[Bug c/108671] New: spurious "defined but not used" warning with static call back function

[galt at soe dot ucsc.edu via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108671

            Bug ID: 108671
           Summary: spurious "defined but not used" warning with static
                    call back function
           Product: gcc
           Version: 9.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: galt at soe dot ucsc.edu
  Target Milestone: ---

Getting an invalid "defined but not used" warning with 
static call back function used in openssl. Other examples online show people
using static functions defined in the same file, so that is normal usage.

cc -O -g -std=c99 -Wall -Wformat -Wimplicit -Wreturn-type -Wuninitialized
-D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_GNU_SOURCE -DMACHTYPE_x86_64
-DUSE_HIC   -Wall -Wformat -Wimplicit -Wreturn-type -Wuninitialized -I../inc
-I../../inc -I../../../inc -I../../../../inc -I../../../../../inc -I../htslib 
-I/include -I/usr/include/libpng16  -o https.o -c https.c

https.c:46:13: warning: ‘openssl_locking_callback’ defined but not used
[-Wunused-function]
   46 | static void openssl_locking_callback(int mode, int n, const char *
file, int line)
      |             ^~~~~~~~~~~~~~~~~~~~~~~~

https.c:41:22: warning: ‘openssl_id_callback’ defined but not used
[-Wunused-function]
   41 | static unsigned long openssl_id_callback(void)
      |                      ^~~~~~~~~~~~~~~~~~~

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

#include "openssl/ssl.h"
#include "openssl/err.h"

#include <sys/socket.h>
#include <unistd.h>
#include <pthread.h>
#include <signal.h>

static unsigned long openssl_id_callback(void)
{
return ((unsigned long)pthread_self());
}

static void openssl_locking_callback(int mode, int n, const char * file, int
line)
{
if (mode & CRYPTO_LOCK)
    pthread_mutex_lock(&mutexes[n]);
else
    pthread_mutex_unlock(&mutexes[n]);
}

void openssl_pthread_setup(void)
{
int i;
int numLocks = CRYPTO_num_locks();
AllocArray(mutexes, numLocks);
for (i = 0;  i < numLocks;  i++)
    pthread_mutex_init(&mutexes[i], NULL);
CRYPTO_set_id_callback(openssl_id_callback);
CRYPTO_set_locking_callback(openssl_locking_callback);
}

void openSslInit()
/* do only once */
{
static boolean done = FALSE;
static pthread_mutex_t osiMutex = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_lock( &osiMutex );
if (!done)
    {
    SSL_library_init();
    ERR_load_crypto_strings();
    ERR_load_SSL_strings();
    OpenSSL_add_all_algorithms();

    openssl_pthread_setup();

    myDataIndex = SSL_get_ex_new_index(0, "myDataIndex", NULL, NULL, NULL);
    done = TRUE;
    }
pthread_mutex_unlock( &osiMutex );
}


So the two callback functions are not called directly,
but they are passed to openssl functions that will call
the functions later.

We have been using this same code for many years up through gcc 4.85.
Now with 4.9 we are seeing this warning for the first time.