Pseudo-terminal not printing output

Question

So here is my code which is designed to open an additional gnome terminal and print the first 20 Fibonacci numbers in the newly opened terminal console:

#include <stdio.h>
#include "apue.h"

int Fibonacci(unsigned int n);

int main() {
    char cmd[256];
    char str[40][256];
    char *name = tempnam(NULL, NULL);
    char *line = "\n";
    FILE *log;

    mkfifo(name, 0777);
    log = fopen(name, "w+");

    for (unsigned int i = 0; i < 20; i++) {
         sprintf(str[(2 * i)], "%s",line);
         fputs(str[(2 * i)], log);
         sprintf(str[(2 * i) + 1], "%u\n", Fibonacci(i));
         fputs(str[(2 * i) + 1], log);
         fflush(NULL);
    }
    if (fork() == 0) {
        sprintf(cmd, "gnome-terminal -e cat %s", name);
        system(cmd);       
        for (unsigned int j = 0; j < 40 ; j++) {    
            fgets(str[j], sizeof(cmd), log);            
        }
        exit(0);
    } else
    if (fork() < 0) {
        perror("fork () error");    
    }
}

int Fibonacci(unsigned int n) {
    if (n == 0) {
        return 0;
    } else
    if (n == 1) {
        return 1;
    } else
    if (n > 1) {
        return Fibonacci(n - 2) + Fibonacci(n - 1);
    }
}

When I compile it, I get the following warning message:

-*- mode: compilation; default-directory: "/usr/lib/gcc/x86_64-linux-gnu/4.8.4/include/" -*-
Compilation started at Sun Sep  3 15:46:28

gcc  -o FIB0   Fibonacci.c
/tmp/ccobnJV9.o: In function `main':
Fibonacci.c:(.text+0x231f): warning: the use of `tempnam' is dangerous, better use `mkstemp'

Compilation finished at Sun Sep  3 15:46:28

When I execute it from the gnome terminal command line, the new terminal pops open but with no output! How could I fix this code to make it work?

When I use

 sprintf(cmd, "xterm -e cat %s", name);

instead of "gnome-terminal", it works correctly. So how does one communicate between gnome terminals using GCC?

Answer 1

Try

sprintf(cmd, "gnome-terminal -e \"cat %s\"", name);

gnone-terminal's man request a string after -e.

Answer 2

There is an extra problem in your forking mechanism:

if (fork() == 0) {
    /* do something in the child */
} else
if (fork() < 0) {   //<--- fork AGAIN!
    perror("fork () error");    
}

The parent process is forked twice!

You should instead store the pid:

int pid = fork();
if (pid == 0) {
    /* do something in the child */
} else
if (pid < 0) {
    perror("fork () error");    
}

Answer 3

Try it in a command line.

If I run

echo hi > ~/temp
gnome-terminal -e cat ~/temp

then I get a terminal running cat (which is initially blank, until I type something and press enter)

If I run

xterm -e cat ~/temp

then I get an xterm that pops up, prints "hi" and closes very quickly.

So it appears gnome-terminal -e cat /some/path is not running cat /some/path but only cat.

If you want to run cat /some/path in a gnome-terminal then you have to use the command:

gnome-terminal -e "cat /some/path"

Notice the extra quote marks.

Answer 4

Either use quotations, as immibis suggested, or use -x instead of -e. See manpage of gnome-terminal for the details.