# Segment fault when handling buffer data

**URL:** <https://gpusph.discourse.group/t/segment-fault-when-handling-buffer-data/143>\
**Category:** Applications\
**Created:** [April 24, 2021, 8:05am UTC](https://gpusph.discourse.group/t/segment-fault-when-handling-buffer-data/143 "2021-04-24T08:05:47Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![RRRRLi](https://yyz2.discourse-cdn.com/free1/user_avatar/gpusph.discourse.group/rrrrli/32/106_2.png) [@RRRRLi](https://gpusph.discourse.group/u/RRRRLi)\
**Post date:** [April 24, 2021, 8:05am UTC](https://gpusph.discourse.group/t/segment-fault-when-handling-buffer-data/143/1 "2021-04-24T08:05:48Z")

</div>

Hello, I’m trying to simulate a pouring scene and compute how many particles are in the cup at the end. I didn’t find an API for post-process for the whole problem but a single frame, i.e. `addPostProcess(CALC_PRIVATE);`. But it reports segment fault when getting particles’ info.

```auto
void Pour::calcPrivate(flag_t options,
    BufferList const& bufread,
    BufferList & bufwrite,
    uint numParticles,
    uint particleRangeEnd,
    uint deviceIndex,
    const GlobalData * const gdata)
{
    //thread per particle
    uint numThreads = BLOCK_SIZE_CALCTEST;
    uint numBlocks = div_up(particleRangeEnd, numThreads);

    const float4 *pos = bufread.getData<BUFFER_POS>();
    const float4 *vel = bufread.getData<BUFFER_VEL>();
    const particleinfo *info = bufread.getData<BUFFER_INFO>();
    const hashKey *particleHash = bufread.getData<BUFFER_HASH>();
    const uint *cellStart = bufread.getData<BUFFER_CELLSTART>();
    const neibdata *neibsList = bufread.getData<BUFFER_NEIBSLIST>();

    int insidenum = 0;
    for(uint i = 0; i<numParticles;i++){
        if (FLUID(info[i])){//<-- Segment Fault (core dumped)
            ......

```

In debug, I can see

```css
i: 0
info[i]: {...}
    x: 0
    y: 0
    z: 0
    w: 0
buread: {...}
    m_map: {...}
    m_keys: 1142461302494
    m_has_pending_state: BufferList::NOT_PENDING
    m_pending_state: ""
    m_updated_buffers: 0
    m_validate_accessfalse
numThreads: 128
numBlocks: 28
pos: 0x7fffc660de00
    x: 0
    y: 0
    z: 0
    w: 0
info: 0x7fffc6637800
    x: 0
    y: 0
    z: 0
    w: 0

```

I haven’t write cuda codes before, so I guess if there’s something wrong with operation about buffer data?  
But I find the same code can work in

```auto
initializeParticles(BufferList &buffers, const uint numParticles){
    ...
}

```

I’m confused. Can anyone help? Thanks in advance!

---

<div class="post-metadata">

**Author:** ![giuseppe.bilotta](https://yyz2.discourse-cdn.com/free1/user_avatar/gpusph.discourse.group/giuseppe.bilotta/32/72_2.png) [@giuseppe.bilotta](https://gpusph.discourse.group/u/giuseppe.bilotta)\
**Post date:** [April 30, 2021, 6:22pm UTC](https://gpusph.discourse.group/t/segment-fault-when-handling-buffer-data/143/2 "2021-04-30T18:22:10Z")

</div>

Hello @RRRRLi —thanks for your interest in GPUSPH, and sorry for the delay in the response.

The `CALC_PRIVATE` post-processing filter is intended for execution on GPU, not on CPU. The buffers in the `bufread` and `bufwrite` lists are device buffers, not host buffers, so in general you cannot access them on host. This is exemplified in the `ProblemExample` test case: the host-side `calcPrivate` function is just a thin wrapper that sets up the launch grid and then invokes a `calcPrivateDevice` kernel that does the actual computation (in the provided example, simply a count of the number of neighbors for it particle).

The reason why accessing the buffer works in `initializeParticle` is because `initializeParticles` is intended to be run on host, and is thus passed a list of host-side buffers (the ones used also for writing. If you only care about producing the additional data on host, when writing, one way to achieve this is to add a `CALLBACKWRITER` to your problem, and override the `writer_callback` method:

```auto
void
YourTestCase::writer_callback(CallbackWriter *,
	uint numParts, BufferList const& bufread, uint node_offset, double t,
	const bool testpoints) const
{
/* here you can read your bufread data, process it without modifying it,
   and then write the output to disk or wherever you want. */
}

```

(I’m afraid we don’t have a test case showing how to use this writer presently, so feel free to ask here on the forum for any additional help.)

---

<div class="post-metadata">

**Author:** ![RRRRLi](https://yyz2.discourse-cdn.com/free1/user_avatar/gpusph.discourse.group/rrrrli/32/106_2.png) [@RRRRLi](https://gpusph.discourse.group/u/RRRRLi)\
**Post date:** [June 1, 2021, 2:58pm UTC](https://gpusph.discourse.group/t/segment-fault-when-handling-buffer-data/143/3 "2021-06-01T14:58:10Z")

</div>

Thanks for your reply! I’ve solved this problem through your method.