Append items to an array with a macro, in C

Question

I have an array (C language) that should be initialized at compile time.

For example:

DECLARE_CMD(f1, arg);
DECLARE_CMD(f2, arg);
         


        

Answer 1

Yes, you can build dynamic arrays at compile time (not at runtime) (and thank's to Mitchel Humpherys), the idea is to declare your callbacks in the same section like this:

EXAMPLE:

Suppose you have three files a.c, b.c main.c and i.h

into i.h

typedef void (*my_func_cb)(void);

typedef struct func_ptr_s {
       my_func_cb cb; /* function callback */
} func_ptr_t;

#define ADD_FUNC(func_cb)                        \
    static func_ptr_t ptr_##func_cb              \
    __attribute((used, section("my_array"))) = { \
        .cb = func_cb,                           \
    }

into a.c

#include "i.h"

static void f1(void) {
   ....
}

ADD_FUNC(f1);

into b.c

#include "i.h"

static void f2(void) {
   ....
}

ADD_FUNC(f2);

into main.c

 #include "i.h"
 
 static void f3(void) {
   ....
 }

 ADD_FUNC(f3);   

 #define section_foreach_entry(section_name, type_t, elem)    \
     for (type_t *elem =                                      \
            ({                                                \
                extern type_t __start_##section_name;         \
                &__start_##section_name;                      \
            });                                               \
            elem !=                                           \
            ({                                                \
                extern type_t __stop_##section_name;          \
                &__stop_##section_name;                       \
            });                                               \
            ++elem)
            

 int main(int argc, char *argv[])
 {
    section_foreach_entry(my_array, func_ptr_t, entry) {
            entry->cb(); /* this will call f1, f2 and f3 */
    }

    return 0;
 }

IMPORTANT

sometimes the compiler optimizes start/end sections variables, it wipes them out, so when you try to use them, you will have a linker error: error LNK2019: unresolved external symbol ...

to fix this problem, i use the following:

1. Try to print your linker script:

   gcc -Wl,-verbose

copy the text between the two:

==================================================

in a file (example lnk.lds), you should see thing like:

/* Script for -z combreloc: combine and sort reloc sections */

OUTPUT_FORMAT("elf64-x86-64", "elf64-x86-64","elf64-x86-64")

........

2. ADD your section to the linker script file lnk.lds after the section .data like this (my defined section is called my_array as in the example):

  __start_my_array = .;
  .my_array :
  {
    *(.my_array)      
  }
  __stop_my_array = .;

3. Compile your program with the updated linker script like this:

   gcc -O3 -Xlinker -T"lnk.lds" file.c -o program

4. If you type strings program | grep "__start_my_array" you should find it.