Linux message sender and receiver execute error(repetition error) - c

Question

I worked in Linux.
I only want to print the message on receiver when Sanders sends.
but, the receiver continues to output same message.
I exactly want to get message in receiver, only if sender sends message.
This code is my code.
I try to conditional, but it's not easy. help
//Sender
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>
int main()
{
int fd;
char message[100];
printf("Enter \"exit\" to stop: \n");
while(1)
{
fd=open("/home/control/work/io_dev",O_RDWR); //open file
if (fd ==-1) {
printf("Device open error");
exit(1);
}
sleep(1);
write(fd,message,sizeof(message));
printf("Sender>>");
fgets(message,sizeof(message),stdin);// input message
write(fd,message, sizeof(message));//write on file
if(strcmp(message,"exit\n")==0)
break;
write(fd,message,sizeof(message));
close(fd); //file close
}
}
//Receiver
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>
int main()
{
int fd;
char message[100];
while(1){
sleep(1);
fd=open("/home/control/work/io_dev",O_RDWR|O_CREAT); // open or create file
if (fd == -1) { //when file open error occurs
printf("Device open error");
exit(1);
}
read(fd, message, sizeof(message));//read file
sleep(1);//wait
if(strcmp(message,"exit\n") ==0)//loop off
break;
if(message[0]!='\0'){
printf("Received>> %s",message);//print
}
write(fd,message,sizeof(message));
close(fd);//file close
sleep(3);
}
}

Answer 1

Delete the first "write(fd,message,sizeof(message));" in sender side.
Delete "if(message[0]!='\0')"in receiver side.

Answer 2

the following proposed code, for receiver
cleanly compiles
performs the desired functionality
properly checks for errors
avoids having to continually open/close the I/O device
echos what was actually received rather than the whole receiver buffer, much of which may contain trash
avoids the use of 'magic' numbers in the code
and now, the proposed code:
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>
#define MAX_MSG_LEN 100
int main( void )
{
int fd;
char message[ MAX_MSG_LEN ];
fd=open("/home/control/work/io_dev",O_RDWR|O_CREAT, 0666 ); // open or create file
if (fd == -1)
{ //when file open error occurs
printf("Device open error");
exit(1);
}
while(1)
{
ssize_t bytesRead = read(fd, message, sizeof(message));//read file
if( bytesRead <0 )
{
// handle error and exit
perror( "read failed" );
close( fd );
break;
}
if( bytesRead == 0 )
{ // sender closed the connection
// clean up and exit
close( fd );
break;
}
message[ bytesRead ] = '\0';
if(strcmp(message,"exit\n") ==0)//loop off
break;
printf("Received>> %s",message);//print
if( write(fd,message,strlen(message)) != strlen( message ) )
{
// handle incomplete message write
}
}
close(fd);//file close
}
HOWEVER, if your trying to 'pace' the read/write operations with multiple readers and writers; then strongly suggest using a mutex to avoid any 'race' conditions and maybe even mkfifo() for creating the file. Remember to (before exiting the program) to destroy the fifo.
If deciding to use a FIFO, then be sure to call unlink( filename ); so the file will be destroyed when the program exits
If you want to keep the repeated open/close in the while() loop, then use O_EXCL rather than O_CREAT so the separate programs cannot step on each other.