Thread: Trouble sharing files between processes.

I'm having trouble sharing files between processes. Here's what I have:

parent.c

Code:

#include <unistd.h>
#include <fcntl.h>

#define OUT STDOUT_FILENO
#define ERR STDERR_FILENO

int main()
{
	int handle, pid;
	char ch;

	if((handle = open("temp", O_CREAT|O_RDWR|O_TRUNC, S_IREAD|S_IWRITE)) < 0)
	{
		write(ERR, "open\n", 5);
		exit(0);
	}

	if((pid = fork()) < 0)
	{
		write(ERR, "fork\n", 5);
		exit(0);
	}
	else if(pid)
	{
		for(ch = 'a'; ch <= 'z'; ch++)
			write(handle, &ch, 1);
	}
	else
	{
		char *file[3];

		sprintf(file, "%d", handle);
		execlp("./child", file, (char *) NULL);
	}

	close(handle);

	exit(0);
}

child.c:

Code:

#include <unistd.h>
#include <fcntl.h>

#define OUT STDOUT_FILENO
#define ERR STDERR_FILENO

int main(int argc, char **argv)
{
	int count = 0;
	int input = atoi(argv[0]);
	char buf;
	int temp;

	while(count < 26)
	{
		if((temp = read(input, &buf, 1)) < 0)
		{
			write(ERR, "write\n", 6);
			exit(0);
		}

		if(temp)
		{
			write(OUT, &buf, 1);
			count++;
		}
	}

	exit(0);
}

The child process just goes into an imfinite loop, which indicates that read is always return 0. When I used pipes, everything worked fine. Please help me out. Thanks.

i haven't tried ur code, but i have a feeling it's because you've used the wrong arguement in the child.

argv[0] is the executable file of the program.

argv[1] is the first arguement given to the program. So try to change

Code:

int input = atoi(argv[0]);

to

Code:

int input = atoi(argv[1]);

I don't think that's it. When I print out argv[0], I get the value of the file descriptor, 3, not value "child." The file descriptor value is the same in the parent as well. I think when you call one of the exec functions, the argument numbering is different.

Bump.

Sorry for bumping; I didn't read the FAQ carefully enough.
The purpose of the assignment was for the child to read the file while the parent is writing to the file. I solved the problem in a desparation attempt by just opening the file, temp, again, as read only; it's not nice because I was supposed to just pass the file descriptor.
When I used pipes, I passed the read of the file descriptor to the child, and everything worked fine then: the parent wrote the write end, and the child read from the read end. Why did it work then?
And my compiler did not complain when I used execlp; it did complain when I used execl.

i have a feeling that it worked with pipes because when you create a pipe, you get 2 separate file descriptors to the same 'file', one for writing and one for reading. I think since it's two separate fd's writing does not modify the position of the reading fd.

If I duplicate a file descriptor using dup(), does modifying the file pointer in one modify the file pointer in another? And does writing in a duplicated file descriptor effect the other file descriptor (i.e. the same file is changed by just using any of file descriptors)?