How can I cast an char array to a function pointer in C?

Question

Is it possible to assign with cast to a function pointer a string or char array and then run it?

I have defined a few functions int f1();, int f2();, and so on

In the main() function I have read a string fct_name and declared a pointer to function int (*pv)();

I need to do something like this:

the fct_name can have values "f1" , "f2" and so on..

pv = (some sort of cast)fct_name;

pv();

My point is I want to avoid conditional instructions in favor of direct assignment (because I have a large number of functions in my program)

The code must obviously run.

"The code must obviously run" is the difficult part here. It might not because of no execute bits, etc. Have fun reading shell code tutorials. — znkr, Dec 14 '11 at 22:02
Oh, your question edit has just completely changed the apparent meaning of your question... — Oliver Charlesworth, Dec 14 '11 at 22:05
Why don't you step back and tell us about your problem rather than your imagined solution. Then we might be able to help. — David Heffernan, Dec 14 '11 at 22:06
I have a large number of functions and each one is related to a command which the user must type as input. I want to avoid something like 20 if statements to check which function to call. I thought it is possible to do that with function pointers in C — Alexandru N. Onea, Dec 14 '11 at 22:14
@user37468: user input is _much_ slower than 20 _if_ statements. — ninjalj, Dec 14 '11 at 22:30
@ninjalj: but most things are more flexible/clean than 20 if statements. — Dan Fego, Dec 14 '11 at 22:34
You can easily create a static table of (constant string, function pointer) pairs and use that... — Chris Dodd, Dec 14 '11 at 22:39
possible duplicate of [Call a function named in a string variable in C](http://stackoverflow.com/questions/1118705/call-a-function-named-in-a-string-variable-in-c) — Ciro Santilli OurBigBook.com, Aug 27 '15 at 14:22

Dan Fego · Answer 1 · 2011-12-14T22:33:52.153

A variant of Carey's answer, in case you're on a *nix system. dlopen() opens up your library. RTLD_LAZY tells the loader to not bother resolving all the library's symbols right away, and to wait for you to try to access them. dlsym() looks up the symbol in question.

Edit: Updated the snippet to better fit your clarification:

#include <dlfcn.h>

int main(int argc, char *argv[])
{
    void *handle = dlopen("libexample.so", RTLD_LAZY);
    if (handle == NULL) {
        // error
    }

    char fct_name[64];

    // read input from terminal here

    void *func = dlsym(handle, fct_name);

    if (func != NULL) {
        // call function here; need to cast as appropriate type
    }
}

libexample.so would be a library with your functions, compiled as a shared library, like so:

gcc -Wall -o libexample.so example.c -shared -fPIC

That being said, if you're going to the trouble of compiling a shared library like this, you'll probably just want to call the functions in your binary. You can do that if you link your library in at compile-time:

gcc -Wall -o test test.c -L. -lexample

-L. tells the linker to look for libraries in the current directory (.) and -lexample tells it to link with a library named "libexample.so". If you do this, you can just call the library functions directly within your program.

Note also -- if you link with `-rdynamic`, you can use `dlsym(RTLD_DEFAULT, name)` to find a function in your executable, without ever calling dlopen... — Chris Dodd, Dec 14 '11 at 22:35

score 1 · Accepted Answer · answered Dec 14 '11 at 22:31

Assuming you don't have an external library and are trying to call functions declared in your executable, you can do a lookup yourself

#define REGISTER_FUNC(name)  {#name, name}

struct funclist
{
    const char* name;
    void (*fp)(void);  //or some other signature
};

struct funclist AllFuncs[] = {
    REGISTER_FUNC(f1),
    REGISTER_FUNC(f2),
    REGISTER_FUNC(f3),
    {NULL,NULL}  //LAST ITEM SENTINEL
 };

Now you can lookup your variable fct_name in AllFuncs. You can use a linear search if the number is small, or insert them all into a hash table for O(1) lookup.

Alternately, if your names really are f1, f2, etc. you can just do

   void (*FuncList)(void)[]  = {NULL, f1,f2,f3};
   ...
   int idx = atol(fct_name+1);
   if (idx && idx < MAX_FUNCS)  
       FuncList[idx]();

score 0 · Answer 3 · answered Dec 14 '11 at 22:05

0

You can't cast a char array to a function just because the array happens to contain the name of a function. What you need to do is put your function(s) in a DLL and then do this:

HMODULE dll = LoadLibrary("foo.dll");
pv func = (pv)GetProcAddress(module, fct_name);

answered Dec 14 '11 at 22:05

Carey Gregory

6,836
2
26
47

Sorry, it had no Linux tag when I saw it. Concept still applies. See Dan's answer. – Carey Gregory Dec 14 '11 at 22:31

How can I cast an char array to a function pointer in C?

3 Answers3