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The Debug.Trace.traceEvent (& traceEventIO) use a code path that unnecessarily uses printf format strings and uses a fixed size 512-byte buffer, hence truncating all user trace messages at that size.

Here is the call path:

• Debug.Trace.traceEvent (& traceEventIO) call traceEvent# primop
• cmm impl of the primop stg_traceEventzh calls C RTS function traceUserMsg
• traceUserMsg calls traceFormatUserMsg(cap, "%s", msg);, using the printf format string
• traceFormatUserMsg uses postUserMsg which eventually calls postLogMsg
• postLogMsg does the printf stuff

Here's what postLogMsg does:

#define BUF 512

void postLogMsg(EventsBuf *eb, EventTypeNum type, char *msg, va_list ap)
{
    char buf[BUF];
    nat size;

    size = vsnprintf(buf,BUF,msg,ap);
    if (size > BUF) {
        buf[BUF-1] = '\0';
        size = BUF;
    }

   ....

This is obviously designed for RTS-internal users, not the user path.

So the problem starts with this bit:

void traceUserMsg(Capability *cap, char *msg)
{
    traceFormatUserMsg(cap, "%s", msg);
}

It just should not use that code path. It should call something that directly posts the message, without any silly printf strings. In fact, the code path from the primop down should really be changed to use an explicit given length, rather than using a null-terminated string and having to call strlen.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
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