Standard C Libraries

Application Programmming Interface to System-Calls

Important File-I/O Functions

int open( char *pathname, int flags, … );

int read( int fd, void *buf, size_t count );

int write( int fd, void *buf, size_t count );

int lseek( int fd, loff_t offset, int whence );

int close( int fd );

UNIX ‘man’ pages

• A convenient online guide to prototypes and semantics of the C linrary functions

• Example of usage:

$ man 2 open

The ‘open’ function

• #include <fcntl.h>

• int open( char *pathname, int flags, … );

• Converts a pathname to a file-descriptor

• File-descriptor is a nonnegative integer

• Used as a file-ID in subsequent functions

• ‘flags’ is a symbolic constant:

O_RDONLY, O_WRONLY, O_RDWR

The ‘close’ function

• #include <unistd.h>

• int close( int fd );

• Breaks link between file and file-descriptor

• Returns 0 on success, or -1 if an error

The ‘write’ function

• #include <unistd.h>

• int write( int fd, void *buf, size_t count );

• Attempts to write up to ‘count’ bytes

• Bytes are taken from ‘buf’ memory-buffer

• Returns the number of bytes written

• Or returns -1 if some error occurred

• Return-value 0 means no data was written

The ‘read’ function

• #include <unistd.h>

• int read( int fd, void *buf, size_t count );

• Attempts to read up to ‘count’ bytes

• Bytes are placed in ‘buf’ memory-buffer

• Returns the number of bytes read

• Or returns -1 if some error occurred

• Return-value 0 means ‘end-of-file’

Notes on ‘read()’ and ‘write()’

• These functions have (as a “side-effect”) the advancement of a file-pointer variable

• They return a negative function-value of -1 if an error occurs, indicating that no actual data could be transferred; otherwise, they return the number of bytes read or written

• The ‘read()’ function normally does not return 0, unless ‘end-of-file’ is reached

The ‘lseek’ function

• #include <unistd.h>

• off_t lseek( int fd, off_t offset, int whence );

• Modifies the file-pointer variable, based on the value of whence:

enum { SEEK_SET, SEEK_CUR, SEEK_END };

• Returns the new value of the file-pointer (or returns -1 if any error occurred)

Getting the size of a file

• For normal files, your application can find out how many bytes belong to a file using the ‘lseek()’ function:

int filesize = lseek( fd, 0, SEEK_END );

• But afterward you need to ‘rewind’ the file if you want to read its data:

lseek( fd, 0, SEEK_SET );

What about ‘pseudo’ files?

• You can use standard library functions to open, read, and close a ‘/proc’ pseudo-file

• You can use ‘lseek’ (except SEEK_END)

• An example is our ‘howfast.cpp’ program

• It measures how fast ‘jiffies’ increments

• It opens, reads, and closes ‘/proc/jiffies’

• And it also uses ‘lseek’ (to rewind this file)

How these system-calls work

Application Program

User-space Kernel-space

C Runtime Library

Operating System Kernel

Module ‘methods’

Special ‘device’ files

• UNIX systems treat hardware-devices as special files, so that familiar functions can be used by application programmers to access these devices (open, read, close)

• But a System Administrator has to create these device-files (in the ‘/dev’ directory)

• There are two categories of device files: ‘character’ devices, and ‘block’ devices

The ‘mknod’ command

• To create the device-node for a character device, an Administrator executes ‘mknod’

root# mknod /dev/scull c 48 0• Here ‘/dev/scull’ is the file’s pathname, ‘c’

indicates that it’s a character-mode device, 48 is its (unique) ‘major’ ID-number, and 0 is its (unique) ‘minor’ ID-number

• Default access-privileges: r w - r - - r - - • Can be modified using ‘chmod’ command

What’s new in 2.6?

• Earlier Linux kernels stored the ‘/dev’ files on the hard disk (so they were persistent)

• The 2.6 kernel stores them in a ram-disk

• So they ‘disappear’ during every shutdown

• You need ‘root’ privileges to re-build them!

• (Fortunately this step can be automated if device-nodes are in ‘/etc/udev/devices’ )

A useful device-driver

• We can create a character-mode driver for the processor’s physical memory (i.e. ram)

• (Our machines have 1-GB of physical ram)

• But another device-file is named ‘/dev/ram’ so ours will be: ‘/dev/dram’ (dynamic ram)

• We’ve picked 253 as its ‘major’ ID-number

• Our SysAdmin setup a device-node using:root# mknod /dev/dram c 253 0

Device knowledge

• Before you can write a device-driver, you must understand how the hardware works

• Usually this means you need to obtain the programmer manual (from manufacturer)

• Nowdays this can often be an obstacle

• But some equipment is standardized, or is well understood (because of its simplicity)

1-GB RAM has ‘zones’

ZONE_NORMAL

ZONE_HIGH

ZONE_LOW

128-MB

16-MB

1024-MB (= 1GB)

Legacy DMA

• Various older devices rely on the PC/AT’s DMA controller to perform data-transfers

• This chip could only use 24-bit addresses• Only the lowest 16-megabytes of physical

memory are ‘visible’ to these devices:224 = 0x01000000 (16-megabytes)

• Linux tries to conserve its use of memory from this ZONE_LOW region for anything except DMA (so it will available if needed)

‘HIGH’ memory

• Linux traditionally tried to ‘map’ as much physical memory as possible into virtual addresses allocated to the kernel-space

• Before the days when systems had 1-GB or more of installed memory, Linux could linearly map ALL of the physical memory into the 1-GB kernel-region:

0xC0000000 – 0xFFFFFFFF• But with 1GB there’s not enough room!

User space(3GB)

The 896-MB limit

Virtual address-space

DRAM (1GB)

Kernel space(1GB)

HIGH MEMORY

linearly mappedPhysical address-space

not-mapped

896-MB

A special pair of kernel-functions named ‘kmap()’ and ‘kunmap()’ can be called by device-drivers to temporarily map pages of physical memory into ‘vacant’ areas within the kernel’s virtual address-space

‘copy_to_user()’

• With kernel 2.6, it is possible to configure the user-space versus kernel-space ‘split’ so that nearly 4GB of physical memory is always linearly mapped into kernel-space

• The configuration-option is CONFIG_4GB• With this option enabled, the user-space

and kernel-space use two different maps• So device-drivers need a special function

to transfer kernel-data to a user’s buffer

Driver-module structure

• We will need three kernel header-files:– #include <linux/module>

// for printk(), register_chrdev(), unregister_chrdev()

– #include <linux/highmem.h>// for kmap(), kunmap(), and ‘num_physpages’

– #include <asm/uaccess.h>// for copy_to_user()

Our ‘dram_size’ global

• Our ‘init_module()’ function will compute the size of the installed physical memory

• It will be stored in a global variable, so it can be accessed by our driver ‘methods’

• It is computed from a kernel global using the PAGE_SIZE constant (=4096 for x86)

dram_size = num_physpages * PAGE_SIZE

‘major’ ID-number

• Our ‘major’ device ID-number is needed when we ‘register’ our device-driver with the kernel (during initialization) and later when we ‘unregister’ our device-driver (during the cleanup procedure):

int my_major = 253; // static ID-assignment

Our ‘file_operations’

• Our ‘dram’ device-driver does not need to implement special ‘methods’ for doing the ‘open()’, ‘write()’, or ‘release()’ operations (the kernel ‘default’ operations will suffice)

• But we DO need to implement ‘read()’ and ‘llseek()’ methods

• Our ‘llseek()’ code here is very standard

• But ‘read()’ is specially crafted for DRAM

Using our driver

• We have provided a development tool on the class website (named ‘fileview.cpp’) which can be used to display the contents of files (including device-files)

• The data is shown in hex and ascii formats

• The arrow-keys can be used for navigation

• The enter-key allows an offset to be typed

• Keys ‘b’, ‘w’, ‘d’ and ‘q’ adjust data-widths

In-class exercise #1

• Install the ‘dram.ko’ device-driver module; then use ‘fileview’ to browse the contents of the processor’s physical memory:

$ fileview /dev/dram

Control Register CR3

• Register CR3 holds the physical-address of the system’s current ‘page-directory’

• The page-directory is an array of 1024 entries, showing how ‘virtual addresses’ are currently ‘mapped’ to physical pages

• With ‘fileview’ you can find and examine this important kernel data-structure – but you must know the value in register CR3

In-class exercise #2

• Use the ‘newinfo’ wizard to quickly create a pseudo-file (named ‘/proc/cr3’) that will allow user-programs to obtain the current value of the Pentium’s CR3 register

• Write a tool (named ‘seepgdir.cpp’) that will read ‘/proc/cr3’ to get the address of the page-directory, then read it from the ‘/dev/dram’ device and print it onscreen