IPC: named FIFO not working properly.

[robgeek]

Hello, guys.

I'm trying to make a simple program which communicates with other programs using named fifo. The first thing i want to do is make a simple server program which creates a random number and writes in a fifo file, and a client program read it. The server is working fine, but my client program is not working properly. He stops in read() function!

What is going on?

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

int main( void ) {

	int number, pipe;

	if( mkfifo("arqfifo", 0666 ) < 0 ) {
		perror("Error: mkfifo( ): ");
		return 1;
	}

	printf("Trying to connect to client.\n");
	if( pipe = open("arqfifo", O_WRONLY ) < 1 ) {
		perror("Error: open( ): ");
		return 1;
	}

	//number = rand() % 100; 
	number = 256;
	printf("Connection succeeded.\n");
	printf("The number is: %d\n", number );

	if( write( pipe, &number, sizeof( int ) ) < 0 ) {
		perror("Error: write( ): ");
		return 1;
	}

	close( pipe );	
	return 0;
}

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

int main( void ) {
	
	int number, pipe;

	printf("Trying to connect to server.\n");
	if( pipe = open("arqfifo", O_RDONLY, 0 ) == -1 ) {
		perror("Error: open( ): ");
		return 1;
	}

	if( read( pipe, &number, sizeof( int ) ) < 0 ) {
		perror("Erro: read( ): ");
		return 1;
	}
	printf("Connection succeeded.\n");
	printf("The number is: %d\n", number );
	close( pipe );	
	return 0;
}

[Dy7SLyTq]

http://linux.die.net/man/3/read
it looks like your arguments are wrong

[mik2718]

You are using two separate terminals?

Here some tests I just did

test1:

terminal 1:

mkfifo myfifo
echo "stuff" > myfifo

this waits because there is no reader from the fifo

terminal 2:

cat myfifo

"stuff" appears, the cat command has pulled it out of the fifo. the echo command in terminal 1 now completes

test 2:

terminal 1:

cat myfifo

cat now waits because there is nothing in the fifo to pull out

terminal 2:

echo "stuff" > myfifo

this puts "stuff" into the fifo - it is immediately pulled out by cat on terminal 1 which now ends waiting

So you need to be careful r.e. synchronisation, if the reader is waiting it may mean that there is nothing in the fifo at that time,

[htirwin]

if( pipe = open("arqfifo", O_RDONLY, 0 ) == -1 )
You should get a warning doing this.

What is happening, is that the return value of open is being compared with -1, and the result of the comparison is true or false, which is then casted to an int, becoming, 1 or 0, for the assignment to pipe. 0, and 1, are standard input and output.

You need parentheses around the assigment.

if( (pipe = open("arqfifo", O_RDONLY, 0 )) == -1 )

And also in other file,

if( (pipe = open("arqfifo", O_WRONLY )) < 1 )

[robgeek]

Thank you, guys. Now everything is working fine. Thank you htirwin, you are right.

Problem fixed.