有个地方没有看懂

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>

 

int main( void )
{
    int ret = 0;

    int in = open( "file1", O_BINARY|O_SEQUENTIAL|O_RDONLY );
    if( in != -1 )
    {
        int out = open( "file8", O_BINARY|O_SEQUENTIAL|O_WRONLY|O_CREAT, S_IRUSR|S_IWUSR );
        if( out != -1 )
        {
            for( ; ; )
            {
                char buf[1024];

                int rc = read( in, buf, (unsigned)sizeof(buf) );
                if( rc < 0 )
                {
                    puts( "failed to read from file8." );
                    ret = 3;
                    break;
                }
                if( rc == 0 )
                    break;

                int wc = write( out, buf, rc );
                if( wc != rc )
                {
                    puts( "failed to write to file8." );
                    ret = 4;
                    break;
                }
            }
            if( ret == 0 )
                puts( "ok" );

            close( out );
        }
        else
        {
            puts( "failed to open file8." );
            ret = 2;
        }

        close( in );
    }
    else
    {
        puts( "failed to open file1." );
        ret = 1;
    }

    return ret;
}