2

i try to write a socket which loads programs and redirects socket io to these. sounds much like inetd but as far as i know, inetd loads the program when its port is requested. i want to have it loaded permanently.

so far so good. writing a socket server is not that tricky but i didn't get the rest working. I basically want to open a pipe(), dup2() it to stdin and stdout and execv() my program.

the problem is, that my called program doesn't get any input.I'll try to show it with a test program. can someone tell me, what's wrong? 

int create_program_fork(int *ios, char const *program) { 
// create pipes to program 
if (pipe(ios) != 0) { 
    return -1; 
} 

// fork to new process 
int f = fork(); 
if (f < 0) { 
    // fork didn't work 
    close(ios[0]); 
    close(ios[1]); 
    return(-1); 
} 
if (f > 0) { 
    // master hasn't much to do here 
    return f; 
} 
// *** Child Process 
// close std** file descriptors 
printf ("executing program"); 
close(STDIN_FILENO); 
close(STDOUT_FILENO); 
// duplicate pipes as std** 
dup2(ios[0], STDIN_FILENO); 
dup2(ios[1], STDOUT_FILENO); 
// close pipes 
close(ios[0]); 
close(ios[1]); 
// call program 
return execvp(program, NULL ); 
} 

int main(int argc, char *argv[]) { 
int ios[2]; 

// call program 
int pid = create_program_fork(ios, "/bin/bash"); 
if (0 != pid){ 
    exit(EXIT_FAILURE); 
} 

char const exit_order[] = "exit\0"; 
char const order[] = ">/tmp/test.txt\0"; 
// do something 
write(ios[1], order, strlen(order)); 
// bash should stop then.. 
write(ios[1], exit_order, strlen(exit_order));   
return 0; 
}
TecDroiD
  • 113
  • 1
  • 6

1 Answers1

1

I see two possible source of trouble:

1) the write part of the pipe is redirected to the child's stdout, so the new process' output is sent back to the input. I suggest to dup only the pipe's read part at the child side. If you want to intercept the child's output, you need another channel (i.e. a new pipe, or simply let both parent and child share the same stdout).

2) the strings you send seem to contain line-oriented commands. It's possible that the child process expects newlines at the end of the strings. This is a very common source of problems. I suggest to check the way the child reads its input. A "\n" at the end of the strings could help (by the way, it's not necessary to explicitly add a "\0" at the end of C strings, since the compiler do it for you. Anyway, strlen won't count the "\0").

Giuseppe Guerrini
  • 4,274
  • 17
  • 32
  • how do you mean the new pipe? – TecDroiD Nov 08 '12 at 21:13
  • If you want to capture the child's output, you must redirect it somewhere in the same way you redirect the input. A solution often used is the creation of a second pipe whose write part is connected to the child's output. Of course, the parent must read from the second pipe to receive the child's output. But it depends on your particular application. The simplest way is to do nothing at all, so the child inherits the parent's output. Anyway, in your code the redirection of child's output on the writing part of the pipe is very weird, since the process eventually sends its output to itself. – Giuseppe Guerrini Nov 08 '12 at 23:02