# PetscOmpCtrlOmpRegionOnMasterBegin
Mark the beginning of an OpenMP library call on master ranks 
## Synopsis
```
#include "petscsys.h"  
PetscErrorCode PetscOmpCtrlOmpRegionOnMasterBegin(PetscOmpCtrl ctrl)
```

## Input Parameter

- ***ctrl -*** a PETSc OMP controller



## Note
Only master ranks can call this function. Call `PetscOmpCtrlGetOmpComms()` to know if this is a master rank.
This function changes CPU binding of master ranks and nthreads-var of OpenMP runtime




## See Also
 `PetscOmpCtrlOmpRegionOnMasterEnd()`, `PetscOmpCtrlCreate()`, `PetscOmpCtrlDestroy()`, `PetscOmpCtrlBarrier()`

## Level
developer

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/sys/utils/mpishm.c.html#PetscOmpCtrlOmpRegionOnMasterBegin">src/sys/utils/mpishm.c</A>


---
[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/sys/utils/mpishm.c)


[Index of all Sys routines](index.md)  
[Table of Contents for all manual pages](/docs/manualpages/index.md)  
[Index of all manual pages](/docs/manualpages/singleindex.md)  
