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| <h4 class="subsection">3.2.1 Memory Allocation in C Programs</h4> |
| |
| <p>The C language supports two kinds of memory allocation through the |
| variables in C programs: |
| |
| <ul> |
| <li><dfn>Static allocation</dfn> is what happens when you declare a static or |
| global variable. Each static or global variable defines one block of |
| space, of a fixed size. The space is allocated once, when your program |
| is started (part of the exec operation), and is never freed. |
| <a name="index-static-memory-allocation-248"></a><a name="index-static-storage-class-249"></a> |
| <li><dfn>Automatic allocation</dfn> happens when you declare an automatic |
| variable, such as a function argument or a local variable. The space |
| for an automatic variable is allocated when the compound statement |
| containing the declaration is entered, and is freed when that |
| compound statement is exited. |
| <a name="index-automatic-memory-allocation-250"></a><a name="index-automatic-storage-class-251"></a> |
| In GNU C, the size of the automatic storage can be an expression |
| that varies. In other C implementations, it must be a constant. |
| </ul> |
| |
| <p>A third important kind of memory allocation, <dfn>dynamic allocation</dfn>, |
| is not supported by C variables but is available via GNU C library |
| functions. |
| <a name="index-dynamic-memory-allocation-252"></a> |
| |
| <h5 class="subsubsection">3.2.1.1 Dynamic Memory Allocation</h5> |
| |
| <p><a name="index-dynamic-memory-allocation-253"></a> |
| <dfn>Dynamic memory allocation</dfn> is a technique in which programs |
| determine as they are running where to store some information. You need |
| dynamic allocation when the amount of memory you need, or how long you |
| continue to need it, depends on factors that are not known before the |
| program runs. |
| |
| <p>For example, you may need a block to store a line read from an input |
| file; since there is no limit to how long a line can be, you must |
| allocate the memory dynamically and make it dynamically larger as you |
| read more of the line. |
| |
| <p>Or, you may need a block for each record or each definition in the input |
| data; since you can't know in advance how many there will be, you must |
| allocate a new block for each record or definition as you read it. |
| |
| <p>When you use dynamic allocation, the allocation of a block of memory is |
| an action that the program requests explicitly. You call a function or |
| macro when you want to allocate space, and specify the size with an |
| argument. If you want to free the space, you do so by calling another |
| function or macro. You can do these things whenever you want, as often |
| as you want. |
| |
| <p>Dynamic allocation is not supported by C variables; there is no storage |
| class “dynamic”, and there can never be a C variable whose value is |
| stored in dynamically allocated space. The only way to get dynamically |
| allocated memory is via a system call (which is generally via a GNU C |
| library function call), and the only way to refer to dynamically |
| allocated space is through a pointer. Because it is less convenient, |
| and because the actual process of dynamic allocation requires more |
| computation time, programmers generally use dynamic allocation only when |
| neither static nor automatic allocation will serve. |
| |
| <p>For example, if you want to allocate dynamically some space to hold a |
| <code>struct foobar</code>, you cannot declare a variable of type <code>struct |
| foobar</code> whose contents are the dynamically allocated space. But you can |
| declare a variable of pointer type <code>struct foobar *</code> and assign it the |
| address of the space. Then you can use the operators ‘<samp><span class="samp">*</span></samp>’ and |
| ‘<samp><span class="samp">-></span></samp>’ on this pointer variable to refer to the contents of the space: |
| |
| <pre class="smallexample"> { |
| struct foobar *ptr |
| = (struct foobar *) malloc (sizeof (struct foobar)); |
| ptr->name = x; |
| ptr->next = current_foobar; |
| current_foobar = ptr; |
| } |
| </pre> |
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