How to call execl() in C with the proper arguments?

Question

i have vlc (program to reproduce videos) if i type in a shell:

/home/vlc "/home/my movies/the movie i want to see.mkv"

it opens up an reproduces the movie.

however, when I run the following program:

#include <unistd.h>

int main(void) {

  execl("/home/vlc", "/home/my movies/the movie i want to see.mkv",NULL);

  return 0;
}

vlc opens up but doesn't reproduce anything. How can I solve this?

Things I tried:

I guessed

execl("/home/vlc", "/home/my movies/the movie i want to see.mkv",NULL);

was equivalent to typing in the shell:

/home/vlc /home/my movies/the movie i want to see.mkv

which doesn't work, so i tried

 execl("/home/vlc", "\"/home/my movies/the movie i want to see.mkv\"",NULL);

and vlc opens up but doesn't reproduce either.

Instead of writing NULL at the end I tried 0, (char*) 0, 1 .... not helpful. Help!!!!

Denson

Why didn't you look at errno after the failed execl? It would have told you more about your problem. What has become of reading the man page for execl carefully? :-) – Rotz

Answer 1

execl("/home/vlc", 
  "/home/vlc", "/home/my movies/the movie i want to see.mkv", 
  (char*) NULL);

You need to specify all arguments, included argv[0] which isn't taken from the executable.

Also make sure the final NULL gets cast to char*.

Details are here: http://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html

Runck

Why does /home/vlc need to be specified twice? – Confederacy

first one is the full path, second is process name, generally "vlc" is enough in the second parameter – Abijah

It should be ... , (char*) NULL); – Strongarm

@alk, indeed. (But I wonder how many current implementations are there out which won't work with a naked NULL as defining NULL as (void*)0 or 0L became popular to make the absence of cast on NULL to work in practice with variadic functions; even passing a naked 0 will work on most if not all 32-bit ABI and, IIRC, some 64-bit one such as the one used by Linux). – Runck

Why do we have to cast NULL? – Eisen

In case NULL is defined as 0 and sizeof(char*) != sizeof(int) – Runck

@Runck That wouldn't conform to POSIX, which requires that NULL is 0 cast to void *. <pubs.opengroup.org/onlinepubs/9699919799/basedefs/stddef.h.html> – Blazer

Answer 2

If you need just to execute your VLC playback process and only give control back to your application process when it is done and nothing more complex, then i suppose you can use just:

system("The same thing you type into console");

Kaleidoscope

Between fixing the slight misunderstanding in the execl arguments and writing a function which will quote correctly so that the shell can undo the quote, I know what I prefer. – Runck

I think i get your point and your preferment is reasonable. But when dealing with such simple needs, is there any other benefits of using execl() instead of system(), besides avoidance of messing with escape sequence backslashes in system()? – Kaleidoscope

There could be security reasons, if you have for example a string created like, sprintf(buf, "ls -la %s", var) which you execute with system(buf), someone could give you a filename, "somefile; rm -rf ~", for example. – Ratsbane