C Programming Absolute Beginner's Guide (3rd Edition)

program can’t just expand the array at runtime. Some programmers (like you) have to change the array
definition and recompile the program before the array can hold more values.

With the heap memory, however, you don’t have to know in advance how much memory you need.
Similar to an accordion, the heap memory your program uses can grow or shrink as needed. If you
need another 100 elements to hold a new batch of customers, your program can allocate that new
batch at runtime without needing another compilation.

Warning

This book doesn’t try to fool you into thinking that this chapter can answer all your

questions. Mastering the heap takes practice—and, in reality, programs that really need

the heap are beyond the scope of this book. Nevertheless, when you finish this chapter,

you’ll have a more solid understanding of how to access the heap than you would get

from most books because of the approach that’s used.

Commercial programs such as spreadsheets and word processors must rely heavily on the heap. After
all, the programmer who designs the program cannot know exactly how large or small a spreadsheet
or word processing document will be. Therefore, as you type data into a spreadsheet or word
processor, the underlying program allocates more data. The program likely does not allocate the data
1 byte at a time as you type because memory allocation is not always extremely efficient when done 1
byte at a time. More than likely, the program allocates memory in chunks of code, such as 100-byte or
500-byte sections.

So why can’t the programmers simply allocate huge arrays that can hold a huge spreadsheet or
document instead of messing with the heap? For one thing, memory is one of the most precious
resources in your computer. As we move into networked and windowed environments, memory
becomes even more precious. Your programs can’t allocate huge arrays for those rare occasions
when a user might need that much memory. Your program would solely use all that memory, and other
tasks could not access that allocated memory.

Note

The heap enables your program to use only as much memory as it needs. When your

user needs more memory (for instance, to enter more data), your program can allocate

the memory. When your user is finished using that much memory (such as clearing a

document to start a new one in a word processor), you can deallocate the memory,

making it available for other tasks that might need it.

How Do I Allocate the Heap?

You must learn only two new functions to use the heap. The malloc() (for memory allocate)