Can't pin down "implicit declaration" problem

[phillipjohnston]

Working on switching over our libc implementation to use your improved fork, and I'm banging my head against the wall with a problem I don't understand (perhaps because it's the end of the day).

I'm compiling the files with -DPRINTF_ALIAS_STANDARD_FUNCTION_NAMES=1 with clang, but I get the warnings below. GCC also has this problem, although it unhelpfully tells me to include stdio.h, which I am already doing.

I have verified that the header is included, and I'm actually in that logic block that defines printf_ to printf. But the only way I can get these warnings to go away is to actually define int printf(const char* format, ...) ATTR_PRINTF(1, 2);, not printf_.

The other oddity I note is that this only happens for printf calls with only a format string.

printf("Overflow case run.\n");

This one in the same file does not generate the warning:

printf("Running stack overflow test program. Canary value: 0x%p\n", (void*)__stack_chk_guard);

Any thoughts on what I'm missing here?

Clang

[9/12] Compiling C object test/stackprotect_test.p/app_stackcheck_main.c.o
../test/app/stackcheck_main.c:21:2: warning: implicit declaration of function 'printf' is invalid in C99 [-Wimplicit-function-declaration]
        printf("Overflow case run.\n");
        ^
../test/app/stackcheck_main.c:27:2: warning: implicit declaration of function 'printf' is invalid in C99 [-Wimplicit-function-declaration]
        printf("Running stack overflow test program. Canary value: 0x%p\n", __stack_chk_guard);
        ^
2 warnings generated.
[10/12] Compiling C object test/stackp...test.p/.._src_crt_stack_protection.c.o
../src/crt/stack_protection.c:45:2: warning: implicit declaration of function 'printf' is invalid in C99 [-Wimplicit-function-declaration]
        printf("Stack overflow detected! Aborting program.\n");
        ^
1 warning generated.

GCC

[1/2] Compiling C object test/stackprotect_test.p/app_stackcheck_main.c.o
../test/app/stackcheck_main.c: In function 'stack_overflows_here':
../test/app/stackcheck_main.c:21:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   21 |  printf("Overflow case run.\n");
      |  ^~~~~~
../test/app/stackcheck_main.c:4:1: note: 'printf' is defined in header '<stdio.h>'; did you forget to '#include <stdio.h>'?
    3 | #include <stdio.h>
  +++ |+#include <stdio.h>
    4 |

[phillipjohnston]

This also appears to be blocking me on the Arduino library integration. If I invoke printf with an argument:

printf("I'm alive! %d\n", counter++);

I get expected serial output:

15:58:35.970 -> I'm alive! 43
15:58:36.963 -> I'm alive! 44
15:58:37.947 -> I'm alive! 45

However, using only text actually results in no output being generated on the device, although I don't see a compiler warning in the Arduino IDE. But I can confirm the sketch is running with an LED blink. I suspect that call is being optimized out even though I don't see a warning.

printf("I'm alive!\n");

[phillipjohnston]

BAH. I found the problem: my build system design caused the command-line arg to be missed on some files. That has been corrected. Now to figure out the Arduino issue - but closing this.