Cast string to function pointer

Yes, this is my "plan B" solution :-)
Isn't there any possibility to perform some kind of direct cast ?

Anyway,
thanks for your suggestion....

Hi darkriser,

you could try to export the functions from you module and use 'GetProcAddress' to get a pointer to the function directly by the name - but this I guess wouldn't be as efficient as holding a map as Infinity08 suggested.

ZOPPO

Example (in C) :

#include <stdio.h>
#include <string.h>
 
typedef struct FunMapNode {
  char name[32];
  void (*funptr)(int);
} FunMapNode;
 
void fun1(int i) {
  printf("fun1(%d) called ...\n", i);
}
 
void fun2(int i) {
  printf("fun2(%d) called ...\n", i);
}
 
FunMapNode funMap[] = {
  { "fun1", fun1 },
  { "fun2", fun2 },
  { "", 0 }
};
 
int main(void) {
  int value = 5;
  char *name = "fun2";
  
  int i = 0;
  while (funMap[i].funptr) {
    if (!strcmp(funMap[i].name, name)) break;
    ++i;
  }
  funMap[i].funptr(value);
 
  return 0;
}
                                              

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>> Isn't there any possibility to perform some kind of direct cast ?

There's no relation between the function name and the function pointer in the binary, unless you explicitly add that relation (by using a map for example).

If, however, you know the strings at compile time, you can just avoid having to look it up. But I assume you'll only know the strings at run time ?

Zoppo,
my primary goal is performance. Therefore I need as optimum solution as possible...
Yes, if there's no possibility to cast function's name to a pointer directly, I'll have to use mapping.

Infinity08,
thanks for your code. Just want to ask - direct casting to a pointer is impossible in C/C++, right?

Infinity08,
yes...at run time, "unfortunately" :-)

I probably should have made the code a bit safer by using :

        if (funMap[i].funptr) {
            funMap[i].funptr(value);
        }
        else {
            printf("unknown function ...\n");
        }

instead of :

        funMap[i].funptr(value);

>> direct casting to a pointer is impossible in C/C++, right?

What do you mean ? Cast a string to a pointer ? A char* is already a pointer, but it won't be the function address - it will be the address of the first character in the string.


>> my primary goal is performance. Therefore I need as optimum solution as possible...

You might want to consider not using strings as function identifiers, but maybe integer values ... Or is that an externally imposed limitation ?

Infinity08: Absolutely no limitations at this stage. What's the difference in using integers as input values?

>> What's the difference in using integers as input values?

A string comparison is more costly than an integer comparison ;) So, you can save a few cycles there ...

>>>> my primary goal is performance.

The time for parsing an algorithm and check it's correctness normally is a multiple of that one of the function calls will need. That includes the lookup in a map as well.

There is an issue when using function pointers for different logical tasks: these functions should (must) have all the same prototype, i. e. the same return type and arguments.

itsmeandnobodyelse: yes, all functions have the same prototype, of course....

Good. I only thought on math functions like exp, power, sin, log where some of them have two arguments.

>> >> What's the difference in using integers as input values?
>>
>> A string comparison is more costly than an integer comparison ;) So, you can save a few cycles there ...

Plus you can of course use the integer directly as an index in an array of function pointers, which avoids having to search for the correct value.

>>>> use the integer directly as an index in an array of function pointers

  enum { F_UNK, F01, F02, F03, F04, F05, F06, F07, F08, F_MAX };

  typedef double (*MyFunc)(double);

  const MyFunc allFuncs[F_MAX] = { NULL, &f01, &f02, &f03, ..., &f08 };

  ...

  fn = atoi(&line_Func[1]);
  if (fn > 0 && fn < F_MAX)
         (arrGide + i)->eleFunc = allFuncs[fn];

I was also looking for a way to do this, and this is how you do it without mapping on Windows. It is just like Zoppo suggested.

I got curious because gtkbuilder does just this, and I was wondering how.

#include <stdio.h>
#include <windows.h>
 
__declspec(dllexport) void foo() {
 
  printf("this is foo\n");
 
}
 
__declspec(dllexport) void bar() {
 
  printf("this is bar\n");
 
}
 
int main(int argc, char **argv) {
 
  char text[256];
  int i;
  HINSTANCE instance;
  FARPROC fun_pointer;
 
  printf("Type the name of the function: ");
  fgets(text,256,stdin);
 
  for ( i = 0 ; text[i] ; i++ ) {
    if ( text[i] == '\r' || text[i] == '\n' ) {
      text[i] = 0;
      break;
    }
  }
 
  if ( ! ( instance = LoadLibrary(argv[0]) ) ) {
    printf("Can't load executable\n");
    exit(0);
  }
 
  if ( ! ( fun_pointer = GetProcAddress(instance,text) ) ) {
    printf("Can't load %s\n",text);
    exit(0);
  }
 
  fun_pointer();
 
  return 0;
 
}
                                              

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