Why doesn't the function printk() use a comma to separate parameters?

Question

An example printk call:

printk(KERN_INFO "Log message.\n");

Perhaps this question is more about C in general, because I've never seen a function in C before that separated parameters without a comma.

How does this work? What does the compiler do with this information? Since the log level is an integer and the message is a pointer to a char array, it must pass them separately.

Answer 1

The printk() function only takes one const char* argument. The KERN_INFO macro expands to "\001" "6", yielding:

printk("\001" "6" "Log message.\n");

The C lexer concatenates adjacent string literal tokens which means that the above is converted into:

printk("\0016Log message.\n");

Answer 2

The log level isn't an integer but a string literal. String literals next to each other are concatenated into a single string literal at compile time.

Answer 3

Because if you search the header files you will see that e.g. KERN_INFO is a macro expanded as a string literal (actually multiple string literals, see e.g. the linked cross-reference), and two string literals next to each-other like that will be concatenated into a single string literal by the compiler.

So the call

printk(KERN_INFO "Log message.\n");

isn't a function call with multiple arguments, it's a function call with a single string literal argument.