		Compares string s1 to s2.  It
						returns 0 if equal, else a
						number != 0.

	char *strcpy(char *s1, char *s2)	Copies string s2 into s1.  In
						other words, s1 = s2.  It
						returns the string.

	int strlen(char *s)			Determines the length of the
						string s.

						Ex: char s[]="hello";
						    int i;

						    i=strlen(s);

						    i has value 5


Miscellaneous Routines
----------------------

	See the C Book, page 225.