User's Guide

+O[no]openmp Command Line Option
The +Oopenmp option is accepted at all optimization levels. The +Oopenmp option enables the
recognition of OpenMP pragmas. Using the +Onoopenmp option will ignore all OpenMP directives
silently.
See Chapter 3: “Pragma Directives and Attributes” (page 96) for more information on OpenMP
pragmas.
_OPENMP Macro
The _OPENMP macro name is defined by OpenMP compliant implementation as the decimal
constant 200203. This macro must not be the subject of #define or #undef preprocessing
directive. Following is an example of conditional compilation:
#ifdef _OPENMP
iam = omp_get_thread_num() + index;
#endif
Environment Variables in OpenMP
The OpenMP environment variables recognized by HP aC++ compiler control the execution of
parallel code. Note that the environment variable names are case sensitive and they must be in
uppercase.
The following environment variables are available in HP aC++ compiler:
• OMP_SCHEDULE
• OMP_NUM_THREADS
• OMP_DYNAMIC
• OMP_NESTED
OMP_SCHEDULE
export OMP_SCHEDULE="kind[,chunk_size]"
setenv OMP_SCHEDULE "kind[,chunk_size]"
where, kind is either of of static, dynamic, or guided.
This environment variable applies to for and parallel for directives that have the schedule
type as runtime. The schedule type and chunk size for all such loops can be set at runtime by setting
this environment variable to any of the recognized schedule types and to an optional chunk_size.
The default value of the environment variable is a static schedule with a chunk_size of 1. If
the optional chunk_size is set, the value must be positive. If chunk_size is not set, a value of
1 is assumed, except for static schedule. For a static schedule, the default chunk_size is
set to the loop iteration space divided by a number of threads applied to the loop.
NOTE: OMP_SCHEDULE is ignored for for and parallel for directives that have a schedule
type other than runtime.
OMP_NUM_THREADS
export OMP_NUM_THREADS=value
setenv OMP_NUM_THREADS value
The OMP_NUM_THREADS environment variable sets the default number of threads to use during
execution. The value of OMP_NUM_THREADS must be a positive integer. Its effect depends on
whether dynamic adjustment of the number of threads is enabled, and its interaction with the
omp_set_num_threads library routine and any num_threads clause on a parallel directive.
The default value is the number of physical processors on the system.
Parallel Programming Using OpenMP 169