c++ - Is args[0] guaranteed to be the path of execution?

Question

This is a fundamental question, but an important one none the less...

When starting a C++ program whose main method has the following common signature:

int main(int argc, char* args[]) {
    //Magic!
    return 0;
}

is args[0] always guaranteed to be the path to the currently running program? What about cross platform (since I am in a Linux environment but may port later on.)?

Answer 1

It is not always. It's the value that you gave the program by the Operation System. For example when starting a program using exec you can set that to an arbitrary value:

int execve(const char *filename, char *const argv[],
           char *const envp[]);

The first parameter is the file to start, and argv will contains argv[0] and all other parameters for main. envp contains the environment variables (not defined by Standard C or C++. This is a posix thing).

More precisely, this is the definition of argv in C++:

An implementation shall not predefine the main function. This function shall not be overloaded. It shall have a return type of type int, but otherwise its type is implementation-defined. All implementations shall allow both of the following definitions of main:

int main() { /* ... */ }

and

int main(int argc, char* argv[]) { /* ... */ }

In the latter form argc shall be the number of arguments passed to the program from the environment in which the program is run. If argc is nonzero these arguments shall be supplied in argv[0] through argv[argc-1] as pointers to the initial characters of null-terminated multibyte strings (NTMBSs) (17.3.2.1.3.2) and argv[0] shall be the pointer to the initial character of a NTMBS that represents the name used to invoke the program or "". The value of argc shall be nonnegative. The value of argv[argc] shall be 0. [Note: it is recommended that any further (optional) parameters be added after argv. ]

It's pretty much up to the implementation what defines a "name used to invoke the program". If you want to get the full path of your executable, you can use GetModuleFileName on Windows, and argv[0] (for getting the name used to execute, may be relative) together with getcwd (for getting the current working directory, trying to make the name absolute).