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trace_printk(9) [centos man page]

TRACE_PRINTK(9) 						   Driver Basics						   TRACE_PRINTK(9)

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. COPYRIGHT
Kernel Hackers Manual 3.10 June 2014 TRACE_PRINTK(9)

Check Out this Related Man Page

vprintf(3int)															     vprintf(3int)

       vprintf, vfprintf, vsprintf - print formatted output of a varargs argument list

       #include <stdio.h>
       #include <varargs.h>

       int vprintf ( format, ap )
       char *format;
       va list ap;

       int vfprintf ( stream, format, ap )
       FILE *stream;
       char *format;
       va list ap;

       int vsprintf ( s, format, ap )
       char *s, *format;
       va list ap;

       The international functions and are similar to the standard I/O functions.

       Likewise,  the  vprintf functions are similar to the printf functions except they are called with an argument list as defined by instead of
       with a variable number of arguments.

       The international functions allow you to use the %digit$ conversion character in place of the % character  you  use  in	the  standard  I/O
       functions.   The digit is a decimal digit n from 1 to 9.  The international functions apply conversions to the nth argument in the argument
       list, rather than to the next unused argument.

       You can use the % conversion character in the international functions.  However, you cannot mix the % conversion character with the %digit$
       conversion character in a single call.

       You  can  indicate  a  field  width or precision by an asterisk (*) instead of a digit string in format strings containing the % conversion
       character. If you use an asterisk, you can supply an integer arg that specifies the field width or precision.  In format strings containing
       the  %digit$  conversion character, you can indicate field width or precision by the sequence *digit$.  You use a decimal digit from 1 to 9
       to indicate which argument contains an integer that specifies the field width or precision.

       The conversion characters and their meanings are identical to

       You must use each digit argument at least once.

       #include <stdio.h>
       #include <varargs.h>

       char *function_name = "vpr";
       char *arg1 = "hello world";
       int arg2 = 2;
       char *arg3 = "study";

       char *i18nfmt = "%1$s %3$d

       test(function_name, i18nfmt, arg1, arg2, arg3);

       va_list args;
       char *fmt;
       char string[1024];


       (void)printf("function %s: ", va_arg(args, char *));

       fmt = va_arg(args, char *);

       (void)vprintf(fmt, args);


See Also
       setlocale(3), scanf(3int), printf(3s), printf(3int), vprintf(3s), putc(3s), scanf(3s), stdio(3s), varargs(3)
       Guide to Developing International Software

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