From: WanLi Niu <niuwl1@chinatelecom.cn>

Fix C++ compilation errors in generated skeleton by adding explicit
pointer casts and use char * subtraction for offset calculation

error: invalid conversion from 'void*' to '<obj_name>*' [-fpermissive]
      |         skel = skel_alloc(sizeof(*skel));
      |                ~~~~~~~~~~^~~~~~~~~~~~~~~
      |                          |
      |                          void*

error: arithmetic on pointers to void
      |         skel->ctx.sz = (void *)&skel->links - (void *)skel;
      |                        ~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~

error: assigning to 'struct <obj_name>__<ident> *' from incompatible type 'void *'
      |                 skel-><ident> = skel_prep_map_data((void *)data, 4096,
      |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                 sizeof(data) - 1);
      |                                                 ~~~~~~~~~~~~~~~~~

error: assigning to 'struct <obj_name>__<ident> *' from incompatible type 'void *'
      |         skel-><ident> = skel_finalize_map_data(&skel->maps.<ident>.initial_value,
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                         4096, PROT_READ | PROT_WRITE, skel->maps.<ident>.map_fd);
      |                                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Minimum reproducer:

	$ cat test.bpf.c
	int val; // placed in .bss section

	#include "vmlinux.h"
	#include <bpf/bpf_helpers.h>

	SEC("raw_tracepoint/sched_wakeup_new") int handle(void *ctx) { return 0; }

	$ cat test.cpp
	#include <cerrno>

	extern "C" {
	#include "test.bpf.skel.h"
	}

	$ bpftool btf dump file /sys/kernel/btf/vmlinux format c > vmlinux.h
	$ clang -g -O2 -target bpf -c test.bpf.c -o test.bpf.o
	$ bpftool gen skeleton test.bpf.o -L  > test.bpf.skel.h
	$ g++ -c test.cpp -I.

Signed-off-by: WanLi Niu <niuwl1@chinatelecom.cn>
Co-developed-by: Menglong Dong <dongml2@chinatelecom.cn>
Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
---
changelog:
v5:
- Use char * arithmetic instead of integer subtraction for offsets as suggested by Andrii Nakryiko
- Use __typeof__() without __cplusplus special casing as suggested by Andrii Nakryiko

v4: https://lore.kernel.org/all/20260105071231.2501-1-kiraskyler@163.com/
- Add a minimum reproducer to demonstrate the issue, as suggested by Yonghong Song

v3: https://lore.kernel.org/all/20260104021402.2968-1-kiraskyler@163.com/
- Fix two additional <obj_name>__<ident> type mismatches as suggested by Yonghong Song

v2: https://lore.kernel.org/all/20251231102929.3843-1-kiraskyler@163.com/
- Use generic (struct %1$s *) instead of project-specific (struct trace_bpf *)

v1: https://lore.kernel.org/all/20251231092541.3352-1-kiraskyler@163.com/
---
 tools/bpf/bpftool/gen.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 993c7d9484a4..2f9e10752e28 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -731,10 +731,10 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
 		{							    \n\
 			struct %1$s *skel;				    \n\
 									    \n\
-			skel = skel_alloc(sizeof(*skel));		    \n\
+			skel = (struct %1$s *)skel_alloc(sizeof(*skel));    \n\
 			if (!skel)					    \n\
 				goto cleanup;				    \n\
-			skel->ctx.sz = (void *)&skel->links - (void *)skel; \n\
+			skel->ctx.sz = (char *)&skel->links - (char *)skel; \n\
 		",
 		obj_name, opts.data_sz);
 	bpf_object__for_each_map(map, obj) {
@@ -755,7 +755,7 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
 		\n\
 		\";							    \n\
 									    \n\
-				skel->%1$s = skel_prep_map_data((void *)data, %2$zd,\n\
+				skel->%1$s = (__typeof__(skel->%1$s))skel_prep_map_data((void *)data, %2$zd,\n\
 								sizeof(data) - 1);\n\
 				if (!skel->%1$s)			    \n\
 					goto cleanup;			    \n\
@@ -857,7 +857,7 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
 
 		codegen("\
 		\n\
-			skel->%1$s = skel_finalize_map_data(&skel->maps.%1$s.initial_value,  \n\
+			skel->%1$s = (__typeof__(skel->%1$s))skel_finalize_map_data(&skel->maps.%1$s.initial_value,\n\
 							%2$zd, %3$s, skel->maps.%1$s.map_fd);\n\
 			if (!skel->%1$s)				    \n\
 				return -ENOMEM;				    \n\
-- 
2.39.1