Home > Articles

This chapter is from the book

This chapter is from the book

Preprocessor Directives and Macros

The preprocessing directives defined by the C99 specification are supported. These include

# non-directive

The defined operator is also included.

The following example demonstrates the use of #if, #elif, #else, and #endif preprocessor macros. In this example, we use the preprocessor macros to determine which arithmetic operation to apply in the kernel. The kernel source is described here:

#define OP_ADD           1
#define OP_SUBTRACT      2
#define OP_MULTIPLY      3
#define OP_DIVIDE        4

kernel void
foo(global float *dst, global float *srcA, global float *srcB)
    size_t id = get_global_id(0);
    dst[id] = srcA[id] + srcB[id];
    dst[id] = srcA[id] – srcB[id];
    dst[id] = srcA[id] * srcB[id];
    dst[id] = srcA[id] / srcB[id];
    dst[id] = NAN;

To build the program executable with the appropriate value for OP_TYPE, the application calls clBuildProgram as follows:

// build program so that kernel foo does an add operation
err = clBuildProgram(program, 0, NULL,
                                "-DOP_TYPE=1", NULL, NULL);

Pragma Directives

The #pragma directive is described as

#pragma pp-tokensopt new-line

A #pragma directive where the preprocessing token OPENCL (used instead of STDC) does not immediately follow pragma in the directive (prior to any macro replacement) causes the implementation to behave in an implementation-defined manner. The behavior might cause translation to fail or cause the translator or the resulting program to behave in a nonconforming manner. Any such pragma that is not recognized by the implementation is ignored. If the preprocessing token OPENCL does immediately follow pragma in the directive (prior to any macro replacement), then no macro replacement is performed on the directive.

The following standard pragma directives are available.

Floating-Point Pragma

The FP_CONTRACT floating-point pragma can be used to allow (if the state is on) or disallow (if the state is off) the implementation to contract expressions. The FP_CONTRACT pragma definition is

#pragma OPENCL FP_CONTRACT on-off-switch
            on-off-switch: one of ON OFF DEFAULT

A detailed description of #pragma OPENCL FP_CONTRACT is found in Chapter 5 in the section "Floating-Point Pragmas."

Compiler Directives for Optional Extensions

The #pragma OPENCL EXTENSION directive controls the behavior of the OpenCL compiler with respect to language extensions. The #pragma OPENCL EXTENSION directive is defined as follows, where extension_ name is the name of the extension:

#pragma OPENCL EXTENSION extension_name: behavior
#pragma OPENCL EXTENSION all : behavior
            behavior: enable or disable

The extension_name will have names of the form cl_khr_<name> for an extension (such as cl_khr_fp64) approved by the OpenCL working group and will have names of the form cl_<vendor_name>_<name> for vendor extensions. The token all means that the behavior applies to all extensions supported by the compiler. The behavior can be set to one of the values given in Table 4.9.

Table 4.9. Optional Extension Behavior Description




Enable the extension extension_name. Report an error on the #pragma OpenCL EXTENSION if the extension_name is not supported, or if all is specified.


Behave (including issuing errors and warnings) as if the extension extension_name is not part of the language definition.

If all is specified, then behavior must revert back to that of the nonextended core version of the language being compiled to.

Warn on the #pragma OPENCL EXTENSION if the extension extension_name is not supported.

The #pragma OPENCL EXTENSION directive is a simple, low-level mechanism to set the behavior for each language extension. It does not define policies such as which combinations are appropriate; these are defined elsewhere. The order of directives matters in setting the behavior for each extension. Directives that occur later override those seen earlier. The all variant sets the behavior for all extensions, overriding all previously issued extension directives, but only if the behavior is set to disable.

An extension needs to be enabled before any language feature (such as preprocessor macros, data types, or built-in functions) of this extension is used in the OpenCL program source. The following example shows how to enable the double-precision floating-point extension:

#pragma OPENCL EXTENSION cl_khr_fp64 : enable
double x = 2.0;

If this extension is not supported, then a compilation error will be reported for double x = 2.0. If this extension is supported, this enables the use of double-precision floating-point extensions in the program source following this directive.

Similarly, the cl_khr_3d_image_writes extension adds new built-in functions that support writing to a 3D image:

#pragma OPENCL EXTENSION cl_khr_fp64 : enable
kernel void my_func(write_only image3d_t img, ...)

    float4 coord, clr;
    write_imagef(img, coord, clr);

The built-in functions such as write_imagef with image3d_t in the preceding example can be called only if this extension is enabled; otherwise a compilation error will occur.

The initial state of the compiler is as if the following directive were issued, telling the compiler that all error and warning reporting must be done according to this specification, ignoring any extensions:

#pragma OPENCL EXTENSION all : disable

Every extension that affects the OpenCL language semantics or syntax or adds built-in functions to the language must also create a preprocessor #define that matches the extension name string. This #define would be available in the language if and only if the extension is supported on a given implementation. For example, an extension that adds the extension string cl_khr_fp64 should also add a preprocessor #define called cl_khr_fp64. A kernel can now use this preprocessor #define to do something like this:

#ifdef cl_khr_fp64
    // do something using this extension
    // do something else or #error


The following predefined macro names are available:

  • __FILE__ is the presumed name of the current source file (a character string literal).
  • __LINE__ is the presumed line number (within the current source file) of the current source line (an integer constant).
  • CL_VERSION_1_0 substitutes the integer 100, reflecting the OpenCL 1.0 version.
  • CL_VERSION_1_1 substitutes the integer 110, reflecting the OpenCL 1.1 version.
  • __OPENCL_VERSION__ substitutes an integer reflecting the version number of the OpenCL supported by the OpenCL device. This reflects both the language version supported and the device capabilities as given in Table 4.3 of the OpenCL 1.1 specification. The version of OpenCL described in this book will have __OPENCL_VERSION__ substitute the integer 110.
  • __ENDIAN_LITTLE__ is used to determine if the OpenCL device is a little endian architecture or a big endian architecture (an integer constant of 1 if the device is little endian and is undefined otherwise).
  • __kernel_exec(X, typen) (and kernel_exec(X, typen)) is defined as
    __kernel __attribute__((work_group_size_hint(X, 1, 1)))     __attribute__((vec_type_hint(typen))).
  • __IMAGE_SUPPORT__ is used to determine if the OpenCL device supports images. This is an integer constant of 1 if images are supported and is undefined otherwise.
  • __FAST_RELAXED_MATH__ is used to determine if the –cl-fast-relaxed-math optimization option is specified in build options given to clBuildProgram. This is an integer constant of 1 if the – cl-fast-relaxed-math build option is specified and is undefined otherwise.

The macro names defined by the C99 specification but not currently supported by OpenCL are reserved for future use.

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