Get a pointer to the current function in C (gcc)?

Question

is there a magic variable in gcc holding a pointer to the current function ?

I would like to have a kind of table containing for each function pointer a set of information.

I know there's a __func__ variable containing the name of the current function as a string but not as a function pointer.

This is not to call the function then but just to be used as an index.

EDIT Basically what i would like to do is being able to run nested functions just before the execution of the current function (and also capturing the return to perform some things.) Basically, this is like __cyg_profile_func_enter and __cyg_profile_func_exit (the instrumentation functions)... But the problem is that these instrumentation functions are global and not function-dedicated.

EDIT In the linux kernel, you can use unsigned long kallsyms_lookup_name(const char *name) from include/linux/kallsyms.h ... Note that the CONFIG_KALLSYMS option must be activated.

Answer 1

void f() {
   void (*fpointer)() = &f;
}

Answer 2

Here's a trick that gets the address of the caller, it can probably be cleaned up a bit. Relies on a GCC extension for getting a label's value.

#include <stdio.h>

#define MKLABEL2(x) label ## x
#define MKLABEL(x) MKLABEL2(x)
#define CALLFOO do { MKLABEL(__LINE__): foo(&&MKLABEL(__LINE__));} while(0)

void foo(void *addr)
{
    printf("Caller address %p\n", addr);
}

int main(int argc, char **argv)
{
    CALLFOO;
    return 0;
}

Answer 3

#define FUNC_ADDR (dlsym(dlopen(NULL, RTLD_NOW), __func__))

And compile your program like

gcc -rdynamic -o foo foo.c -ldl

Answer 4

I also had the problem that I needed the current function's address when I created a macro template coroutine abstraction that people can use like modern coroutine language features (await and async). It compensates for a missing RTOS when there is a central loop which schedules different asynchronous functions as (cooperative) tasks. Turning interrupt handlers into asynchronous functions even causes race conditions like in a preemptive multi-tasking system.

I noticed that I need to know the caller function's address for the final return address of a coroutine (which is not return address of the initial call of course). Only asynchronous functions need to know their own address so that they can pass it as hidden first argument in an AWAIT() macro. Since instrumenting the code with a macro solution is as simple as just defining the function it suffices to have an async-keyword-like macro.

This is a solution with GCC extensions:

#define _VARGS(...) _VARGS0(__VA_ARGS__)
#define _VARGS0(...) ,##__VA_ARGS__

typedef union async_arg async_arg_t;
union async_arg {
    void (*caller)(void*);
    void *retval;
};
#define ASYNC(FUNCNAME, FUNCARGS, ...)            \
void FUNCNAME (async_arg_t _arg _VARGS FUNCARGS)  \
GENERATOR(                                        \
    void (*const THIS)(void*) = (void*) &FUNCNAME;\
    static void (*CALLER)(void*),                 \
    CALLER = _arg.caller;                         \
    __VA_ARGS__                                   \
)

#define GENERATOR(INIT,...) {                     \
    __label__ _entry, _start, _end;               \
    static void *_state = (void*)0;               \
    INIT;                                         \
_entry:;                                          \
    if (_state - &&_start <= &&_end - &&_start)   \
        goto *_state;                             \
    _state = &&_start;                            \
_start:;                                          \
    __VA_ARGS__;                                  \
_end: _state = &&_entry;                          \
}

#define AWAIT(FUNCNAME,...) ({          \
    __label__ _next;                    \
    _state = &&_next;                   \
    return FUNCNAME((async_arg_t)THIS,##__VA_ARGS__);\
    _next: _arg.retval;                 \
})

#define _I(...) __VA_ARGS__
#define IF(COND,THEN) _IF(_I(COND),_I(THEN))
#define _IF(COND,THEN) _IF0(_VARGS(COND),_I(THEN))
#define _IF0(A,B) _IF1(A,_I(B),)
#define _IF1(A,B,C,...) C

#define IFNOT(COND,ELSE) _IFNOT(_I(COND),_I(ELSE))
#define _IFNOT(COND,ELSE) _IFNOT0(_VARGS(COND),_I(ELSE))
#define _IFNOT0(A,B) _IFNOT1(A,,_I(B))
#define _IFNOT1(A,B,C,...) C

#define IF_ELSE(COND,THEN,ELSE) IF(_I(COND),_I(THEN))IFNOT(_I(COND),_I(ELSE))

#define WAIT(...) ({                 \
    __label__ _next;                 \
    _state = &&_next;                \
    IF_ELSE(_I(__VA_ARGS__),         \
        static __typeof__(__VA_ARGS__) _value;\
        _value = (__VA_ARGS__);      \
        return;                      \
        _next: _value;               \
    , return; _next:;)               \
})

#define YIELD(...) do {              \
    __label__ _next;                 \
    _state = &&_next;                \
    return IF(_I(__VA_ARGS__),(__VA_ARGS__));\
_next:;                              \
} while(0)

#define RETURN(VALUE) do {   \
    _state = &&_entry;       \
    if (CALLER != 0)         \
        CALLER((void*)(VALUE +0));\
    return;                  \
} while(0)

#define ASYNCALL(FUNC, ...) FUNC ((void*)0,__VA_ARGS__)

I know, a more portable (and maybe secure) solution would use the switch-case statement instead of label addresses but I think, gotos are more efficient than switch-case-statements. It also has the advantage that you can use the macros within any other control structures easily and break will have no unexpected effects.

You can use it like this:

#include <stdint.h>
int spi_start_transfer(uint16_t, void *, uint16_t, void(*)());
#define SPI_ADDR_PRESSURE 0x24

ASYNC(spi_read_pressure, (void* dest, uint16_t num),
    void (*callback)(void) = (void*)THIS;    //see here! THIS == &spi_read_pressure
    int status = WAIT(spi_start_transfer(SPI_ADDR_PRESSURE,dest,num,callback));
    RETURN(status);
)

int my_gen() GENERATOR(static int i,
    while(1) {
        for(i=0; i<5; i++)
            YIELD(i);
    }
)

extern volatile int a;
ASYNC(task_read, (uint16_t threshold),
    while(1) {
        static uint16_t pressure;
        int status = (int)AWAIT(spi_read_pressure, &pressure, sizeof pressure);
        if (pressure > threshold) {
            a = my_gen();
        }
    }
)

You must use AWAIT to call asynchronous functions for return value and ASYNCALL without return value. AWAIT can only be called by ASYNC-functions. You can use WAIT with or without value. WAIT results in the expression which was given as argument, which is returned AFTER the function is resumed. WAIT can be used in ASYNC-functions only. Keeping the argument with WAIT wastes one new piece of static memory for each WAIT() call with argument though so it is recommended to use WAIT() without argument. It could be improved, if all WAIT calls would use the same single static variable for the entire function.

It is only a very simple version of a coroutine abstraction. This implementation cannot have nested or intertwinned calls of the same function because all static variables comprise one static stack frame.

If you want to solve this problem, you also need to distinguish resuming an old and starting a new function call. You can add details like a stack-frame queue at the function start in the ASYNC macro. Create a custom struct for each function's stack frame (which also can be done within the macro and an additional macro argument). This custom stack frame type is loaded from a queue when entering the macro, is stored back when exiting it or is removed when the call finishes. You could use a stack frame index as alternative argument in the async_arg_t union. When the argument is an address, it starts a new call or when given a stack frame index it resumes an old call. The stack frame index or continuation must be passed as user-defined argument to the callback that resumes the coroutine.

Answer 5

I think you could build your table using strings (the function names) as keys, then look up by comparing with the __func__ builtin variable.

To enforce having a valid function name, you could use a macro that gets the function pointer, does some dummy operation with it (e.g. assigning it to a compatible function type temporary variable) to check that it's indeed a valid function identifier, and then stringifies (with #) the function name before being used as a key.

UPDATE:

What I mean is something like:

typedef struct {
  char[MAX_FUNC_NAME_LENGTH] func_name;
  //rest of the info here
} func_info;

func_info table[N_FUNCS];

#define CHECK_AND_GET_FUNC_NAME(f) ({void (*tmp)(int); tmp = f; #f})

void fill_it()
{
  int i = -1;
  strcpy(table[++i].func_name, CHECK_AND_GET_FUNC_NAME(foo));
  strcpy(table[++i].func_name, CHECK_AND_GET_FUNC_NAME(bar));
  //fill the rest
}

void lookup(char *name) {
  int i = -1;
  while(strcmp(name, table[++i]));
  //now i points to your entry, do whatever you need
}

void foo(int arg) {
  lookup(__func__);
  //do something
}

void bar(int arg) {
  lookup(__func__);
  //do something
}

(the code might need some fixes, I haven't tried to compile it, it's just to illustrate the idea)

Answer 6

If you went for C++ the following information might help you:

Objects are typed, functors are functions wrapped as objects, RTTI allows the identification of type at runtime.

Functors carry a runtime overhead with them, and if this is a problem for you I would suggest hard-coding the knowledge using code-generation or leveraging a OO-heirarchy of functors.

Answer 7

static const char * const cookie = __FUNCTION__;

__FUNCTION__ will be stored at the text segment at your binary and a pointer will always be unique and valid.

Answer 8

No, the function is not aware of itself. You will have to build the table you are talking about yourself, and then if you want a function to be aware of itself you will have to pass the index into the global table (or the pointer of the function) as a parameter.

Note: if you want to do this you should have a consistent naming scheme of the parameter.

Answer 9

If you want to do this in a 'generic' way, then you should use the facilities you already mention (__cyg_profile_func*) since that is what they are designed for. Anything else will have to be as ad hoc as your profile.

Honestly, doing things the generic way (with a filter) is probably less error prone than any new method that you will insert on-the-fly.

Answer 10

You can capture this information with setjmp(). Since it saves enough information to return to your current function, it must include that information in the provided jmp_buf.

This structure is highly nonportable, but you mention GCC explicitly so that's probably not a blocking issue. See this GCC/x86 example to get an idea how it roughly works.

Answer 11

If you want to do code generation I would recomend GSLGen from Imatix. It uses XML to structure a model of your code and then a simple PHP like top-down generation language to spit out the code -- it has been used to generate C code.

I have personally been toying arround with lua to generate code.

Answer 12

If all you need is a unique identifier for each function, then at the start of every function, put this:

static const void * const cookie = &cookie;

The value of cookie is then guaranteed to be a value uniquely identifying that function.

Answer 13

Another option, if portability is not an issue, would be to tweak the GCC source-code... any volunteers?!