CptS 360 (System Programming) Unit 5: Low-Level I/O Bob Lewis - - PowerPoint PPT Presentation

Unit 5: Low-Level I/O

CptS 360 (System Programming) Unit 5: Low-Level I/O

Bob Lewis

School of Engineering and Applied Sciences Washington State University

Spring, 2020

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

Motivation

◮ All file and device (and some network) input/output

ultimately uses these routines.

◮ Most of them map one-to-one to kernel system calls. ◮ This is the functionality all I/O drivers have to provide.

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

References

◮ Stevens & Rago Ch. 3

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

Introduction

◮ Recall system/utility library interface diagram. ◮ These all work with file descriptors.

◮ Q: What is the C type used for a file descriptor? ◮ Q: What are the typical values of file descriptors?

◮ All programs start with 3 open file descriptors.

◮ Q: What are they and what do they correspond to? Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

• pen(2) and creat(2)

Opens or creates a file for reading or writing. prototypes: #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> int open(const char *pathname, int flags); int open(const char *pathname, int flags, mode_t mode); int creat(const char *pathname, mode_t mode); creat() is equivalent to open() with flags set to O_CREAT|O_WRONLY|O_TRUNC

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

common open(2) flags

◮ O_RDONLY, O_WRONLY, or O_RDWR

read-only, write-only, read/write (choose one)

◮ O_APPEND

append

◮ O_CREAT

create file if it doesn’t exist

◮ O_EXCL

exclusive access (with O_CREAT, make sure file is created by caller)

◮ O_TRUNC

truncate to length 0

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

less-used open(2) flags

◮ O_NOCTTY

if opening a device, don’t make it the control tty

◮ O_NONBLOCK or O_NDELAY

nonblocking open/access – caller will never wait

◮ O_SYNC

sync for each physical I/O (generally not a good idea – why?)

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

The mode argument

◮ Remember “ugo” for mode flags. ◮ Primitive (early UNIX) security notion divides the world into

three classes:

◮ user ◮ members of the user’s group ◮ ”other”

◮ mode is only used when O_CREAT is one of the flags.

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

mode is a mask (in octal)

also set in 00400 user read S_IRUSR S_IRWXU 00200 user write S_IWUSR S_IRWXU 00100 user execute S_IXUSR S_IRWXU 00040 group read S_IRGRP S_IRWXG 00020 group write S_IWGRP S_IRWXG 00010 group execute S_IXGRP S_IRWXG 00004

• thers read

S_IROTH S_IRWXO 00002

• thers write

S_IWOTH S_IRWXO 00001

• thers execute

S_IXOTH S_IRWXO

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

umask(2)

Sets the calling process’s “file creation mask”. prototype: #include <sys/types.h> #include <sys/stat.h> mode_t umask(mode_t mask);

◮ Set before open()/creat(). ◮ “umask” is a process attribute, but it affects open() and

creat().

◮ The umask bitmask says: “Turn these mode bits off, even if

the application says to turn them on.”

◮ File creation mask = (mode & ~umask) where mode is the

argument to open() or creat().

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

close(2)

protoype: #include <unistd.h> int close(int fd);

◮ pretty straightforward ◮ Q: Why not close fd’s 0, 1, and 2?

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

lseek(2)

Randomly positions a file for reading or writing. prototype: #include <sys/types.h> #include <unistd.h>

• ff_t lseek(int fildes, off_t offset, int whence);

◮ Q: Can you lseek()

◮ standard input (filedes == 0)? ◮ standard output (filedes == 1)?

◮ whence should be one of:

◮ SEEK_SET ◮ SEEK_CUR ◮ SEEK_END

◮ Take a look at lseek64(3).

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

read(2) and write(2)

These are the primary low-level I/O functions. read() prototype: #include <unistd.h> ssize_t read(int fd, void *buf, size_t count); Note: returned value may not equal count. write() prototype: #include <unistd.h> ssize_t write(int fd, const void *buf, size_t count);

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

I/O Efficiency

Q: What’s are the advantages of these routines over those in the Standard I/O library (TBD)? Vice Versa?

◮ No buffering (with some exceptions). ◮ Direct control to user. ◮ Performance could go either way.

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

File Sharing

◮ Two processes on the same system can share the same file. ◮ OK for one writer, N readers. ◮ Not so for > 1 writer.

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

Simultaneous Operations

◮ Consider two processes doing an lseek() to EOF, followed by

write().

◮ Workaround: O_APPEND

◮ Simultaneous file creation if file doesn’t already exist...

◮ resolved with O_CREAT and O_EXCL flags to open(). Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

dup(2) and dup2(2)

Duplicates one file descriptor to another. prototype: #include <unistd.h> int dup(int oldfd); int dup2(int oldfd, int newfd);

◮ dup2(foo, bar);

closes ”bar” before dupping it

◮ Handy: Lets you direct two already-open files (e.g. standard

• utput and standard error) to the same place.

◮ This is how

$ myprog 2>&1 | less works.

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

fcntl(2)

Miscellaneous “file control” functions. prototype: #include <unistd.h> #include <fcntl.h> int fcntl(fd, cmd[, arg]);

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

fcntl(2) Commands

cmd can be:

◮ F_DUPFD

duplicates fd as the lowest file descriptor (“dup(foo)” is equivalent to “fcntl(foo, F_DUPFD, 0)”)

◮ F_SETFD

sets the close-on-exec (which is?) flag to arg

◮ F_GETFD

returns the close-on-exec flag

◮ F_SETFL

sets descriptors (O_APPEND, O_NONBLOCK, O_ASYNC only) ...and other stuff having to do with signalling I/O and other kinds

• f (open) file related flags.

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

ioctl(2)

Device-related controls (more so than fctnl(2)). prototype: #include <sys/ioctl.h> int ioctl(int df, int cmd[, ... ]);

◮ disk labels ◮ device control (e.g., CD/DVD eject) ◮ mag tape control (e.g., BPI) ◮ socket I/O ◮ terminal I/O

• esp. character-at-a-time I/O (e.g. stty)

This is a catch-all function for the device driver.

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

/dev/fd/*

◮ On Linux (mainly). ◮ Allows standardized access to standard input, standard

• utput, and standard error, even though the executable might

not permit it.

◮ Occasionally useful. ◮ Q: Why does

$ ls /dev/fd list four file descriptors, and $ ls /dev/fd/0 works, but $ ls /dev/fd/3

• fails. (You have to think through what’s going on.)

Bob Lewis WSU CptS 360 (Spring, 2020)

Unit 5: Low-Level I/O

Summary

Two related questions:

◮ Why are there so few device-related system calls per device? ◮ What does it mean to implement a device driver?

Bob Lewis WSU CptS 360 (Spring, 2020)