Don't Start from Scratch, Use Libraries

Libraries are very important in C because the C language supports only the most basic features that it needs. For example, C doesn't contain input-output (I/O) functions to read from the keyboard and write to the screen. Anything that extends beyond the basics must be written by a programmer. If the chunk of code is useful to multiple different programs, it's often put into a library to make it easily reusable.
In our discussion of C so far, we've already seen one library, the standard I/O (stdio) library. The #include line at the beginning of the program instructed the C compiler to loaded the library from its header file named stdio.h. C maintainers include standard C libraries for I/O, mathematical functions, time manipulation and common operations on certain data structures, such as a string of characters. Search the Web or your favorite C programming guide for information about the C89 standard library and the updates and additions in C99.
You, too, can write C libraries. By doing so, you can split your program into reusable modules. This modular approach not only makes it easy to include the same code in multiple programs, but it also makes for shorter program files which are easier to read, test and debug.
To use the functions within a header file, add a #include line for it at the beginning of your program. For standard libraries, put the name of the library's corresponding header file between greater-than and less-than signs (). For libraries you create yourself, put the name of the file between double quotes. Unlike statements in other parts of your C program, you don't have to put a semicolon at the end of each line. The following shows including one of each type of library:
#include
#include "mylib.h"
A comprehensive C programming source should provide the instructions you need to write your own libraries in C. The function definitions you'll write are not any different whether they're in a library or in your main program. The difference is that you'll compile them separately in something called an object file (with a name ending in .o), and you'll create a second file, called a header file (with a name ending in .h) which contains the function prototypes corresponding to each function in the library. It's the header file you'll reference in your #include line in each main program that uses your library, and you'll include the object file as an argument in the compiler command each time you compile that program.
The C features we've explored so far are typical in other programming languages, too. Next, though, we'll talk about how C manages your computer's memory.

Some Pointers about Pointers in C

When your C program is loaded into memory (typically the random-access memory, or RAM, in your computer), each piece of the program is associated with an address in memory. This includes the variables you're using to hold certain data. Each time your program calls a function, it loads that function and all of its associated data into memory just long enough to run that function and return a value. If you pass parameters to the function, C automatically makes a copy of the value to use in the function.
Sometimes when you run a function, though, you want to make some permanent change to the data at its original memory location. If C makes a copy of data to use in the function, the original data remains unchanged. If you want to change that original data, you have to pass a pointer to its memory address (pass by reference) instead of passing its value to the function (pass by value).
Pointers are used everywhere in C, so if you want to use the C language fully you have to have a good understanding of pointers. A pointer is a variable like other variables, but its purpose is to store the memory address of some other data. The pointer also has a data type so it knows how to recognize the bits at that memory address.
When you look at two variables side-by-side in C code, you may not always recognize the pointer. This can be a challenge for even the most experienced C programmers. When you first create a pointer, though, it's more obvious because there must be an asterisk immediately before the variable name. This is known as the indirection operator in C. The following example code creates an integer i and a pointer to an integer p:
int i;
int *p;
Currently there is no value assigned to either i or p. Next, let's assign a value to i and then assign p to point to the address of i.
i = 3;
p = &i;
Here you can see the ampersand (&) used as the address operator immediately before i, meaning the "address of i." You don't have to know what that address is to make the assignment. That's good, because it will likely be different every time you run the program! Instead, the address operator will determine the address associated with that variable while the program is running. Without the address operator, the assignment p=i would assign p the memory address of 3, literally, rather than the memory address of the variable i.
Next, let's look at how you can use pointers in C code and the challenges you'll want to be prepared for.

Using Pointers Correctly in C

Once you have a pointer, you can use that in place of a variable of the same data type in operations and function calls. In the following example, the pointer to i is used instead of i within a larger operation. The asterisk used with the p (*p) indicates that the operation should use the value that p is pointing to at that memory address, not the memory address itself:
int b;
b = *p + 2;
Without pointers, it's nearly impossible to divide tasks into functions outside of main in your C program. To illustrate this, consider you've created a variable in main called h that stores the user's height to the nearest centimeter. You also call a function you've written named setHeight that prompts the user to set that height value. The lines in your main function might look something like this:
int h;
setHeight(h); /* There is a potential problem here. */
This function call will try to pass the value of h to setHeight. However, when the function finishes running, the value of h will be unchanged because the function only used a copy of it and then discarded it when it finished running.
If you want to change h itself, you should first ensure that the function can take a pointer to an existing value rather than a new copy of a value. The first line of setHeight, then, would use a pointer instead of a value as its parameter (note the indirection operator):
setHeight(int *height) { /* Function statements go here */ }
Then, you have two choices for calling setHeight. The first is to use the address operator for h as the passed parameter (&h). The other is to create a separate pointer to h and pass that instead. The following shows both options:
setHeight(&h); /* Pass the address of h to the function */
int *p;
p = &h;
setHeight(p); /* Pass a separate pointer to the address of h to the function */
The second option reveals a common challenge when using pointers. The challenge is having multiple pointers to the same value. This means that any change in that one value affects all its pointers at once. This could be a good or bad thing, depending on what you're trying to accomplish in your program. Again, mastering the use of pointers is an important key to mastering C programming. Practice with pointers as much as possible so you'll be ready to face these challenges.
The C features we've explored so far are typical in other programming languages, too. Next, though, we'll look at C's demands for careful memory management.