#include <fcntl.h> 
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define LEN_MSG 20
//#include "mopen.h"
char dev[ 80 ] = "/dev/mynull";
int prepare( char *test ) { 
int df; 
if( ( df = open( dev, O_RDWR ) ) < 0 )  printf( "open device error: %m\n" ); 
int res, len = strlen( test ); 
if( ( res = write( df, test, len ) ) != len ) 
printf( "write device error: %m\n" ); 
else 
printf( "prepared %d bytes: %s\n", res, test ); 
return df; 
}
void test( int df ) { 
char buf[ LEN_MSG + 1 ]; 
int res;
printf( "------------------------------------\n" ); 
do { 
   if( ( res = read( df, buf, LEN_MSG ) ) > 0 ) { 
   buf[ res ] = '\0'; 
   printf( "read %d bytes: %s\n", res, buf ); 
   }
   else if( res < 0 ) 
   printf( "read device error: %m\n" ); 
   else 
   printf( "read end of stream\n" ); 
} while ( res > 0 ); 
printf( "------------------------------------\n" ); 
} 
int main( int argc, char *argv[] ) { 
//strcat( dev, DEVNAM ); 
int df1, df2;                   // разные дескрипторы одного устройства 
df1 = prepare( "1111111" ); 
test( df1 ); 
close( df1 );
df2 = prepare( "22222222" ); 
test( df2 ); 
close( df2); 
return EXIT_SUCCESS; 
};