# Retrieving an array from a Ptr{Ptr{Cvoid}}

**URL:** <https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611>\
**Category:** New to Julia\
**Tags:** array, c\
**Created:** [January 27, 2023, 9:08am UTC](https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611 "2023-01-27T09:08:52Z")\
**Posts on this page:** 12\
**Page:** 1

<div class="post-metadata">

**Author:** ![mmv](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mmv/32/26323_2.png) [@mmv](https://discourse.julialang.org/u/mmv)\
**Post date:** [January 27, 2023, 9:08am UTC](https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611/1 "2023-01-27T09:08:52Z")

</div>

Hi!

I have a Julia wrapper function that calls a C function from a shared library. This C function updates an output structure `databuf`, which is an instance of

```julia
mutable struct MTKt_DataBuffer
    nline::Cint
    nsample::Cint
    datasize::Cint
    datatype::MTKt_DataType
    imported::MTKt_boolean
    data::MTKt_DataBufferType
    vdata::Ptr{Ptr{Cvoid}}
    dataptr::Ptr{Cvoid}
end

```

Upon returning from the `ccall`,

```julia
julia> databuf
MTKt_DataBuffer(512,
    2048,
    2,
    JMtk15.MTKe_uint16,
    JMtk15.MTK_FALSE,
    MTKt_DataBufferType((0x00, 0x00, 0x00, 0x00, 0x00, 0x42, 0x43, 0xba)),
    Ptr{Ptr{Nothing}} @0x00007fcfba434200,
    Ptr{Nothing} @0x000000014b250000)

julia> databuf.nline
512

julia> databuf.nsample
2048

julia> databuf.datatype
MTKe_uint16::MTKt_DataType = 0x00000006

```

How do I retrieve the 2D array vdata? I’ve tried `unsafe_wrap`, but I don’t seem to get what I was expecting (namely an array of 512 lines and 2048 columns UInt16 values):

```julia
julia> outm = unsafe_wrap(Array, databuf.vdata::Ptr{Ptr{Nothing}}, (databuf.nline, databuf.nsample); own = false)
512×2048 Matrix{Ptr{Nothing}}:
 Ptr{Nothing} @0x000000014b250000 … Ptr{Nothing} @0x0000000000000000
 Ptr{Nothing} @0x000000014b251000 Ptr{Nothing} @0x0000000000000000
 Ptr{Nothing} @0x000000014b252000 Ptr{Nothing} @0x00007fcfbb96a9c0
 Ptr{Nothing} @0x000000014b253000 Ptr{Nothing} @0x000000014aa94210
 Ptr{Nothing} @0x000000014b254000 Ptr{Nothing} @0x000000010b3beb60
 Ptr{Nothing} @0x000000014b255000 … Ptr{Nothing} @0x00007fcfbb96a9a8
 Ptr{Nothing} @0x000000014b256000 Ptr{Nothing} @0x000000014be386e0
 Ptr{Nothing} @0x000000014b257000 Ptr{Nothing} @0x0300000000000006
 Ptr{Nothing} @0x000000014b258000 Ptr{Nothing} @0x000000014be387e0
 Ptr{Nothing} @0x000000014b259000 Ptr{Nothing} @0x0000000000000000
 ⋮ ⋱  
 Ptr{Nothing} @0x000000014b447000 Ptr{Nothing} @0x0000000000000000
 Ptr{Nothing} @0x000000014b448000 Ptr{Nothing} @0x0000000000000000
 Ptr{Nothing} @0x000000014b449000 … Ptr{Nothing} @0x00007fcfbac33d20
 Ptr{Nothing} @0x000000014b44a000 Ptr{Nothing} @0x0000000000000000
 Ptr{Nothing} @0x000000014b44b000 Ptr{Nothing} @0x000000010b3aef60
 Ptr{Nothing} @0x000000014b44c000 Ptr{Nothing} @0x0000000000000000
 Ptr{Nothing} @0x000000014b44d000 Ptr{Nothing} @0x00007fcfbac34210
 Ptr{Nothing} @0x000000014b44e000 … Ptr{Nothing} @0x0300000000000007
 Ptr{Nothing} @0x000000014b44f000 Ptr{Nothing} @0x0000000000000000

julia> outm[100, 100]
Ptr{Nothing} @0x0000000000000000

```

Lastly, how to deal with a Ptr{Ptr{Ptr{Cvoid}}}? Thanks for suggestions…

---

<div class="post-metadata">

**Author:** ![GunnarFarneback](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gunnarfarneback/32/1827_2.png) [@GunnarFarneback](https://discourse.julialang.org/u/GunnarFarneback)\
**Post date:** [January 27, 2023, 9:37am UTC](https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611/2 "2023-01-27T09:37:11Z")

</div>

Do you have any documentation on what `vdata` means? My best guess is that

```julia
vp = unsafe_wrap(Array, databuf.vdata, databuf,nline)

```

gives you pointers to individual line arrays and that

```julia
[unsafe_wrap(Array, Ptr{T}(p), databuf.nsample) for p in vp]

```

gives you all the line data, assuming you set `T` to whatever datatype `MTKe_uint16::MTKt_DataType = 0x00000006` happens to encode.

---

<div class="post-metadata">

**Author:** ![barucden](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/barucden/32/26154_2.png) [@barucden](https://discourse.julialang.org/u/barucden)\
**Post date:** [January 27, 2023, 9:42am UTC](https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611/3 "2023-01-27T09:42:10Z")

</div>

Why not pass the type to `unsafe_wrap`?

```julia
unsafe_wrap(Array{UInt16, 2}, databuf.vdata, (databuf.nline, databuf.nsample); own=false)

```

Also note that C uses row-major ordering whereas Julia column-major so it might be necessary to switch the dimension order.

---

<div class="post-metadata">

**Author:** ![Gnimuc](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gnimuc/32/2194_2.png) [@Gnimuc](https://discourse.julialang.org/u/Gnimuc)\
**Post date:** [January 27, 2023, 10:18am UTC](https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611/4 "2023-01-27T10:18:56Z")

</div>

As I answered in the DM message, `vdata` needs to be `unsafe_load`ed once and then we can use `unsafe_wrap` to extract the array.

---

<div class="post-metadata">

**Author:** ![mmv](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mmv/32/26323_2.png) [@mmv](https://discourse.julialang.org/u/mmv)\
**Post date:** [January 27, 2023, 10:30am UTC](https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611/5 "2023-01-27T10:30:00Z")

</div>

There must still be something missing:

@barucden:

```julia
julia> outm = unsafe_wrap(Array{UInt16, 2}, databuf.vdata, (databuf.nline, databuf.nsample); own=false)
ERROR: StackOverflowError:

```

or @Gnimuc:

```julia
julia> outm1 = unsafe_load(databuf.vdata)
Ptr{Nothing} @0x000000014b250000

julia> outm2 = unsafe_wrap(Array{UInt16, 2}, outm1, (databuf.nline, databuf.nsample); own=false)
ERROR: StackOverflowError:

```

---

<div class="post-metadata">

**Author:** ![Gnimuc](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gnimuc/32/2194_2.png) [@Gnimuc](https://discourse.julialang.org/u/Gnimuc)\
**Post date:** [January 27, 2023, 10:35am UTC](https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611/6 "2023-01-27T10:35:22Z")

</div>

[https://nasa.github.io/MISR-Toolkit/Mtk\_struct.html](https://nasa.github.io/MISR-Toolkit/Mtk_struct.html)

> MTKt\_DataBuffer

The MTKt\_DataBuffer structure is used to return geo-located data planes (“arrays”) of MISR data from the MtkReadData routine. **It is designed to handle any C datatype. It does this using a union pointer data field which points to the same data pointed to by the dataptr field. The dataptr field points to the main data array which is stored in row-major form.** The number of bytes of the data plane can be determined using buf.nline, buf.nsample and sizeof the datatype field. It can be used for bulk transfers. There is also a row vector (Illiffe vector) pointing to each row of the main array to support C bracket index notation for a 2D-array. It can be ignored. The structure does maintain dynamic memory, so use MtkDataBufferFree when finished with this structure.  
Type Definition:

```julia

typedef enum {
  MTKe_void=0,
  MTKe_char8,
  MTKe_uchar8,
  MTKe_int8,
  MTKe_uint8,
  MTKe_int16,
  MTKe_uint16,
  MTKe_int32,
  MTKe_uint32,
  MTKe_int64,
  MTKe_uint64,
  MTKe_float,
  MTKe_double
} MTKt_DataType;

typedef union {
  void **v;
  MTKt_char8 **c8;
  MTKt_uchar8 **uc8;
  MTKt_int8 **i8;
  MTKt_uint8 **u8;
  MTKt_int16 **i16;
  MTKt_uint16 **u16;
  MTKt_int32 **i32;
  MTKt_uint32 **u32;
  MTKt_int64 **i64;
  MTKt_uint64 **u64;
  MTKt_float **f;
  MTKt_double **d;
} MTKt_DataBufferType;

typedef struct {
  int nline; /**< Number of lines */
  int nsample; /**< Number of samples */
  int datasize; /**< Data element size (bytes) */
  MTKt_DataType datatype; /**< Data type (enumeration) */
  MTKt_DataBufferType data; /**< Data type access union */
  void **vdata; /** < Row major 2D array with Illiffe vector */
  void *dataptr; /**< Pointer data buffer */
} MTKt_DataBuffer;
Example Usage:

#include “MisrToolkit.h”

MTKt_status status;
MTKt_Region region = MTK_REGION_INIT;
MTKt_DataBuffer buf = MTKT_DATABUFFER_INIT;
MTKt_MapInfo mapinfo = MTKT_MAPINFO_INIT;

status = MtkSetRegionByPathBlockRange(32, 65, 125, &region);
if (status != MTK_SUCCESS) error;

status = MtkReadData(“MISR_file.hdf”, “MISR_gridname”, “MISR_fieldname”, region, &buf, &mapinfo)
if (status != MTK_SUCCESS) error;

for (l = 0; l < buf.nline; l++)
   for (s = 0; s < buf.nsample; s++)
       printf(“Data Buffer[%d][%d]: %f\n”, l, s, buf.data.f[l][s]);

MtkDataBufferFree(&buf);

```

---

<div class="post-metadata">

**Author:** ![Gnimuc](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gnimuc/32/2194_2.png) [@Gnimuc](https://discourse.julialang.org/u/Gnimuc)\
**Post date:** [January 27, 2023, 10:38am UTC](https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611/7 "2023-01-27T10:38:26Z")

</div>

This suggests we can directly use `unsafe_wrap` `dataptr` instead of the mysterious `vdata`.

---

<div class="post-metadata">

**Author:** ![mmv](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mmv/32/26323_2.png) [@mmv](https://discourse.julialang.org/u/mmv)\
**Post date:** [January 27, 2023, 10:42am UTC](https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611/8 "2023-01-27T10:42:52Z")

</div>

Still getting the error message:

```julia
julia> outm3 = unsafe_wrap(Array{UInt16, 2}, databuf.dataptr, (databuf.nline, databuf.nsample); own=false)
ERROR: StackOverflowError:

```

---

<div class="post-metadata">

**Author:** ![GunnarFarneback](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gunnarfarneback/32/1827_2.png) [@GunnarFarneback](https://discourse.julialang.org/u/GunnarFarneback)\
**Post date:** [January 27, 2023, 12:45pm UTC](https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611/9 "2023-01-27T12:45:21Z")

</div>

> [@barucden](#):
>
> Why not pass the type to `unsafe_wrap`?
> 
> ```julia
> unsafe_wrap(Array{UInt16, 2}, databuf.vdata, (databuf.nline, databuf.nsample); own=false)
> 
> ```

Because it gives `StackOverflowError` and has no moral support in the docstring?

> `unsafe_wrap(Array, pointer::Ptr{T}, dims; own = false)`
> 
> Wrap a Julia Array object around the data at the address given by pointer, without  
> making a copy. **The pointer element type T determines the array element type.**

---

<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [January 27, 2023, 1:46pm UTC](https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611/10 "2023-01-27T13:46:08Z")

</div>

> [@mmv](#):
>
> `outm3 = unsafe_wrap(Array{UInt16, 2}, databuf.dataptr, (databuf.nline, databuf.nsample); own=false)`

`outm3 = unsafe_wrap(Array, Ptr{UInt16}(databuf.dataptr), (databuf.nline, databuf.nsample); own=false)`

---

<div class="post-metadata">

**Author:** ![mmv](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mmv/32/26323_2.png) [@mmv](https://discourse.julialang.org/u/mmv)\
**Post date:** [January 27, 2023, 1:49pm UTC](https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611/11 "2023-01-27T13:49:30Z")

</div>

Thanks a lot, @stevengj: That worked (albeit in hexadecimal):

```julia
julia> outm3 = unsafe_wrap(Array, Ptr{UInt16}(databuf.dataptr), (databuf.nline, databuf.nsample); own=false)
512×2048 Matrix{UInt16}:
 0xffeb 0x1c54 0x068c 0x0fbc 0xffeb … 0xffeb 0x1810 0x1324 0x14e4
 0xffeb 0x1d1c 0x0698 0x1038 0xffeb 0xffeb 0x1860 0x1300 0x14d8
 0xffeb 0x1d28 0x06ac 0x109c 0xffeb 0xffeb 0x1b38 0x12d0 0x128c
 0xffeb 0x1d0c 0x06b4 0x11fc 0xffeb 0xffeb 0x1c0c 0x12cc 0x1114
 0xffeb 0x1ca0 0x07f4 0x1130 0xffeb 0xffeb 0x1b64 0x12dc 0x10f8
 0xffeb 0x1be8 0x0968 0x1074 0xffeb … 0xffeb 0x1bac 0x12bc 0x11a4
 0xffeb 0x1d08 0x07dc 0x10bc 0xffeb 0xffeb 0x1b8c 0x125c 0x12d8
 0xffeb 0x1e60 0x06d8 0x10f8 0xffeb 0xffeb 0x1b20 0x1294 0x0de4
 0xffeb 0x1e68 0x06c4 0x1184 0xffeb 0xffeb 0x1aac 0x12a8 0x0b20
 0xffeb 0x1d90 0x06c4 0x1218 0xffeb 0xffeb 0x1a20 0x12dc 0x1098
      ⋮ ⋱ ⋮          
 0x1bcc 0x06dc 0x0d98 0xffeb 0x1ad8 0x1970 0x13cc 0x124c 0xffeb
 0x1cc8 0x06bc 0x0fdc 0xffeb 0x1b90 0x1980 0x1410 0x1298 0xffeb
 0x1cc0 0x06b0 0x1198 0xffeb 0x1c04 … 0x1910 0x1428 0x1330 0xffeb
 0x1cdc 0x06a8 0x1184 0xffeb 0x1cb8 0x1900 0x13f8 0x1408 0xffeb
 0x1d20 0x06a4 0x1208 0xffeb 0x1bbc 0x196c 0x13e8 0x14d4 0xffeb
 0x1cc4 0x068c 0x1348 0xffeb 0x1ab8 0x1978 0x1440 0x1410 0xffeb
 0x1b74 0x06a8 0x1340 0xffeb 0x1a48 0x1a04 0x1414 0x1334 0xffeb
 0x1ad0 0x069c 0x124c 0xffeb 0x19fc … 0x1c20 0x13a4 0x134c 0xffeb
 0x1b50 0x068c 0x10c4 0xffeb 0x1aa8 0x19a0 0x1360 0x14a0 0xffeb

```

---

<div class="post-metadata">

**Author:** ![GunnarFarneback](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gunnarfarneback/32/1827_2.png) [@GunnarFarneback](https://discourse.julialang.org/u/GunnarFarneback)\
**Post date:** [January 27, 2023, 1:51pm UTC](https://discourse.julialang.org/t/retrieving-an-array-from-a-ptr-ptr-cvoid/93611/12 "2023-01-27T13:51:52Z")

</div>

> [@mmv](#):
>
> (albeit in hexadecimal):

That’s just how Julia prints unsigned integers.