Re: _init() and _fini() for dynamically loaded libraries?

[Ronald Landheer-Cieslak <ronald at landheer dot com>]

gcc supports a couple of special attributes for init and fini - type 
functions: __attribute__((constructor)) and __attribute__((destructor))

If you add 
void _init() __attribute__((constructor));
void _fini() __attribute__((destructor));

somewhere to the top of mylib.c, your example will work as expected.

HTH

rlc


On Wed, 19 Mar 2003, Daniel Daboul wrote:
> Does Cygwin support initialization and cleanup for libraries loaded
> dynamically via dlopen() in a way similar to Linux and Solaris?  If
> yes, how can I get it to work?  I attached a simple test case below,
> with comments telling how to compile/link.  The output under cygwin
> is:
> 
> calculating
> doubled in library: 2.468000
> 
> So it basically works, but neither _init() nor _fini() get
> executed. Under Solaris the output is:
> 
> initializing
> calculating
> doubled in library: 2.468000
> finishing
> 
> (There I link with "ld -Bshareable mylib.o -o libmylib.dll" and "gcc
> -Wall -o caller caller.c -ldl".)  Solaris declares _init and _fini in
> <sys/modctl.h> (actually returning int), which does not exists in my
> cygwin installation, but I'm not sure this is enough to conclude that
> it's unsupported.  I didn't find help in the documentation or
> mail-archives. -- Daniel
> 
> /* mylib.c
>    gcc -Wall -c mylib.c
>    gcc -shared -Wl,--export-all mylib.o -o libmylib.dll
> */
> #include <stdio.h>
> double dub(double in1) {printf("calculating\n");return 2*in1;}
> void _init() {printf("initializing\n");}
> void _fini() {printf("finishing\n");}
> 
> /* caller.c
>    gcc -Wall -o caller caller.c
>    ./caller
> */
> #include <stdio.h>
> #include <stdlib.h>
> #include <dlfcn.h>
> int main() {
>     void *handle;
>     double (*pdub)(double);
>     char *error;
>     handle = dlopen("./libmylib.dll",RTLD_LAZY);
>     if(!handle) {fprintf(stderr,"%s\n",dlerror()); exit(1);}
>     pdub =dlsym(handle,"dub");
>     if((error = dlerror())!=NULL) {fprintf(stderr,"%s\n",error); exit(1);}
>     printf("doubled in library: %f\n",(*pdub)(1.234));
>     dlclose(handle);
>     return 0;
> }
> 
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