Kernel logging: APIs and implementation

Kernel API

Logging within the kernel is performed using the printk function

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

The kernel code simply defines the log level as the first argument of the message, as illustrated in the following example for a critical message:

printk( KERN_CRIT "Error code %08x.\n", val );

Note that the first argument is not an argument at all, as no comma (,) separates the level (KERN_CRIT) from the format string.

Table 1. Log levels, symbolics, and uses

Note that if a caller does not provide a log level within printk, a default of KERN_WARNING is automatically used.

At this point, you‘ve explored the API used to insert log messages into the kernel ring buffer. Now, let‘s look at the method used to migrate data from the kernel into the host.

## Printk source code

Kernel logging and interface

Access to the log buffer is provided at the core through the multi-purpose syslog system call.

The syslog call serves as the input/output (I/O) and control interface to the kernel‘s log message ring buffer.

User space applications

References

• Kernel logging: APIs and implementation

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