Using execvp with Input,Output and redirection

Question

In a C program, let's say i wann use Exec functions for executing a given program, for example if i wanna just try ls -l i'll do something like

args[0]="ls";
args[1]="-l";
args[2]=NULL;

...
execvp("ls", args);

and it's all fine. Now what if i wanna also add the redirection to a file (or to stderr)? I'm stuck, it's obvious that adding >log.txt as a 3rd entry in the array won't work, but I don't know how to proceed.

And also, what if I wanna pass some Input parameters? What if i wanna execute a GCC command like "gcc -o out in redirection>log.txt" ?

[update from comment:]

It's a C program that simulate a shell which can "run strings", string that contains a command, a list o parameters, input and a redirection.

"what if I wanna pass some Input parameters?" - isn't the args array there for exactly that purpose? (if you want to pipe text to and from stdin/stdout, you'll need popen().) — The Paramagnetic Croissant
– The Paramagnetic Croissant, Commented Apr 27, 2014 at 10:38
@OliCharlesworth Is there a guarantee not only the stream but also the file-descriptor is the same after freopen? — Deduplicator
– Deduplicator, Commented Apr 27, 2014 at 10:50

Deduplicator · Accepted Answer · 2014-04-27 10:46:14Z

1

Just set up your file descriptors as the exec-d process shall find them and then do the exec.
For that you need open, dup2 and close.

All functions in the exec-family just replace the current process with whatever one you say.

answered Apr 27, 2014 at 10:46

Deduplicator

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2 Comments

ro.nin Over a year ago

thank you for the answer, i think i'm gettin the output part. This also applies to the input thing? Changing file descriptor of stdIn to the file i want to be the input.c of (for instance) gcc?

Deduplicator Over a year ago

Yes, just set up whatever environment you want the new executable to inherit.

alk · Accepted Answer · 2014-04-27 10:50:52Z

0

Run the command in a shell:

char * args[] = {
  "sh",
  "-c",
  "ls -l >out.ls 2>err.ls <in.ls",
  NULL
};

...

execvp(args[0], args); 
perror("execvp() failed");

edited Apr 27, 2014 at 10:50

answered Apr 27, 2014 at 10:43

alk

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2 Comments

ro.nin Over a year ago

Sorry to ask but i'm newbie, i didn't hinted in the OP that all this has to be written in a C program, do this solution works for what i'm doing? I't a C program that simulate a shell which can "run strings", string that contains a command, a list o parameters, input and a redirection. Suppose i have to study more for the SH thing, by now i never seen sh -c ...stuff

alk Over a year ago

@ro.nin: For such requirements an approach as described by Deduplicator's answer (stackoverflow.com/a/23322407/694576) suit better.

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