Thread: popen problem

Hi,

I have been learning about pipes in C recently. Here is an example of some code that works fine:

Code:

// some lines are omitted
	switch(fork()) {
		case -1:

		perror("Fork");
		exit(1);

		case 0:

		dup2(p[1], fileno(stdout));
		close(p[0]);
		close(p[1]);
		execl("/usr/bin/last", "last", 0);

		_exit(0);

		default:

		printf("parent\n");

		dup2(p[0], fileno(stdin));
		close(p[1]);
		close(p[0]);

		execl("/bin/sort", "sort", 0);
	}

Now this does exactly what it is supposed to do, last | sort , I'm trying to do all this with popen instead. Here is my attempt at that.

Code:

int main(void)
{
	FILE *fr, *fw;
	int i, j=0;

	char buffer[BUFSIZE];

	fr = popen("last", "r");
	fw = popen("sort", "w");

	while (fgets(buffer, sizeof(buffer), fr))
		fprintf(fw, "%s\n", buffer);

	pclose(fr);
	pclose(fw);
	
	return 0;
}

This almost works perfectly, except I get 10000 blank lines printing to stdout first. I know it's 10000 because I through in a counter at one point. After the blank lines it outputs semi-properly. every 10 lines or so a line that's not sorted properly appears. Thanks for any help/advice on this.

thanks, it actually works fine with fprintf after removing the \n
I will try with fputs as well