printk(9) [suse man page]

PRINTK(9)							   Driver Basics							 PRINTK(9)

NAME

       printk - print a kernel message

SYNOPSIS

       int printk(const char * fmt, ...);

ARGUMENTS

       fmt
	   format string

       ...
	   variable arguments

DESCRIPTION

       This is printk. It can be called from any context. We want it to work.

       We try to grab the console_sem. If we succeed, it's easy - we log the output and call the console drivers. If we fail to get the semaphore
       we place the output into the log buffer and return. The current holder of the console_sem will notice the new output in release_console_sem
       and will send it to the consoles before releasing the semaphore.

       One effect of this deferred printing is that code which calls printk and then changes console_loglevel may break. This is because
       console_loglevel is inspected when the actual printing occurs.

SEE ALSO

       printf(3)

       See the vsnprintf documentation for format string extensions over C99.

COPYRIGHT

Kernel Hackers Manual 2.6.					     July 2010								 PRINTK(9)

TRACE_PRINTK(9) 						   Driver Basics						   TRACE_PRINTK(9)

NAME

       trace_printk - printf formatting in the ftrace buffer

SYNOPSIS

       trace_printk(fmt, ...);

ARGUMENTS

       fmt
	   the printf format for printing

       ...
	   variable arguments

NOTE

       __trace_printk is an internal function for trace_printk and the ip is passed in via the trace_printk macro.

       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.

       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 trace_printk is used)

       A little optization trick is done here. If there's only one argument, there's no need to scan the string for printf formats. The trace_puts
       will suffice. But how can we take advantage of using trace_puts when trace_printk 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 "()" 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's
       bigger, use do_trace_printk otherwise, optimize it to trace_puts. Then just let gcc optimize the rest.

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Kernel Hackers Manual 3.10					     June 2014							   TRACE_PRINTK(9)