'\" t
.\"     Title: printk
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.78.1 <http://docbook.sf.net/>
.\"      Date: May 2018
.\"    Manual: Driver Basics
.\"    Source: Kernel Hackers Manual 3.16.56
.\"  Language: English
.\"
.TH "PRINTK" "9" "May 2018" "Kernel Hackers Manual 3\&.16\&" "Driver Basics"
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.SH "NAME"
printk \- print a kernel message
.SH "SYNOPSIS"
.HP \w'__visible\ int\ printk('u
.BI "__visible int printk(const\ char\ *\ " "fmt" ", " "\&.\&.\&." ");"
.SH "ARGUMENTS"
.PP
\fIfmt\fR
.RS 4
format string
.RE
.PP
\fI\&.\&.\&.\fR
.RS 4
variable arguments
.RE
.SH "DESCRIPTION"
.PP
This is
\fBprintk\fR\&. It can be called from any context\&. We want it to work\&.
.PP
We try to grab the console_lock\&. If we succeed, it\*(Aqs 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
\fBconsole_unlock\fR; and will send it to the consoles before releasing the lock\&.
.PP
One effect of this deferred printing is that code which calls
\fBprintk\fR
and then changes console_loglevel may break\&. This is because console_loglevel is inspected when the actual printing occurs\&.
.SH "SEE ALSO"
.PP
printf(3)
.PP
See the
\fBvsnprintf\fR
documentation for format string extensions over C99\&.
.SH "COPYRIGHT"
.br