:orphan:
# ISAllGather
Given an index set `IS` on each processor, generates a large index set (same on each processor) by concatenating together each processors index set. 
## Synopsis
```
#include "petscis.h"  
PetscErrorCode ISAllGather(IS is, IS *isout)
```
Collective


## Input Parameter

- ***is -*** the distributed index set



## Output Parameter

- ***isout -*** the concatenated index set (same on all processors)





## Notes
`ISAllGather()` is clearly not scalable for large index sets.

The `IS` created on each processor must be created with a common
communicator (e.g., `PETSC_COMM_WORLD`). If the index sets were created
with `PETSC_COMM_SELF`, this routine will not work as expected, since
each process will generate its own new `IS` that consists only of
itself.

The communicator for this new `IS` is `PETSC_COMM_SELF`


## See Also
 [](sec_scatter), `IS`, `ISCreateGeneral()`, `ISCreateStride()`, `ISCreateBlock()`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/vec/is/is/utils/iscoloring.c.html#ISAllGather">src/vec/is/is/utils/iscoloring.c</A>


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


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