__attribute__((constructor (200)) does not seem to work with Teensy 4.1

[markster]

I recently purchased a Teensy 4.1 and am porting an application that heavily leverages constructor attributes. When running the following application under LInux, this snippet:

#include <stdio.h>

int foo;

int main(int argc, char *argv[])
{
	printf("foo is %d\n", foo);
}

void __attribute__((constructor (200))) init_foo(void)
{
	foo = 5;
}

Compiles and produces the following (expected output)

[markster@portamarko ~]$ gcc -o footest footest.c
[markster@portamarko ~]$ ./footest 
foo is 5
[markster@portamarko ~]$

That is, the funciton "init_foo" is called before main.

However, running a similar snippet of code under teensyduino (or using Makefiles):

int foo;

void setup() {
  while (!Serial);
  Serial.print("foo has the value ");
  Serial.println(foo);
}

// the loop routine runs over and over again forever:
void loop() {
}

void __attribute__((constructor (200))) init_foo(void)
{
  foo = 5;
}

Produces the incorrect output:

foo has the value 0

Any thoughts on what I need to do to get the constructors to be called?

Mark

 

[luni]

Don't ask me why but it works without the priority argument. I.e.,

void __attribute__((constructor )) init_foo(void)
{
  foo = 5;
}

Are you sure that this is even supposed to work in c++ code? Seems to be relevant for C only?

 

[markster]

Don't ask me why but it works without the priority argument. I.e.,

void __attribute__((constructor )) init_foo(void)
{
  foo = 5;
}

Are you sure that this is even supposed to work in c++ code? Seems to be relevant for C only?

Luni,

Thank you so much for the prompt reply. I can confirm your results. In my case, the code I am porting over is in C, but has the same result.

Based on your idea, I copied two directory trees from arduino_build to compare the binaries being built with and without priority. Using "objdump -t" on each object file and comparing the results, I see the following:

[markster@portamarko xfs]$ diff -uNr ~/without.txt ~/with.txt 
--- /home/markster/without.txt	2021-02-03 07:24:15.363077354 -0600
+++ /home/markster/with.txt	2021-02-03 07:24:07.905046480 -0600
@@ -1,5 +1,5 @@
 
-arduino_without_priority/sketch/Blink.ino.cpp.o:     file format elf32-little
+arduino_with_priority/sketch/Blink.ino.cpp.o:     file format elf32-little
 
 SYMBOL TABLE:
 00000000 l    df *ABS*	00000000 Blink.ino.cpp
@@ -14,8 +14,8 @@
 00000000 l    d  .text.startup._Z8init_foov	00000000 .text.startup._Z8init_foov
 00000000 l       .text.startup._Z8init_foov	00000000 $t
 00000008 l       .text.startup._Z8init_foov	00000000 $d
-00000000 l    d  .init_array	00000000 .init_array
-00000000 l       .init_array	00000000 $d
+00000000 l    d  .init_array.00200	00000000 .init_array.00200
+00000000 l       .init_array.00200	00000000 $d
 00000000 l    d  .bss.foo	00000000 .bss.foo
 00000000 l       .bss.foo	00000000 $d
 00000000 l    d  .rodata.str1.4	00000000 .rodata.str1.4
[markster@portamarko xfs]$

So, importantly, we can see it's going from .init_array to .init_array.00200. With a quick scan of the linker script, it doesn't appear that there is a KEEP for things with stuff after init_array, so I added this line to the linker script:

               KEEP (*(.init_array.*))

and voila, my constructors run. However, I am not sure if this should be differently in order to ensure they are placed in the order of priority. Any thoughts?

Mark

 

[luni]

Interesting! However, linker scripts are definitely not my core competence. But I'm sure someone knowing more about this stuff will jump in to help.