Home > Articles

A Word on C

Memory Management

Proper memory management is critical in application development. Careful attention has been paid in previous code samples to ensure that the compiler always implicitly provided the memory space necessary.

Memory management can be implicit or explicit. C provides built-in functions enabling a programmer to allocate and free memory explicitly. It is also possible that the necessary memory is implied by the manner in which a variable is declared.

Samples of associating memory implicitly with a variable include

char str1[10];

where the variable array str1, upon declaration, has sufficient space for storing 10 characters.

Consider also

char *buf = "No error";

where the variable buf is declared with space enough to hold 8 characters as implied by the initialization combined with the declaration.

Memory associated with variables can be taken from one of several areas available to an application. Variables declared after a start-of-body marker reside on the stack. (See Chapter 1 for a review of automatic variables and the stack.) The stack is volatile memory changing constantly during program execution.


A programmer affects the stack through the manner in which variables are declared. In other words, the programmer can never explicitly allocate or free stack memory.

The memory associated with the variable buf in the previous code fragment resides on the stack until it is out of scope. After it is no longer visible, the stack frame holding the reference to buf is removed from the stack and a new frame uses the memory.

Contrast the declaration of the two character pointers in the following example:

char *buf = "No error",

The variables buf and token both point to characters with the contents of buf being initialized at the moment of its declaration. Based on buf's initialization, you know both its size (length) and contents. Because no memory is associated with token, it has neither a length nor valid contents. Token is a character pointer, but as of yet points to nothing (garbage).

The cumbersome task of memory management thus begins.

A valid statement is to assign the memory associated with buf to token.

token = buf;

Because the variables are compatible (both character pointers) the assignment is legal and logical; now, token refers to valid memory (the implicit memory given to buf from the stack). Employing token follows the same rule as using buf, namely, the size of the memory provided cannot be exceeded (eight characters).

Furthermore, when the variables go out of scope, their contents are no longer valid because a new stack frame will begin using the memory that was once reserved for them.

Consider this incorrect code sample in which the memory associated with an automatic variable is returned to the calling function:

1: // forward declaration
2: char *someFunc( void );
4: // entry point of the program
5: int main( int argc, char *argv[] )
6: {
7:   char *badString = someFunc();
8:   printf("The value of str is: " );
8a:   sleep( 1 ); // new stack frame
 9:   printf( "'%s'\n", badString );
10:   return( 0 );
11: }
13: char *someFunc( void )
14: {
15:   char buf[15];
16:   strcpy( buf, "Error ahead" );
17:   // return the memory provided from the
18:   // the stack for the variable buf
19:   return( buf );
20: }

As described in the previous example, when the function main calls someFunc, a new frame is placed on the stack to manage all aspects of the function call. Specifics such as the function arguments, the return address of the calling function, the return value, and the local (automatic) variables, as well as temporary storage needed during function evaluation, are part of the stack frame.

Therefore, the memory used to store the value of buf in the function someFunc resides in the stack frame managing the function call. When the function returns, the frame is removed and all associated memory is made available for subsequent function calls.


A distinction must be made between returning pointers (addresses) from a function and returning values.

The caution being issued in the previous example is for returning addresses to data values whose location is on the stack and therefore in volatile memory.

The variable buf is a pointer to a value and not the value itself. Returning the value presents no danger because a copy of the data is made before returning it to the calling function.

Returning a value is not always practical, however, because the size of a data structure can adversely affect program performance.


Returning and employing memory from the stack creates memory errors that will likely result in a program crash when the stack frame is no longer active.

In the previous example, if main did not exit immediately but instead invoked another function, a new frame would be placed on the stack and the memory previously associated with buf (still pointed to by badString) would be reused. The new function owning the memory location would store a new value, possibly a value that is not character data. This would ensure that the value of badString is not as expected and a program crash could soon occur.

To avoid the problem of returning memory contained on the stack, you must explicitly allocate memory associated with pointer variables or reserve them in non-stack memory if their use extends beyond a single function.

The static Keyword

The keyword static serves several purposes in the C programming language. One of its uses is to force the memory associated with a variable to come from non-stack memory. This is beneficial because the memory will be persistent through the life of the program and not just the life of the function, as we saw with the previous example.

Use of the static keyword follows the form

static data type variable name;

Applying this to the previous example to correct the imminent program crash would require that the declaration of the variable buf change from

15:   char buf[15];


15:   static char buf[15];

As stated previously, the static keyword informs the compiler that the memory associated with the variable must not be taken from the stack. Instead, persistent memory is used and therefore the contents of buf are maintained during the life of the program. In addition to the memory address being safely returned to the calling function with the line

19:   return( buf );

the contents of the variable will be maintained in consecutive calls to the function. More examples of static automatic variables will be seen in the Graphics Editor project.


Memory associated with static variables can never be returned or unreserved by a program. Therefore, memory reserved as static permanently increases the size of a program.


You can also use the static keyword beyond making a variable's memory persistent to limit the variable's scope.

» As described earlier in this chapter, in the section "Variable Scope" on page 88, variables declared outside of a function body are global variables.

Global variables are visible (in scope) from the moment of their declaration until the end of the file. Global variables can be made visible to other files through use of the keyword extern.

The extern Keyword

The keyword extern can preface any variable declaration as a method of informing the compiler that the variable's actual declaration is external to the current file.

The declaration

extern int sum; // references a previous declaration of sum

tells the compiler that use of the variable sum is by merit of a previous declaration in another file. By using extern, the variable is global (visible) to multiple files.

However, if the original (actual) declaration employed the static keyword, visibility would be limited to the file in which it was declared. In other words, when you use the keyword static on a global variable, the variable cannot be declared externally through use of the extern keyword in another file.

Similarly, when functions are prefaced with the static keyword, their visibility, too, is limited to the file in which they are declared even if a prototype for the function exists elsewhere.

A function defined as

static int add2Nums( int num1, int num2 )
  return( num1 + num2);

can only be invoked by functions in the same file, which contains this definition because the static keyword limits its visibility.

Using the static keyword in the context of ensuring that a variable's memory is persistent during program execution is one method of managing memory that doesn't employ the stack. Another method of managing memory is to dynamically request it from an area known as the heap.


InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020