Question

I would like to make a system call to a Linux program from my C++ code, but I would like to check whether the program is installed on the user’s machine first.

In Ubuntu, I can determine whether a package associated with that program was installed using a system call like dpkg -s gifsicle and parse its output. gifsicle here is the program name.

However, it’s possible that the program (e.g. gifsicle) was compiled from source and thus does not appear in the Ubuntu package repository.

What’s a good programmatic way of determining whether a program (e.g. gifsicle) is available on the system executing the C++ code?

Answer 1

There is no standard package manager for Linux, so dpkg is definitely the wrong answer.

For security and correctness reasons, it is probably unwise to rely on the user’s PATH to locate an executable. So you probably should already be using the fully-qualified path (e.g. /usr/bin/gifsicle) in your call to system.

If so, the easy answer to your question is:

if (access("/usr/bin/gifsicle", X_OK) == 0) {
    system("/usr/bin/gifsicle -my -args");
}
else if (errno == EACCESS) {
    /* gifsicle not found */
}
else {
    /* access() failed!  Operating system is broken or Windows (or both) */
}

(Bonus points if you put /usr/bin/gifsicle into a variable)

The harder — but arguably “more correct” — answer is to avoid system and do fork + execl yourself, checking the execl to see if it results in ENOENT or similar. Communicating the failure back to the parent process could be annoying, though.