'\" t
.\"     Title: trace_printk
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: January 2017
.\"    Manual: Driver Basics
.\"    Source: Kernel Hackers Manual 4.8.15
.\"  Language: English
.\"
.TH "TRACE_PRINTK" "9" "January 2017" "Kernel Hackers Manual 4\&.8\&." "Driver Basics"
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.SH "NAME"
trace_printk \- printf formatting in the ftrace buffer
.SH "SYNOPSIS"
.HP \w'trace_printk('u
.BI "trace_printk(" "fmt" ", " "\&.\&.\&." ");"
.SH "ARGUMENTS"
.PP
\fIfmt\fR
.RS 4
the printf format for printing
.RE
.PP
\fI\&.\&.\&.\fR
.RS 4
variable arguments
.RE
.SH "NOTE"
.PP
__trace_printk is an internal function for trace_printk and the
\fIip\fR
is passed in via the trace_printk macro\&.
.PP
This function allows a kernel developer to debug fast path sections that printk is not appropriate for\&. By scattering in various printk like tracing in the code, a developer can quickly see where problems are occurring\&.
.PP
This is intended as a debugging tool for the developer only\&. Please refrain from leaving trace_printks scattered around in your code\&. (Extra memory is used for special buffers that are allocated when
\fBtrace_printk\fR
is used)
.PP
A little optization trick is done here\&. If there\*(Aqs only one argument, there\*(Aqs no need to scan the string for printf formats\&. The
\fBtrace_puts\fR
will suffice\&. But how can we take advantage of using
\fBtrace_puts\fR
when
\fBtrace_printk\fR
has only one argument? By stringifying the args and checking the size we can tell whether or not there are args\&. __stringify((__VA_ARGS__)) will turn into
\(lq()\e0\(rq
with a size of 3 when there are no args, anything else will be bigger\&. All we need to do is define a string to this, and then take its size and compare to 3\&. If it\*(Aqs bigger, use
\fBdo_trace_printk\fR
otherwise, optimize it to
\fBtrace_puts\fR\&. Then just let gcc optimize the rest\&.
.SH "COPYRIGHT"
.br