using exec with only some parameters of argv (using C)

Question

I need to have an input that looks like

./a.out <exe> <arg1> ... <argn> <others_stuff>

where <exe> <arg1> ... <argn> is the input that I must execute as a separate process (the objective is to save the exe's output into a txt).

Saving output into a txt file isn't a problem, I just have to redirect stdout (using dup2, freopen, or something similar).

The problem is to execute just a portion of argv! Because exec's family functions (they are so many!) let to give as input whole argv, or specifying each arg. I'm writing over here because I can't solve the problem, so I hope you're going to help me (I googled everywhere with no success).

EDIT: I forgot to say that i cannot use system for execute the command!

@unwind whynotpostthatcommentintheformofapropersentence.com? — gcbenison
– gcbenison, Commented May 28, 2012 at 13:38

Dave · Accepted Answer · 2012-05-28 14:16:32Z

4

If you want to use a contiguous portion of argv, you have two options, you can (as you have tried) create a new arg array, properly filling it as so:

  char *params[argc-2];
  memcpy(params, argv+1, sizeof params);
  params[argc-3] = NULL;

  execvp(*params, params);

You could just smash argv

  argv[argc-3] = NULL;
  execvp(argv[1], argv+1);

Or if you don't have too many args, you can use execlp:

  execlp(argv[0], argv[0], argv[3], argv[2], argv[4], NULL);

edited May 28, 2012 at 14:16

answered May 28, 2012 at 13:56

Dave

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6 Comments

Andrea Over a year ago

I used the second one, because after that i don't need argv anymore. It worked properly with "ls" command, but with "mkdir" doesn't. It says "missing argument for mkdir" or something like that (english isn't my mother tongue)

ugoren Over a year ago

Smashing argv looks simplest. "smashing" sounds bad, but doing it after fork and before execvp means it can't break any other uses of argv.

Dave Over a year ago

@Black27 use a simple "printargs" program to make sure you're using the args you want.

Andrea Over a year ago

Thank you for answering, I solved the problem! Simply I did a while iteration from 1 to argc-2 instead argc-1...What a stupid mistake!

gcbenison Over a year ago

@dave In this approach is there any requirement that params reside in the heap, or could params be a local variable?

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gcbenison · Accepted Answer · 2012-05-28 13:01:10Z

1

Since exec accepts an argv argument as a char* array terminated by a NULL pointer, you can just use the existing argv and set the member after the last one you want to pass to NULL.

This does destroy argv - if that's a problem, you can copy it first (you'll have to allocate some memory for the new copy...)

answered May 28, 2012 at 13:01

gcbenison

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