petsc-3.14.5 2021-03-03
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# PCGetOperators

Gets the matrix associated with the linear system and possibly a different one associated with the preconditioner.

### Synopsis

    #include "petscksp.h"
    PetscErrorCode  PCGetOperators(PC pc,Mat *Amat,Mat *Pmat)

Not collective, though parallel Mats are returned if the PC is parallel

### Input Parameter

| pc | - the preconditioner context

### Output Parameters

| Amat | - the matrix defining the linear system
| Pmat | - the matrix from which the preconditioner is constructed, usually the same as Amat.

### Notes

Does not increase the reference count of the matrices, so you should not destroy them

Alternative usage: If the operators have NOT been set with KSP/PCSetOperators() then the operators are created in PC and returned to the user. In this case, if both operators mat and pmat are requested, two DIFFERENT operators will be returned. If only one is requested both operators in the PC will be the same (i.e. as if one had called KSP/PCSetOperators() with the same argument for both Mats). The user must set the sizes of the returned matrices and their type etc just as if the user created them with MatCreate(). For example,

            KSP/PCGetOperators(ksp/pc,&Amat,NULL); is equivalent to

              set size, type, etc of Amat

            MatCreate(comm,&mat);

            KSP/PCSetOperators(ksp/pc,Amat,Amat);

            PetscObjectDereference((PetscObject)mat);

              set size, type, etc of Amat

and

            KSP/PCGetOperators(ksp/pc,&Amat,&Pmat); is equivalent to

              set size, type, etc of Amat and Pmat

            MatCreate(comm,&Amat);

            MatCreate(comm,&Pmat);

            KSP/PCSetOperators(ksp/pc,Amat,Pmat);

            PetscObjectDereference((PetscObject)Amat);

            PetscObjectDereference((PetscObject)Pmat);

              set size, type, etc of Amat and Pmat

The rational for this support is so that when creating a TS, SNES, or KSP the hierarchy of underlying objects (i.e. SNES, KSP, PC, Mat) and their livespans can be completely managed by the top most level object (i.e. the TS, SNES, or KSP). Another way to look at this is when you create a SNES you do not NEED to create a KSP and attach it to the SNES object (the SNES object manages it for you). Similarly when you create a KSP you do not need to attach a PC to it (the KSP object manages the PC object for you). Thus, why should YOU have to create the Mat and attach it to the SNES/KSP/PC, when it can be created for you?

### See Also

PCSetOperators(), KSPGetOperators(), KSPSetOperators(), PCGetOperatorsSet()

### Level

intermediate

### Location

src/ksp/pc/interface/precon.c

### Examples

src/ksp/ksp/tutorials/ex15f.F90.html
Index of all PC routines
Table of Contents for all manual pages
Index of all manual pages