[LCP]system funtion or fork or exec

[Gonzalo Sanchez]

thanks for your help mehul.  I found out that I could use ssh without password.  However I am gonna try the first option that you mention..just for fun..and see.

thanks again

-----Original Message-----
From: mehul radheshyam choube [mailto:mrc_cprog at rediffmail.com]
Sent: Friday, February 14, 2003 2:25 AM
To: linuxcprogramming at lists.linux.org.au
Cc: Gsanchez at roinenetwork.com
Subject: Re: [LCP]system funtion or fork or exec


hi sanchez,
         i m sorry i m late to answer maybe by this time u must 
have solved ur
problem. now ur case is a classic example for Interprocess 
Communication(IPC).
i had a similar problem a few months back and got it solved. i 
will explain u
using the problem i had. i wanted to do the following :-

         I wanted to write wrappers to an existing command-line 
based cvs
commands The existing cvs login cmd goes like this
$ cvs login
(Logging in to mehul at 192.168.0.25)
CVS password: (enter password here)
$ messages..... eg login sucessful or not.

         Basically my program needs to issue the cvs login 
command, wait for the
password prompt, sends the password and wait for the login 
messages.

         now this can be done in 3 ways. choose one which is 
suitable for u.

1. using popen().

    #include <stdio.h>

    FILE *popen(const char *command, const char *type);

   eg.
         FILE *fpCmd = popen("cvs login", "w");
         if(!fpCmd) {
                 fwrite( password, 1, strlen(password), fpCmd);
         }/* if(!fpCmd) */

2. using fork(), execvp(), pipe() & select() combination.

    1)  Create a pair of pipes.  One will be for the parent app to 
send
        instructions to the child, the other will be for the child 
to respond to
        the parent.
    2)  Fork the app
    3a) In the child, set the parent->child pipe to be stdin, and 
the
        child->parent to be stdout
    3b) In the parent, send any data you need to through the 
parent->child pipe.
    4a) Have the child execute the command with exec.  The output 
will be sent
        on stdout which is now a pipe going to the parent.
    4b) Have the parent wait with select until there is data on 
the
        child->parent pipe.  Read the data in, and parse the 
output message.
        Take the appropriate actions.
    5b) If there are more than 1 output statements that need 
parsing, goto 4b.i


3. using pseudo-terminal concept. i went for this option. the 
reason is some
    utilities use terminals to when they interact. cvs uses 
terminals so above
    two options failed. using pseudo-terminal programming i was 
able to overcome
    the problem. i have attached a tarred file 
pseudo-terminals.tgz it contains
    info related to pseudo-terminals. if u want more info u can 
always go for
    search on google.com.

    eg.
         i have attached a tarred file pseudo_code.tgz t contains 
code that i use to
         execute cvs cmds using pseudo-terminals.
         u can modify execute_cvs_cmd.cpp into C code.

         i will be glad if above information is useful to u. this 
is best to my
knowledge. i m sorry if it has any short commings. i think this 
info can at
least give u a start so that u can go ahead.

mehul.