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    SubjectRe: [PATCH v2 1/2] tracing/events: make __string() more general
    Steven Rostedt wrote:
    >
    > On Tue, 26 May 2009, Li Zefan wrote:
    >
    >> This changes __string(..., src) to __string(..., len), thus makes it more
    >> general, and then block TRACE_EVENT() can use it.
    >>
    >> Also introduce __fetch_str(), which is used in TP_assign() and returns
    >> the address of the string, while __get_str() is used in TP_print().
    >>
    >> [ Impact: TRACE_EVENT api change ]
    >
    > What's the reason for the change?
    >
    > Is it to handle NULL pointers? If so, lets change the ftrace.h to handle
    > it instead.
    >

    No, not for this purpose.

    With __string(..., src), we need to have a source string, but sometimes no
    source string is available, and what we know is how big the string will be.

    Take block trace events for example, I want to convert an unsigned char array
    into a string, after this patch, this can be done this way:

    TP_STRUCT_entry(
    __string( cmd, rq->cmd_len * 3 )
    )

    TP_fast_assign(
    construct_str( cmd )
    )

    TP_prink("%s\n", get_str(cmd))

    Seems Christoph also runs into this problem while doing his XFS tracing.
    (and NULL str ptr that I'm not suffering)

    > This is why I like the Impact line really be a "rational" line.
    > "TRACE_EVENT api change" is meaningless to me.
    >

    So what should this impact be. :(


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