command line options for child process

[Vandana]

Hi,

I would like to know how to redirect the stdin of the parent
process to the child process.

In my program on the command line, I pass the input file. For ex:

../a.out (command line options) < in.txt

In the C Program, I create a copy of the argv, this is because the
child process needs to be run with small modifications in the command
line parameters. Now I create the copy, make the modifications, then
after the fork, for the child process I use the modified command line
parameters. Is there a cleaner way to do this. Also, how do I make the
child process read the input from in.txt?

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

char *argv_parent[];
char *argv_child[];

/*create a copy of the argv */

argv_parent = duplicate(argv);
argv_child = {"xx",yy", NULL};

process_id = fork()

if (process_id == 0) {
/* this is child process */
argv = argv_child;

} else
/* parent process */
argv = argv_parent;
return 0;
}

I run the program like this
../a.out (command line options) < in.txt

My program seems to work, but I dont know how to pass the input file
(in.txt) to the child process.
Could someone please help me.

Thanks,
Vandana

 

[Eric Sosman]

Hi,

I would like to know how to redirect the stdin of the parent
process to the child process.

In my program on the command line, I pass the input file. For ex:

./a.out (command line options)< in.txt

In the C Program, I create a copy of the argv, this is because the
child process needs to be run with small modifications in the command
line parameters. Now I create the copy, make the modifications, then
after the fork, for the child process I use the modified command line
parameters. Is there a cleaner way to do this. Also, how do I make the
child process read the input from in.txt?

Although you're writing in C, your question is not about
the C language but about the environment your program runs in.
A Unix-oriented forum like comp.unix.programmer will be able to
give you better help.

<off-topic>

1) If all the parent does is modify the command-line and
then launch the child, fork() doesn't seem necessary: Just do
the execXX() in the parent.

2) All the parent's file descriptors are inherited by the
child, including descriptor 0. The only likely problem is if
the parent has already read something from stdin, and the I/O
library has read a little extra and is holding it in a buffer:
The buffer and its contents, if any, are *not* inherited. But
if you haven't touched stdin, there should be no difficulty.

3) If I'm wrong about either of (1) and (2), check with the
experts in comp.unix.programmer to find out about it. Follow-ups
set.

</off-topic>