# The Linux Kernel API

## Data Types

### Doubly Linked Lists

!Iinclude/linux/list.h

## Basic C Library Functions

When writing drivers, you cannot in general use routines which are from the C Library. Some of the functions have been found generally useful and they are listed below. The behaviour of these functions may vary slightly from those defined by ANSI, and these deviations are noted in the text.

### String Conversions

!Elib/vsprintf.c

### String Manipulation

!Elib/string.c

### Bit Operations

!Iarch/x86/include/asm/bitops.h

## Basic Kernel Library Functions

The Linux kernel provides more basic utility functions.

### Bitmap Operations

!Elib/bitmap.c !Ilib/bitmap.c

### Command-line Parsing

!Elib/cmdline.c

### CRC Functions

!Elib/crc7.c !Elib/crc16.c !Elib/crc-itu-t.c !Elib/crc32.c !Elib/crc-ccitt.c

### idr/ida Functions

!Pinclude/linux/idr.h idr sync !Plib/idr.c IDA description !Elib/idr.c

## Memory Management in Linux

### The Slab Cache

!Iinclude/linux/slab.h !Emm/slab.c

### User Space Memory Access

!Iarch/x86/include/asm/uaccess_32.h !Earch/x86/lib/usercopy_32.c

### More Memory Management Functions

!Emm/readahead.c !Emm/filemap.c !Emm/memory.c !Emm/vmalloc.c !Imm/page_alloc.c !Emm/mempool.c !Emm/dmapool.c !Emm/page-writeback.c !Emm/truncate.c

## Kernel IPC facilities

### IPC utilities

!Iipc/util.c

## FIFO Buffer

### kfifo interface

!Iinclude/linux/kfifo.h

## relay interface support

Relay interface support is designed to provide an efficient mechanism for tools and facilities to relay large amounts of data from kernel space to user space.

### relay interface

!Ekernel/relay.c !Ikernel/relay.c

## Module Support

### Module Loading

!Ekernel/kmod.c

### Inter Module support

Refer to the file kernel/module.c for more information.

## Hardware Interfaces

### Interrupt Handling

!Ekernel/irq/manage.c

### DMA Channels

!Ekernel/dma.c

### Resources Management

!Ikernel/resource.c !Ekernel/resource.c

### MTRR Handling

!Earch/x86/kernel/cpu/mtrr/main.c

### PCI Support Library

!Edrivers/pci/pci.c !Edrivers/pci/pci-driver.c !Edrivers/pci/remove.c !Edrivers/pci/search.c !Edrivers/pci/msi.c !Edrivers/pci/bus.c !Edrivers/pci/access.c !Edrivers/pci/irq.c !Edrivers/pci/htirq.c !Edrivers/pci/probe.c !Edrivers/pci/slot.c !Edrivers/pci/rom.c !Edrivers/pci/iov.c !Idrivers/pci/pci-sysfs.c

### PCI Hotplug Support Library

!Edrivers/pci/hotplug/pci_hotplug_core.c

## Firmware Interfaces

### DMI Interfaces

!Edrivers/firmware/dmi_scan.c

### EDD Interfaces

!Idrivers/firmware/edd.c

## Security Framework

!Isecurity/security.c !Esecurity/inode.c

## Audit Interfaces

!Ekernel/audit.c !Ikernel/auditsc.c !Ikernel/auditfilter.c

## Accounting Framework

!Ikernel/acct.c

## Block Devices

!Eblock/blk-core.c !Iblock/blk-core.c !Eblock/blk-map.c !Iblock/blk-sysfs.c !Eblock/blk-settings.c !Eblock/blk-exec.c !Eblock/blk-flush.c !Eblock/blk-lib.c !Eblock/blk-tag.c !Iblock/blk-tag.c !Eblock/blk-integrity.c !Ikernel/trace/blktrace.c !Iblock/genhd.c !Eblock/genhd.c

## Char devices

!Efs/char_dev.c

## Miscellaneous Devices

!Edrivers/char/misc.c

## Clock Framework

The clock framework defines programming interfaces to support software management of the system clock tree. This framework is widely used with System-On-Chip (SOC) platforms to support power management and various devices which may need custom clock rates. Note that these "clocks" don't relate to timekeeping or real time clocks (RTCs), each of which have separate frameworks. These struct clk instances may be used to manage for example a 96 MHz signal that is used to shift bits into and out of peripherals or busses, or otherwise trigger synchronous state machine transitions in system hardware.

Power management is supported by explicit software clock gating: unused clocks are disabled, so the system doesn't waste power changing the state of transistors that aren't in active use. On some systems this may be backed by hardware clock gating, where clocks are gated without being disabled in software. Sections of chips that are powered but not clocked may be able to retain their last state. This low power state is often called a retention mode. This mode still incurs leakage currents, especially with finer circuit geometries, but for CMOS circuits power is mostly used by clocked state changes.

Power-aware drivers only enable their clocks when the device they manage is in active use. Also, system sleep states often differ according to which clock domains are active: while a "standby" state may allow wakeup from several active domains, a "mem" (suspend-to-RAM) state may require a more wholesale shutdown of clocks derived from higher speed PLLs and oscillators, limiting the number of possible wakeup event sources. A driver's suspend method may need to be aware of system-specific clock constraints on the target sleep state.

Some platforms support programmable clock generators. These can be used by external chips of various kinds, such as other CPUs, multimedia codecs, and devices with strict requirements for interface clocking.

!Iinclude/linux/clk.h