Special handling of /dev/null in open and write?

Question

according to a hint from another thread I want to analyze pointers, if their dereferencing would cause an segmentation faults or not. The idea is to write code like this:

bool IsPointerValid( void* pPointer )
{
    // when opening "/tmp/hugo" here, it works fine... but not with /dev/null??
    int iFD = open( "/dev/null", O_WRONLY );
    int iBytesWritten = write( iFD, pPointer, 4 );
    close( iFD );

    return iBytesWritten > 0;
}

But whatevery I pass to IsPointerValid(..), it returns always true - because iBytesWritten is always 4. But when opening "/tmp/testfile" or a fifo, it works like expected: Passing the NULL pointer to write(..), it returns -1.

What is the reason for this special treating of "/dev/null"?

Thanks,

Charly

Answer 1

Because it's a special file, so it brings its own definition of read and write. (The linux kernel implements polymorphism using function pointers in the driver layer). Apparently the version of write provided by the /dev/null device doesn't use the pointer you pass in.