在我的C ++程序中代替对“系统”的调用

Question

I'm trying to find find a substitute for a call to "system" (from stdlib.h) in my C++ program. So far I've been using it to call g++ in my program to compile and then link a variable number of source files in a directory chosen by the user.

Here I've got an example how the command could approximately look like: "C:/mingw32/bin/g++.exe -L"C:\\mingw32\\lib" [...]"

However, I have the problem that (at least with the MinGW compiler I'm using) I get the error "Command line is too long" when the command string gets too long. In my case it was about 12000 characters long. So I probably need another way to call g++. Additionally, I've read that you generally shouldn't use "system" anyway : http://www.cplusplus.com/forum/articles/11153/

So I'm in need for some substitute (that should also be as platform independent as possible, because I want the program to run on Windows and Linux). I've found one candidates that would generally look quite well suited:

• _execv / execv :

  Platform independent, but:

  a) http://linux.die.net/man/3/exec says "The exec() family of functions replaces the current process image with a new process image". So do I need to call "fork" first so that the C++ program isn't terminated? Is fork also available on Windows/MSVC ?

  b) Using "system", I've tested whether the return value was 0 to see if the source file could be compiled. How would this work with exec? If I understand the manpage correctly, will it only return the success of creating the new process and not the status of g++? And with which function could I suspend my program to wait for g++ to finish and get the return value?

All in all, I'm not quite sure how I should handle this. What are your suggestions? How do multiplatform programs like Java (Runtime.getRuntime().exec(command)) or the Eclipse C++ IDE internally solve this? What would you suggest me to do to call g++ in an system independent way - with as many arguments as I want?

EDIT: Now I'm using the following code - I've only tested it on Windows yet, but at least there it seems to work as expected. Thanks for your idea, jxh! Maybe I'll look into shortening the commands by using relative paths in the future. Then I would have to find a platform independent way of changing the working directory of the new process.

#ifdef WIN32
int success = spawnv(P_WAIT, sCompiler.c_str(), argv);
#else
pid_t pid;
switch (pid = fork()) {
case -1:
    cerr << "Error using fork()" << endl;
    return -1;
    break;
case 0:
    execv(sCompiler.c_str(), argv);
    break;
default:
    int status;
    if (wait(&status) != pid) {
        cerr << "Error using wait()" << endl;
        return -1;
    }
    int success = WEXITSTATUS(status);
}
#endif

Answer 1

You might get some traction with some of these command line options if all your files are in (or could be moved to) one (or a small number) of directories. Given your sample path to audio.o, this would reduce your command line by about 90%.

-Ldir
Add directory dir to the list of directories to be searched for `-l'.

From: https://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_3.html#SEC17

-llibrary
Search the library named library when linking.

It makes a difference where in the command you write this option; the linker searches processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o' searches library z' after file foo.o' but before bar.o'. If bar.o' refers to functions in z', those functions may not be loaded.

The linker searches a standard list of directories for the library, which is actually a file named `liblibrary.a'. The linker then uses this file as if it had been specified precisely by name.

The directories searched include several standard system directories plus any that you specify with `-L'.

Normally the files found this way are library files--archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fashion. The only difference between using an -l' option and specifying a file name is that -l' surrounds library with lib' and .a' and searches several directories.

From: http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_3.html

Answer 2

Here's another option, perhaps closer to what you need. Try changing the directory before calling system(). For example, here's what happens in Ruby...I'm guessing it would act the same in C++.

> system('pwd')
/Users/dhempy/cm/acts_rateable
=> true
> Dir.chdir('..')
=> 0
> system('pwd')
/Users/dhempy/cm
=> true

Answer 3

If none of the other answers pan out, here's another. You could set an environment variable to be the path to the directory, then use that variable before each file that you link in.

I don't like this approach much, as you have to tinker with the environment, and I don't know if that would actually affect the command line limit. It may be that the limit applies after interpolating the command. But, something to thing about, regardless.