shlibs/dynload.c

+/* dynload.c
+
+   Usage: dynload library-path function-name
+
+   Demonstrate dynamic loading of libraries. The program loads the
+   named library and then executes the named function in that library.
+*/
 #include <dlfcn.h>
 #include "tlpi_hdr.h"
 
 int
 main(int argc, char *argv[])
 {
     void *libHandle;            /* Handle for shared library */
     void (*funcp)(void);        /* Pointer to function with no arguments */
     const char *err;
 
     if (argc != 3 || strcmp(argv[1], "--help") == 0)
         usageErr("%s lib-path func-name\n", argv[0]);
 
     /* Load the shared library and get a handle for later use */
 
     libHandle = dlopen(argv[1], RTLD_LAZY);
     if (libHandle == NULL)
         fatal("dlopen: %s", dlerror());
 
     /* Search library for symbol named in argv[2] */
 
     (void) dlerror();                           /* Clear dlerror() */
-    *(void **) (&funcp) = dlsym(libHandle, argv[2]);
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpedantic"
+    funcp = (void (*)(void)) dlsym(libHandle, argv[2]);
+#pragma GCC diagnostic pop
+
+    /* In the book, instead of the preceding line, the code uses a
+       rather clumsy looking cast of the form:
+
+           *(void **) (&funcp) = dlsym(libHandle, argv[2]);
+
+       This was done because the ISO C standard does not require compilers
+       to allow casting of pointers to functions back and forth to 'void *'.
+       (See TLPI pages 863-864.) SUSv3 TC1 and SUSv4 accepted the ISO C
+       requirement and proposed the clumsy cast as the workaround. However,
+       the 2013 Technical Corrigendum to SUSv4 requires implementations
+       to support casts of the more natural form (now) used in the code
+       above. However, various current compilers (e.g., gcc with the
+       '-pedantic' flag) may still complain about such casts. Therefore,
+       we use a gcc pragma to disable the warning.
+
+       Note that this pragma is available only since gcc 4.6, released in
+       2010. If you are using an older compiler, the pragma will generate
+       an error. In that case, simply edit this program to remove the
+       lines above that begin with '#pragma".
+
+       See also the erratum note for page 864 at
+       http://www.man7.org/tlpi/errata/. */
+
     err = dlerror();
     if (err != NULL)
         fatal("dlsym: %s", err);
 
     /* Try calling the address returned by dlsym() as a function
        that takes no arguments */
 
     (*funcp)();
 
     dlclose(libHandle);                         /* Close the library */
 
     exit(EXIT_SUCCESS);
 }