On 01/01/14 05:19, Mike Frysinger wrote: > On Tuesday 31 December 2013 07:42:09 noname@xxxxxxxxxxxxx wrote: >> dlopen C code example contains following comment: >> >> /* Writing: cosine = (double (*)(double)) dlsym(handle, "cos"); >> ... */ >> >> "Writing:" should be replaced with "Warning:". > > both are correct > > however, i wonder if the comment should be inverted. the latest POSIX spec > requires that the natural cast work. > http://pubs.opengroup.org/onlinepubs/9699919799/functions/dlsym.html#tag_16_96_07 > > Linux man pages document the normal Linux environment, and that means POSIX, > not strict ISO C. Thanks for your comments Mike. It's been at the back of my mind for a while now to do something about this. I applied the patch below. Look okay to you? Cheers, Michael diff --git a/man3/dlopen.3 b/man3/dlopen.3 index dd54120..47ca55f 100644 --- a/man3/dlopen.3 +++ b/man3/dlopen.3 @@ -466,15 +466,22 @@ main(int argc, char **argv) dlerror(); /* Clear any existing error */ - /* Writing: cosine = (double (*)(double)) dlsym(handle, "cos"); - would seem more natural, but the C99 standard leaves - casting from "void *" to a function pointer undefined. - The assignment used below is the POSIX.1\-2003 (Technical - Corrigendum 1) workaround; see the Rationale for the - POSIX specification of dlsym(). */ + cosine = (double (*)(double)) dlsym(handle, "cos"); - *(void **) (&cosine) = dlsym(handle, "cos"); -.\" But in fact "gcc -O2 -Wall" will complain about the preceding cast. + /* According to the C99 standard, casting 'void *' to a function pointer + as shown above is forbidden. POSIX.1-2003 and POSIX.2008 followed + C99's requirement, and proposed the following workaround: + + *(void **) (&cosine) = dlsym(handle, "cos"); + + This (clumsy) cast conforms with C99 and will avoid any compiler + warnings. + + The 2013 Technical Corrigendum to POSIX.1-2008 (a.k.a. POSIX.1-2013) + improved matters by requiring that conforming implementations support + casting 'void *' to a function pointer. Nevertheless, some compilers + (e.g., gcc with the '-pedantic' option) may complain about the cast + used in this program. */ -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
