GeezerVison

printk is a printf-like function of the Linux kernel interface for formatting and writing kernel log entries.^[1] Since the C standard library (which contains the ubiquitous printf-like functions) is not available in kernel mode, printk provides for general-purpose output in the kernel.^[2] Due to limitations of the kernel design, the function is often used to aid debugging kernel mode software.^[1]

printk can be called from anywhere in the kernel except during early stages of the boot process, before the system console is initialized.^[3] The alternative function early_printk is implemented on some architectures and is used identically to printk but during the early stages of the boot process.^[3]

printk has the same syntax as printf, but somewhat different semantics. Like printf, printk accepts a format C-string argument and a list of value arguments.^[1] Both format text based on the input parameters and with significantly similar behavior, but there are also significant differences.^[1] The printk function prototype (which matches that of printf) is:

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

The features different from printf are described below.

printk(KERN_INFO "Message: %s", arg)

The function tries to lock the semaphore controlling access to the Linux system console.^[1][8] If it succeeds, the output is logged and the console drivers are called.^[1] If it is not possible to acquire the semaphore the output is placed into the log buffer, and the current holder of the console semaphore will notice the new output when they release the console semaphore and will send the buffered output to the console before releasing the semaphore.^[1]

One effect of this deferred printing is that code which calls printk and then changes the log levels to be printed may break. This is because the log level to be printed is inspected when the actual printing occurs.^[1]

• printk format reference