Sams Teach Yourself C in 21 Days

Decrementing You can subtract an integer from a pointer in order to point to a dif-

ferent memory location.

Differencing You can subtract one pointer from another pointer in order to deter-

mine how far apart they are.

Comparison Valid only with two pointers that point to the same array.

Pointer Cautions ..................................................................................................

When you’re writing a program that uses pointers, you must avoid one serious error:

using an uninitialized pointer on the left side of an assignment statement. For example,

the following statement declares a pointer to type int:

int *ptr;

This pointer isn’t yet initialized, so it doesn’t point to anything. To be more exact, it

doesn’t point to anything known. An uninitialized pointer has some value; you just don’t

know what it is. In many cases, it is zero. If you use an uninitialized pointer in an assign-

ment statement, this is what happens:

*ptr = 12;

The value 12 is assigned to whatever address ptrpoints to. That address can be almost

anywhere in memory—where the operating system is stored or somewhere in the pro-

gram’s code. The 12 that is stored there might overwrite some important information, and

the result can be anything from strange program errors to a full system crash.

The left side of an assignment statement is the most dangerous place to use an uninitial-

ized pointer. Other errors, although less serious, can also result from using an uninitial-

ized pointer anywhere in your program, so be sure your program’s pointers are properly

initialized before you use them. You must do this yourself; don’t assume that the com-

piler will do this for you.

210 Day 9

TABLE9.1 continued

Operation Description

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