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trace_printk(9) [centos man page]

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

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VSNPRINTF(9)						     Basic C Library Functions						      VSNPRINTF(9)

NAME
vsnprintf - Format a string and place it in a buffer SYNOPSIS
int vsnprintf(char * buf, size_t size, const char * fmt, va_list args); ARGUMENTS
buf The buffer to place the result into size The size of the buffer, including the trailing null space fmt The format string to use args Arguments for the format string DESCRIPTION
This function follows C99 vsnprintf, but has some extensions: pS output the name of a text symbol with offset ps output the name of a text symbol without offset pF output the name of a function pointer with its offset pf output the name of a function pointer without its offset pB output the name of a backtrace symbol with its offset pR output the address range in a struct resource with decoded flags pr output the address range in a struct resource with raw flags pM output a 6-byte MAC address with colons pMR output a 6-byte MAC address with colons in reversed order pMF output a 6-byte MAC address with dashes pm output a 6-byte MAC address without colons pmR output a 6-byte MAC address without colons in reversed order pI4 print an IPv4 address without leading zeros pi4 print an IPv4 address with leading zeros pI6 print an IPv6 address with colons pi6 print an IPv6 address without colons pI6c print an IPv6 address as specified by RFC 5952 pIS depending on sa_family of 'struct sockaddr *' print IPv4/IPv6 address piS depending on sa_family of 'struct sockaddr *' print IPv4/IPv6 address pU[bBlL] print a UUID/GUID in big or little endian using lower or upper case. %*ph[CDN] a variable-length hex string with a separator (supports up to 64 bytes of the input) pc print a cpumask as comma-separated list n is ignored ** Please update Documentation/printk-formats.txt when making changes ** The return value is the number of characters which would be generated for the given input, excluding the trailing '', as per ISO C99. If you want to have the exact number of characters written into buf as return value (not including the trailing ''), use vscnprintf. If the return is greater than or equal to size, the resulting string is truncated. If you're not already dealing with a va_list consider using snprintf. COPYRIGHT
Kernel Hackers Manual 3.10 June 2014 VSNPRINTF(9)
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