Pass a string containing a function name and its arguments to another function.

Question

I would like to pass a string to a function in C in which the function will parse out a function name, the arguments for the function, and its datatypes, and then call the function for me. What is the best approach?

Answer 1

If you want to write a function with a format string and variable arguments like:

int function(const char* strFormat, ... )
{
  //parse out the format using regex or something
  //then store the data into the variable aruments
  //or create a string concatenating everything
}

like, say printf, sprintf, or scanf does,

then the best thing for you to do is look at some good tutorials.

http://msdn.microsoft.com/en-us/library/fxhdxye9(v=vs.80).aspx

http://www.cprogramming.com/tutorial/c/lesson17.html

If you are wanting to actually pass a function name for the function to call, along with its arguments, you either need to implement some form of reflection or introspection in your c code, a really complex switch statement which calls the functions for you based upon the string value, or write some complex macros to act as a sort of a secondary compiler.

glib's gobject is an excellent example of introspection in c. http://developer.gnome.org/gobject/stable/

something simple without introspection may be:

void* function (const char* strFunctionName, ... )
{
  if(!strcmp(strFunctionName, "functionA"))
  {
    //use va_list to parse out the arguments for the function.
    functionA(//each of the arguments from va_list);
  }
  else if(!strcmp(strFunctionName, "functionB"))
  {
    //use va_list to parse out the arguments for the function.
    functionB(//args from va_list);
  }
  ...
}

If you have something more specific in mind, please specify in your question.

Answer 2

I'm not sure if this is standard, but you could use __PRETTY_FUNCTION__ in C++.

e.g:

void foo(int a, int b)
{
    cout << __PRETTY_FUNCTION__ << endl;
}

outputs:

void foo(int, int)

Answer 3

If you want to make:

char *funct = "foo(bar)";

to actually call the function foo(), then its impossible, C just doesn't work that way. I would recommend that:

a) you sit down and rethink your application, because such behaviour shouldn't be needed for anything.

b) try a language with such capabilities, such as objective-c

c) if you really, really, really wanna do it... then make a string parser and lots of if's, or function pointers. Something in the lines of:

1. Get string in the form of "function(var1, var2, ..., varN)"
2. Get the name of the function (ie. everything before '(' )
3. Get all the parameters (strtok() for example)
4. Identify the function comparing constant names with your string ( strcmp() )
5. If the parameters are numeric ( isdigit() or sscanf( string, "%d", &parameter ) > 0 ) convert them to a usable primitive (atoi(), atof(), etc or sscanf())
6. Pass the params to your function.

Answer 4

If you want to execute a code from a string (I'm just guessing, your question is hard to understand), then there are many options available: from embedding the whole Clang and LLVM into your code (and making sure its JIT is aware of all your declarations you're going to export) to embedding a simple scripting language (like Lua or Guile) and providing wrappers for all the functions you want to call this way.