netdev - Re: [PATCH bpf-next] libbpf: make sure bpf headers are c++ include-able

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Date:   Tue, 20 Nov 2018 13:24:07 -0800
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Y Song <ys114321@...il.com>
Cc:     Stanislav Fomichev <sdf@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH bpf-next] libbpf: make sure bpf headers are c++
 include-able

On 11/20, Y Song wrote:
> On Tue, Nov 20, 2018 at 10:19 AM Stanislav Fomichev <sdf@...gle.com> wrote:
> >
> > Wrap headers in extern "C", to turn off C++ mangling.
> > This simplifies including libbpf in c++ and linking against it.
> >
> > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> > ---
> >  tools/lib/bpf/bpf.h    | 9 +++++++++
> >  tools/lib/bpf/libbpf.h | 9 +++++++++
> 
> Do you want to add tools/lib/bpf/btf.h as well? it has some functions
> which could be used outside libbpf as well.
Sure, I can do that in v2.

> >  2 files changed, 18 insertions(+)
> >
> > diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> > index 26a51538213c..9ea3aec82d8a 100644
> > --- a/tools/lib/bpf/bpf.h
> > +++ b/tools/lib/bpf/bpf.h
> > @@ -27,6 +27,10 @@
> >  #include <stdbool.h>
> >  #include <stddef.h>
> >
> > +#ifdef __cplusplus
> > +extern "C" {
> > +#endif
> 
> Some (but not all) __cplusplus extern wrappers wraps standard include as well
> like "#include <stdbool.h>", "#include <stddef.h>". probably does not
> matter here
> since they may not have function prototype. But just want to point it out so you
> are aware of this and may double check.
Standard headers should be safe, they are usually wrapped into
__BEGIN_DECLS/__END_DECLS (which is extern "C" {}). For <linux/bpf.h>
I don't think we need an extern, because it only defines enums/structs
and not something we can link against (global vars/functions).

> > +
> >  #ifndef LIBBPF_API
> >  #define LIBBPF_API __attribute__((visibility("default")))
> >  #endif
> > @@ -128,4 +132,9 @@ LIBBPF_API int bpf_load_btf(void *btf, __u32 btf_size, char *log_buf,
> >  LIBBPF_API int bpf_task_fd_query(int pid, int fd, __u32 flags, char *buf,
> >                                  __u32 *buf_len, __u32 *prog_id, __u32 *fd_type,
> >                                  __u64 *probe_offset, __u64 *probe_addr);
> > +
> > +#ifdef __cplusplus
> > +} /* extern "C" */
> > +#endif
> > +
> >  #endif /* __LIBBPF_BPF_H */
> > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> > index b1686a787102..74e57e041705 100644
> > --- a/tools/lib/bpf/libbpf.h
> > +++ b/tools/lib/bpf/libbpf.h
> > @@ -16,6 +16,10 @@
> >  #include <sys/types.h>  // for size_t
> >  #include <linux/bpf.h>
> >
> > +#ifdef __cplusplus
> > +extern "C" {
> > +#endif
> > +
> >  #ifndef LIBBPF_API
> >  #define LIBBPF_API __attribute__((visibility("default")))
> >  #endif
> > @@ -335,4 +339,9 @@ int libbpf_nl_get_qdisc(int sock, unsigned int nl_pid, int ifindex,
> >                         libbpf_dump_nlmsg_t dump_qdisc_nlmsg, void *cookie);
> >  int libbpf_nl_get_filter(int sock, unsigned int nl_pid, int ifindex, int handle,
> >                          libbpf_dump_nlmsg_t dump_filter_nlmsg, void *cookie);
> > +
> > +#ifdef __cplusplus
> > +} /* extern "C" */
> > +#endif
> > +
> >  #endif /* __LIBBPF_LIBBPF_H */
> > --
> > 2.19.1.1215.g8438c0b245-goog
> >