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dlopen.3: Update remarks on cast needed when assigning dlsym() return value 

POSIX.1-2013 eases life when casting the dlsym() return value to a
function pointer

Reported-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2014-01-02 14:38:53 +13:00
parent a9deb5e056
commit 299653455f
1 changed files with 16 additions and 9 deletions
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25
man3/dlopen.3
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@ -32,7 +32,7 @@
.\" Modified by Walter Harms: dladdr, dlvsym
.\" Modified by Petr Baudis <pasky@suse.cz>, 2008-12-04: dladdr caveat
.\"
.TH DLOPEN 3 2013-12-30 "Linux" "Linux Programmer's Manual"
.TH DLOPEN 3 2014-01-02 "Linux" "Linux Programmer's Manual"
.SH NAME
dladdr, dlclose, dlerror, dlopen, dlsym, dlvsym \- programming interface to
dynamic linking loader
@ -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. */
error = dlerror();
if (error != NULL) {