Subject: Re: [PATCH bpf-next 4/9] libbpf: add libbpf_swap_print to get previous print func

From: Stanislav Fomichev <[email-masked]>
Date: Fri, 26 Jul 2019 14:28:18 -0700

On 07/26, Andrii Nakryiko wrote:
> libbpf_swap_print allows to restore previously set print function.
> This is useful when running many independent test with one default print
> function, but overriding log verbosity for particular subset of tests.
Can we change the return type of libbpf_set_print instead and return
the old function from it? Will it break ABI?

> Signed-off-by: Andrii Nakryiko <[email-masked]>
> ---
>  tools/lib/bpf/libbpf.c   | 8 ++++++++
>  tools/lib/bpf/libbpf.h   | 1 +
>  tools/lib/bpf/libbpf.map | 5 +++++
>  3 files changed, 14 insertions(+)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 8741c39adb1c..0c254b6c9685 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -79,6 +79,14 @@ void libbpf_set_print(libbpf_print_fn_t fn)
>  	__libbpf_pr = fn;
>  }
>  
> +libbpf_print_fn_t libbpf_swap_print(libbpf_print_fn_t fn)
> +{
> +	libbpf_print_fn_t old_print_fn = __libbpf_pr;
> +
> +	__libbpf_pr = fn;
> +	return old_print_fn;
> +}
> +
>  __printf(2, 3)
>  void libbpf_print(enum libbpf_print_level level, const char *format, ...)
>  {
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 5cbf459ece0b..4e0aa893571f 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -58,6 +58,7 @@ typedef int (*libbpf_print_fn_t)(enum libbpf_print_level level,
>  				 const char *, va_list ap);
>  
>  LIBBPF_API void libbpf_set_print(libbpf_print_fn_t fn);
> +LIBBPF_API libbpf_print_fn_t libbpf_swap_print(libbpf_print_fn_t fn);
>  
>  /* Hide internal to user */
>  struct bpf_object;
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index f9d316e873d8..e211c38ddc43 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -184,3 +184,8 @@ LIBBPF_0.0.4 {
>  		perf_buffer__new_raw;
>  		perf_buffer__poll;
>  } LIBBPF_0.0.3;
> +
> +LIBBPF_0.0.5 {
> +	global:
> +		libbpf_swap_print;
> +} LIBBPF_0.0.4;
> -- 
> 2.17.1
>