Alternative way to obtain argc and argv of a process - c++

Question

I'm looking for alternative ways to obtain the command line parameters argc and argv provided to a process without having direct access to the variables passed into main().
I want to make a class that is independent of main() so that argc and argv don't have to be passed explicitly to the code that uses them.
EDIT: Some clarification seems to be in order. I have this class.
class Application
{
int const argc_;
char const** const argv_;
public:
explicit Application(int, char const*[]);
};
Application::Application(int const argc, char const* argv[]) :
argc_(argc),
argv_(argv)
{
}
But I'd like a default constructor Application::Application(), with some (most probably) C code, that pulls argc and argv from somewhere.

Answer 1

On Linux, you can get this information from the process's proc file system, namely /proc/$$/cmdline:
int pid = getpid();
char fname[PATH_MAX];
char cmdline[ARG_MAX];
snprintf(fname, sizeof fname, "/proc/%d/cmdline", pid);
FILE *fp = fopen(fname);
fgets(cmdline, sizeof cmdline, fp);
// the arguments are in cmdline

Answer 2

The arguments to main are defined by the C runtime, and the only standard/portable way to obtain the command line arguments. Don't fight the system. :)
If all you want to do is to provide access to command line parameters in other parts of the program with your own API, there are many ways to do so. Just initialise your custom class using argv/argc in main and from that point onward you can ignore them and use your own API. The singleton pattern is great for this sort of thing.
To illustrate, one of the most popular C++ frameworks, Qt uses this mechanism:
int main(int argc, char* argv[])
{
QCoreApplication app(argc, argv);
std::cout << app.arguments().at(0) << std::endl;
return app.exec();
}
The arguments are captured by the app and copied into a QStringList. See QCoreApplication::arguments() for more details.
Similarly, Cocoa on the Mac has a special function which captures the command line arguments and makes them available to the framework:
#import <Cocoa/Cocoa.h>
int main(int argc, char *argv[])
{
return NSApplicationMain(argc, (const char **)argv);
}
The arguments are then available anywhere in the app using the NSProcessInfo.arguments property.
I notice in your updated question that your class directly stores a copy of argc/argv verbatim in its instance:
int const argc_;
char const** const argv_;
While this should be safe (the lifetime of the argv pointers should be valid for the full lifetime of the process), it is not very C++-like. Consider creating a vector of strings (std::vector<std::string>) as a container and copy the strings in. Then they can even be safely mutable (if you want!).
I want to make a class that is independent of main() so that argc and argv don't have to be passed explicitly to the code that uses them.
It is not clear why passing this info from main is somehow a bad thing that is to be avoided. This is just how the major frameworks do it.
I suggest you look at using a singleton to ensure there is only one instance of your Application class. The arguments can be passed in via main but no other code need know or care that this is where they came from.
And if you really want to hide the fact that main's arguments are being passed to your Application constructor, you can hide them with a macro.

Answer 3

To answer the question in part, concerning Windows, the command line can be obtained as the return of the GetCommandLine function, which is documented here, without explicit access to the arguments of the main function.

Answer 4

I totally agree with #gavinb and others. You really should use the arguments from main and store them or pass them where you need them. That's the only portable way.
However, for educational purposes only, the following works for me with clang on OS X and gcc on Linux:
#include <stdio.h>
__attribute__((constructor)) void stuff(int argc, char **argv)
{
for (int i=0; i<argc; i++) {
printf("%s: argv[%d] = '%s'\n", __FUNCTION__, i, argv[i]);
}
}
int main(int argc, char **argv)
{
for (int i=0; i<argc; i++) {
printf("%s: argv[%d] = '%s'\n", __FUNCTION__, i, argv[i]);
}
return 0;
}
which will output:
$ gcc -std=c99 -o test test.c && ./test this will also get you the arguments
stuff: argv[0] = './test'
stuff: argv[1] = 'this'
stuff: argv[2] = 'will'
stuff: argv[3] = 'also'
stuff: argv[4] = 'get'
stuff: argv[5] = 'you'
stuff: argv[6] = 'the'
stuff: argv[7] = 'arguments'
main: argv[0] = './test'
main: argv[1] = 'this'
main: argv[2] = 'will'
main: argv[3] = 'also'
main: argv[4] = 'get'
main: argv[5] = 'you'
main: argv[6] = 'the'
main: argv[7] = 'arguments'
The reason is because the stuff function is marked as __attribute__((constructor)) which will run it when the current library is loaded by the dynamic linker. That means in the main program it will run even before main and have a similar environment. Therefore, you're able to get the arguments.
But let me repeat: This is for educational purposes only and shouldn't be used in any production code. It won't be portable and might break at any point in time without warning.

Answer 5

In Windows, if you need to get the arguments as wchar_t *, you can use CommandLineToArgvW():
int main()
{
LPWSTR *sz_arglist;
int n_args;
int result;
sz_arglist = CommandLineToArgvW(GetCommandLineW(), &n_args);
if (sz_arglist == NULL)
{
fprintf(stderr, _("CommandLineToArgvW() failed.\n"));
return 1;
}
else
{
result = wmain(n_args, sz_arglist);
}
LocalFree(sz_arglist);
return result;
}
This is very convenient when using MinGW because gcc does not recognize int _wmain(int, wchar_t *) as a valid main prototype.

Answer 6

Passing values doesn't constitute creating a dependency. Your class doesn't care about where those argc or argv values come out of - it just wants them passed. You may want to copy the values somewhere, though - there's no guarantee that they are not changed (the same applies to alternate methods like GetCommandLine).
Quite the opposite, in fact - you're creating a hidden dependency when you use something like GetCommandLine. Suddenly, instead of a simple "pass a value" semantics, you have "magically take their inputs from elsewhere" - combined with the aforementioned "the values can change at any time", this makes your code a lot more brittle, not to mention impossible to test. And parsing command line arguments is definitely one of the cases where automated testing is quite beneficial. It's a global variable vs. a method argument approach, if you will.

Answer 7

In C/C++, if main() doesn't export them, then there isn't a direct way to access them; however, that doesn't mean there isn't an indirect way. Many Posix-like systems use the elf format which passes argc, argv and envp on the stack in order to be initialized by _start() and passed into main() via normal calling convention. This is typically done in assembly (because there is still no portable way to get the stack pointer) and put in a "start file", typically with some variation of the name crt.o.
If you don't have access to main() so that you can just export the symbols, you probably aren't going to have access to _start(). So why then, do I even mention it? Because of that 3rd parameter envp. Since environ is a standard exported variable that does get set during _start() using envp. On many ELF systems, if you take the base address of environ and walk it backwards using negative array indices you can deduce the argc and argv parameters. The first one should be NULL followed by the last argv parameter until you get to the first. When the pointed to value cast to long is equal to the negative of your negative index, you have argc and the next (one more than your negative index) is argv/argv[0].

Answer 8

There are few common scenarios with functions requiring arguments of type int argc, char *argv[] known to me. One such obvious example is GLUT, where its initializing function is taking over these arguments from main(), which is kind of "nested main" scenario. This may or may not be your desired behavior. If not, as there is no convention for naming these arguments, as long as your function has its argument parser, and you know what you're doing, you can do whatever you need, hardcoded:
int foo = 1;
char * bar[1] = {" "};
or read from user input or generated otherwise, AFAIK.
int myFunc( int foo, char *bar[]){
//argument parser
{… …}
return 0;
}
Please, see this SO post.

Answer 9

The most portable way would be to use a global variable for storing the parameters. You can make this less ugly by using a Singleton (like your class in the question, but a singleton initialized by main) or similar a Service Locator which is basically just the same: Create an object in main, pass and store params statically, and have another or the same class access them.
Non-Portable ways are using GetCommandLine in Windows, accessing /proc/<pid>/cmdline or (/proc/self/cmdline), or using compiler-specific extensions like __attribute__((constructor))
Note that getting the command line in function via an equivalent of GetCommandLine is not possible (TLDR: Commandline is not passed to the Linux kernel, but already parsed and split by the invoking process (e.g. shell))

Answer 10

Here is a proper c++ way of doing so:
#include <iostream>
#include <fstream>
#include <unistd.h>
#include <sstream>
#include <vector>
using namespace std;
template <typename T>
string to_str(T value ){
ostringstream ss;
ss << value;
return ss.str();
}
int main(int argc, char** argv){
ifstream reader("/proc/" + to_str(getpid()) + "/cmdline", ios::binary);
vector<unsigned char> buffer(istreambuf_iterator<char>(reader), {});
int length = buffer.size();
for(int i = 0; i < length; i++){
if(!buffer[i]){
cout << endl;
}else{cout << buffer[i];}
}
return 0;
}

Answer 11

It sounds like what you want is a global variable; what you should do is just pass argc and argv as parameters.

Answer 12

An instructor had a challenge to use the gcc option nostartfiles and then try to access argc and argv. The nostartfiles option causes argc and argv to not be populated. This was my best solution for 64 bit Linux as it accesses argc and argv directly using the base pointer:
// Compile with gcc -nostartfiles -e main args.c -o args
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]) // argc and argv are not available when compiled with -nostartfiles
{
register void *rbp asm ("rbp");
printf("argc is %ld\n", *(unsigned long *)(rbp + 8));
for(int count = 0 ; count < *(unsigned long *)(rbp + 8) ; count++)
{
printf("argv[%d] is %s\n", count, *(char **)(rbp + 16 + count * 8));
}
exit(0);
}