execve with path search?

Question

I want to execute a program from my code, and supply it with environment variables and arguments. AFAICT, execve is the right choice.

But, execve receives a path argument, not a filename, meaning it expects the first argument to be a path to the executable.

I know I can parse $PATH myself to find the path, but really, is there no other choice? Has no one else implemented it somewhere for me to use?

Answer 1

Some systems may provide execvpe(). A Google search for 'execvpe' shows a variety of options, including at least one implementation (considerably more complex than what follows, but it includes most of execvp() in its own code).

For those that do not, you can provide it for yourself:

int execvpe(const char *program, char **argv, char **envp)
{
    char **saved = environ;
    int rc;
    environ = envp;
    rc = execvp(program, argv);
    environ = saved;
    return rc;
}

You probably could survive without rc (just forcibly returning -1) since execvp() only ever returns -1 (and it only ever returns on an error).

You probably do not even have to worry about thread safety in this code. The normal scenario where it is used will be in a child process just after a fork(), and at that point, there is only one thread in the process. If you think you may use it when there are multiple threads around, then you need to think rather carefully about whether it is safe to modify the global environment even briefly. Clearly, if the execvp() succeeds, there won't be a problem (all the threads will be abruptly terminated). If the execvp() fails, then maybe one of the other threads would see the modified environment and might make bad decisions based on that. In which case, you would need to protect the environment appropriately (and that probably involves (mutual exclusion) locking in getenv(), setenv() and putenv() as well as in execvpe()).

(The implementation of execvpe() that I found avoids the thread-safety issues by implementing execvp() logic and then using execve() to execute the program.)

Normally, if execvpe() returns, the process will exit, so very often reinstating the environment is not going to affect the program. However, it is better to be safe than sorry.