OpenMP3.1-CCard

OpenMP API 3.1 C/C++ Page 1 © 2011 OpenMP ARB OMP0711C Directives OpenMP 3.1 API C/C++ Syntax Quick Reference Card OpenMP Application Program Interface (API) is a portable, scalable model that gives shared-memory parallel programmers a simple and flexible interface for developing parallel applications for platforms ranging from the desktop to the supercomputer. OpenMP supports multi-platform shared-memory parallel programming in C/C++ and Fortran on all architectures, including Unix platforms and Windows NT platforms. A separate OpenMP reference card for Fortran is also available. [n.n.n] refers to sections in the OpenMP API Specification available at www.openmp.org. An OpenMP executable directive applies to the succeeding structured block or an OpenMP Construct. A structured-block is a single statement or a compound statement with a single entry at the top and a single exit at the bottom. Parallel [2.4] The parallel construct forms a team of threads and starts parallel execution. #pragma omp parallel [clause[ [, ]clause] ...] structured-block clause: if(scalar-expression) num_threads(integer-expression) default(shared | none) private(list) firstprivate(list) shared(list) copyin(list) reduction(operator: list) Loop [2.5.1] The loop construct specifies that the iterations of loops will be distributed among and executed by the encountering team of threads. #pragma omp for [clause[ [, ]clause] ...] for-loops clause: private(list) firstprivate(list) lastprivate(list) reduction(operator: list) schedule(kind[, chunk_size]) collapse(n) ordered nowait kind: • static: Iterations are divided into chunks of size chunk_size. Chunks are assigned to threads in the team in round-robin fashion in order of thread number. • dynamic: Each thread executes a chunk of iterations then requests another chunk until no chunks remain to be distributed. • guided: Each thread executes a chunk of iterations then requests another chunk until no chunks remain to be assigned. The chunk sizes start large and shrink to the indicated chunk_size as chunks are scheduled. • auto: The decision regarding scheduling is delegated to the compiler and/or runtime system. • runtime: The schedule and chunk size are taken from the run-sched-var ICV. Sections [2.5.2] The sections construct contains a set of structured blocks that are to be distributed among and executed by the encountering team of threads. #pragma omp sections [clause[[,] clause] ...] { [#pragma omp section] structured-block [#pragma omp section structured-block] ... } clause: private(list) firstprivate(list) lastprivate(list) reduction(operator: list) nowait Single [2.5.3] The single construct specifies that the associated structured block is executed by only one of the threads in the team (not necessarily the master thread), in the context of its implicit task. #pragma omp single [clause[ [, ]clause] ...] structured-block clause: private(list) firstprivate(list) copyprivate(list) nowait Parallel Loop [2.6.1] The parallel loop construct is a shortcut for specifying a parallel construct containing one or more associated loops and no other statements. #pragma omp parallel for [clause[ [, ]clause] ...] for-loop clause: Any accepted by the parallel or for directives, except the nowait clause, with identical meanings and restrictions. Parallel Sections [2.6.2] The parallel sections construct is a shortcut for specifying a parallel construct containing one sections construct and no other statements. #pragma omp parallel sections [clause[ [, ]clause] ...] { [#pragma omp section] structured-block [#pragma omp section structured-block] ... } clause: Any of the clauses accepted by the parallel or sections directives, except the nowait clause, with identical meanings and restrictions. Task [2.7.1] The task construct defines an explicit task. The data environment of the task is created according to the data-sharing attribute clauses on the task construct and any defaults that apply. #pragma omp task [clause[ [, ]clause] ...] structured-block clause: if(scalar-expression) final(scalar-expression) untied default(shared | none) mergeable private(list) firstprivate(list) shared(list) Taskyield [2.7.2] The taskyield construct specifies that the current task can be suspended in favor of execution of a different task. #pragma omp taskyield Master [2.8.1] The master construct specifies a structured block that is executed by the master thread of the team. There is no implied barrier either on entry to, or exit from, the master construct. #pragma omp master structured-block Critical [2.8.2] The critical construct restricts execution of the associated structured block to a single thread at a time. #pragma omp critical [(name)] structured-block Barrier [2.8.3] The barrier construct specifies an explicit barrier at the point at which the construct appears. #pragma omp barrier Taskwait [2.8.4] The taskwait construct specifies a wait on the completion of child tasks of the current task. #pragma omp taskwait Atomic [2.8.5] The atomic construct ensures that a specific storage location is updated atomically, rather than exposing it to the possibility of multiple, simultaneous writing threads. #pragma omp atomic [read | write | update | capture] expression-stmt #pragma omp atomic capture structured-block where expression-stmt may be one of the following forms: if clause is... expression-stmt: read v = x; write x = expr; update or is not present x++; x--; ++x; --x; x binop = expr; x = x binop expr; capture v = x++; v = x--; v = ++x; v = --x; v = x binop= expr; and structured-block may be one of the following forms: {v = x; x binop= expr;} {x binop= expr; v = x;} {v = x; x = x binop expr;} {x = x binop expr; v = x;} {v = x; x++;} {v = x; ++x;} {++x; v = x;} {x++; v = x;} {v = x; x--;} {v = x; --x;} {--x; v = x;} {x--; v = x;} Flush [2.8.6] The flush construct executes the OpenMP flush operation, which makes a thread’s temporary view of memory consistent with memory, and enforces an order on the memory operations of the variables. #pragma omp flush [(list)] Ordered [2.8.7] The ordered construct specifies a structured block in a loop region that will be executed in the order of the loop iterations. This sequentializes and orders the code within an ordered region while allowing code outside the region to run in parallel. #pragma omp ordered structured-block Threadprivate [2.9.2] The threadprivate directive specifies that variables are replicated, with each thread having its own copy. #pragma omp threadprivate(list) list: A comma-separated list of file-scope, namespace- scope, or static block-scope variables that do not have incomplete types. Most common form of the for loop: for(var = lb; var relational-op b; var += incr) Simple Parallel Loop Example The following example demonstrates how to parallelize a simple loop using the parallel loop construct. void simple(int n, float *a, float *b) { int i; #pragma omp parallel for for (i=1; i