How to "fread" a binary file

- When you want to read 2 blocks, use this array float a[2][400];.
- To read binary data, use binary mode in fopen().
- Use sizeof() for the number of array elements and size of one element. So when you change the size of the array or use another type you must not change the code for reading.
- Because an array in C is allready a pointer, you don't need the &-operator for a[i] in fread().
- When the data blocks are in serial order, you don't need a fseek() between the fread() calls. You even don't need a loop, you could read all data within one fread() call.
- The long(3600+i*400*4) is not a C typecast. It is a C++ copy constructor. This would compile in a C++ compiler but a C typecast is written (long)3600+i*400*4.
- Do some tests for error handling.

Finaly, it could look like this:

float a[2][400];
FILE *fp = fopen( "a.txt","rb");
if( NULL != fp)
{
   size_t nElements = sizeof(a[0]) / sizeof(a[0][0]);
   for( int i=0; i<2; i++)
   {
      long lOffset = 3600 + i * sizeof(a[0]);
      if( 0 == fseek( fp, lOffset, SEEK_SET))
      {
         if( nElements != fread( a[i], sizeof(a[0][0]), nElements, fp))
         {
            // error handling
            break;
         }
      }
      else
      {
         // error handling
         break;
      }
   }
}

What happens when you run the code?

I'm not a C programmer but my C Quick Reference guide tells me that you are using the fread function incorrectly.

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

Each invocation of fread in the inner loop is reading 400 data elements of size 4 bytes, i.e. 1600 bytes. I think you meant to write
fread(&a[j][i],4,1,fp);

There is also a syntax error in the line containing fseek which you'll need to correct before you can compile the code successfully.

Alan.

thank you all!
I have learned a lot!!
hehe,the next I will compile the code again!
Thank you all!