How to implement a circular buffer using a file?

Question

My application (C program) opens two file handles to the same file (one in write and one in read mode). Two separate threads in the app read from and write to the file. This works fine. Since my app runs on embedded device with a limited ram disk size, I would like write FileHandle to wrap to beginning of file on reaching max size and the read FileHandle to follow like a circular buffer. I understand from answers to this question that this should work. However as soon as I do fseek of write FileHandle to beginning of file, fread returns error. Will the EOF get reset on doing fseek to beginning of file? If so, which function should be used to cause write file position to get set to 0 without causing EOF to be reset.

EDIT/UPDATE: I tried couple of things:

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1. Based on @neodelphi I used pipes this works. However my usecase requires I write to a file. I receive multiple channels of live video surveilance stream that needs to be stored to harddisk and also read back decoded and displayed on monitor.

2. Thanks to @Clement suggestions on doing ftell I fixed a couple of bugs in my code and wrap works for the reader however, the data read appears to be stale data since write are still buffered but reader reads stale content from hard disk. I cant avoid buffering due to performance considerations (I get 32Mbps of live data that needs to be written to harddisk). I have tried things like flushing writes only in the interval from when write wraps to when read wraps and truncating the file (ftruncate) after read wraps but this doesnt solve the stale data problem.

3. I am trying to use two files in ping-pong fashion to see if this solves the issue but want to know if there is a better solution

Answer 1

You should have something like that :

// Write
if(ftell(WriteHandle)>BUFFER_MAX) rewind (WriteHandle);
fwrite(WriteHandle,/* ... */);

// Read (assuming binary)
readSize = fread (buffer,1,READ_CHUNK_SIZE,ReadHandle);
if(readSize!=READ_CHUNK_SIZE){
    rewind (ReadHandle);
    if(fread (buffer+readSize,1,READ_CHUNK_SIZE-readSize,ReadHandle)!=READ_CHUNK_SIZE-readSize)
        ;// ERROR !
}

Not tested, but it gives an idea. The write should also handle the case BUFFER_MAX is not modulo WRITE_CHUNK_SIZE.

Also, you may read only if you are sure that the data has already been written. But I guess you already do that.