Allocating pointer to a pointer-CodePudding

I have 5 buffers and 20 frames to write in them. Being one frame per buffer, at a certain moment i will have to overwrite buffers with the newest frame.

At random moments i need to read the oldest frame(its id and data) from all the buffers.

I am obliged to use a pointer to a pointer for my buffers, but since i suck at pointers, not even the allocation works, giving me a SEGMENTATION FAULT and not sure why.

What i have until now:

 void fakeFrame(uint16_t *data)
 {
    for (auto i = 0; i < 1440; i  )
       for (auto j = 0; j < 1440; j  )
           data[(i * 1440)   j] = std::rand()%2;
       
  }
    

int main()
{
        uint16_t         **p_frameBuffers;
        uint32_t          *p_frameIdxs;
        uint16_t           wrIdx = 0;
        uint16_t           reIdx = 0;
        uint16_t           currentFrameCounter = 0;
        uint16_t           nbBuffers = 5;

        for(auto i =0; i< nbBuffers; i  )
        {
           p_frameBuffers[i] = (uint16_t*)malloc(1440*1440*2);
        }


        while(currentFrameCounter <= 20)
        {
             wrIdx  ;
             wrIdx %= nbBuffers;

    
             if(wrIdx == reIdx)
             {
                    std::cout<<"i passed the limit";
             }
             currentFrameCounter  ;

             p_frameIdxs[wrIdx] = currentFrameCounter;
             fakeFrame(p_frameBuffers[wrIdx]);
         }

         std::cout<<"\n";


         return 0;
 }

CodePudding user response：

I can see a few different problems with this code here.

You declare the long-form of the function for fakeFrame() in the beginning of the program, when the standard is usually to declare the function header first.

This is like a warning to the program that a function is about to be used, and that it's not part of a class or anything. Just standalone.

Example:

#include <iostream>

void fakeFrame();

int main()
{
    return 0;
}

void fakeFrame()
{
for (auto i = 0; i < 1440; i  )
       for (auto j = 0; j < 1440; j  )
           data[(i * 1440)   j] = std::rand()%2;
}

You're also using some of these 16 and 32 bit unsigned ints as if they were arrays, so I was deeply confused about that. Did you mean to set them as arrays?

You also have some variables being declared in a non-array context but being used as arrays. I'm not deeply familiar with the uint variable/object types but I know they aren't usually meant to function as standalone arrays.

Also, no variable called m_pFrameBuffers is actually declared in the code you provided. Plus this is also used as an array, so it should really be declared as one.

I hope this provides at least some insight into what's not working. I'm actually kind of surprised that the void function ran before, it's improperly formatted.

CodePudding user response：

In the end this is what did it: the pointer to a pointer is actually an array of pointers (which i did not know, of course)

p_frameBuffers = (uint16_t**)malloc((sizeof(uint16_t*)*nbBuffers));

for(auto i = 0; i < nbBuffers; i  )
{
    p_frameBuffers[i] = (uint16_t*)malloc(1440*1440*2);
}