Linux System Programming Ch 2

Front 1

What is the system call for opening a file

Back 1

"int open (const char *name, int flags, mode_t mode); "

Front 2

What is returned on success of the open system call?

Back 2

The file descriptor

Front 3

The three main flag arguments to open are?

Back 3

"O_RDONLY, O_WRONLY, or O_RDWR"

Front 4

O_TRUNC is an additional flag that means what?

Back 4

Truncate to a length of zero (start over)

Front 5

The mode argument is used for what?

Back 5

Specifying file ownership

Front 6

What is O_CREAT?

Back 6

"If the file denoted by name does not exist, the kernel will create it. If the file already exists, this flag has no effect

Front 7

The creat function corresponds to what flags in open?

Back 7

O_WRONLY | O_CREAT | O_TRUNC

Front 8

What does the creat signature look like?

Back 8

"int creat (const char *name, mode_t mode);"

Front 9

The main system call for reading a file is?

Back 9

read

Front 10

The signature for read is ?

Back 10

"ssize_t read (int fd, void *buf, size_t len); "

Front 11

"On success, read returns what?"

Back 11

The number of bytes successfully read.

Front 12

"On error, read returns?"

Back 12

-1

Front 13

By default, what will happen if a call is made for len bytes, but no bytes are available for reading

Back 13

The call will block

Front 14

Non-blocking reads are possible using...?

Back 14

Open the file with O_NONBLOCK and handle the EAGAIN errror

Front 15

What does the signature of write look like?

Back 15

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

Front 16

What is append mode?

Back 16

"Writes always occur that the end of the file, not where the fd currently points. Even works with two concurrent writers"

Front 17

"When a write returns, where is your data? On disk?"

Back 17

"No, a write only takes your buffer and writes it into a kernel buffer. "

Front 18

What is "writeback"

Back 18

When the kernel takes all the "dirty" buffers and optimally writes them to disk

Front 19

What if prior to writeback a read is issued for that data?

Back 19

The request is then served from memory and not disk!

Front 20

Can multiple processes dirty a buffer? Are buffers associated with a process?

Back 20

Yes multiple processes can dirty a buffer. Buffers are not associated with a particular process

Front 21

What is the maximum time a buffer can remain in memory before it is written out to disk?

Back 21

"It's configurable, found in .../proc/sys/vm/dirty_expire_centiseconds"

Front 22

Why would you want to use synchronized I/O?

Back 22

If you care about when the data hits the disk.

Front 23

How does fsync work?

Back 23

Writes all dirty data buffers to disk for a given file

Front 24

What is the signature of fsync?

Back 24

int fsync(int fd);

Front 25

What is sync

Back 25

"Flushes ALL buffers, not just for one fd"

Front 26

"Which runs faster, sync or fsync?"

Back 26

"fsync, sync can take several minutes on a busy machine."

Front 27

What is direct I/O?

Back 27

"Bypassing the Kernel's I/O system (caching, buffers, etc...) and doing this all in user space. Some database systems roll their own I/O system."

Front 28

How do you do direct I/O?

Back 28

provide the O_DIRECT flag to open()

Front 29

What does the O_DIRECT flag do?

Back 29

Bypasses kernel buffers and loads data directly into the user-space buffer provided. All operations are synchronous (won't return until completed)

Front 30

What is the signature of the function that closes a file?

Back 30

int close (int fd);

Front 31

What does a call to close() do?

Back 31

Unmaps the file descriptor from the file and disassociates the process from the file.

Front 32

"When close() is call, will a file be flushed to disk?"

Back 32

No. Closing has no bearing on when this is done.

Front 33

Can you move the file position around for a given file descriptor?

Back 33

"Yes, using lseek()"

Front 34

What is the signature for lseek()

Back 34

"off_t lseek (int fd, off_t pos, int origin); "

Front 35

what are the three values for lseek() origin?

Back 35

"SEEK_CUR, SEEK_END, SEEK_SET "

Front 36

How would you set the current file position to be at char 1875?

Back 36

"ret = lseek (fd, (off_t) 1875, SEEK_SET); "

Front 37

What is a 'hole'?

Back 37

A padding of zeros (if you write passed the end of the file)

Front 38

What is the pread function used for?

Back 38

A variation on the read function that takes a file position as an argument.

Front 39

Do pread and pwrite permanently change the file position?

Back 39

"No, upon completion the file position is not updated."

Front 40

Why is pread/pwrite preferable to lseek?

Back 40

"Thread safe, it's possible for one app thread to call lseek(), then before it gets a chance to call read/write another app thread calls lseek and changes the position"

Front 41

What system call would you use to truncate a file if you have a file descriptor?

Back 41

ftruncate()

Front 42

Can you use ftruncate() or truncate() to make a file larger?

Back 42

"Yes, if so, it is padded out with zeros."

Front 43

Do the truncate calls update the current file position?

Back 43

No

Front 44

"These flags are defined where...O_RDONLY, O_WRONLY, or O_RDWR"

Back 44

fcntl.h

Front 45

The functions for using variable arguments to a function is found in...

Back 45

stdarg.h

Front 46

The structure that contains the list of variable arguments is of type...?

Back 46

va_list

Front 47

"To initialize va_list, you need to call...?"

Back 47

va_start()

Front 48

How do you retrieve the value for the next argument?

Back 48

"va_arg(va_list, type)"

Front 49

What function ends the program?

Back 49

"exit(1) (1 means error, 0 means success)"

Front 50

Blocking on more than one file descriptor at a time (without the use of threads is known as what?

Back 50

Multiplexed I/O

Front 51

Why is multiplexed i/o preferable to non-blocking i/o for multiple file descriptors?

Back 51

Non-blocking i/o causes you to read in a loop, continuously tying up resourses. Multiplexed i/o blocks, thus freeing up the processor.

Front 52

What does select's signature look like?

Back 52

int select (int n,
fd_set *readfds,
fd_set *writefds,
fd_set *exceptfds,
struct timeval *timeout);

Front 53

What are the three interfaces for multiplexed i/o?

Back 53

Select, poll and epoll

Front 54

What is the timeout argument used for in select?

Back 54

If the timeout arg is not null, the call will return after that time period if no fd's are ready for I/O

Front 55

The first arg to select (n), what is that?

Back 55

n is equal to the value of the highest valued file descriptor in any set, plus one

Front 56

T or F, the timeval must be reinitialized after every call?

Back 56

True, it must be reinitialized

Front 57

The file descriptor sets used by select are implemented using what data structure?

Back 57

bit arrays

Front 58

How many bytes would you need to represent a range of 0 to 1024 file descriptors (1024 is the default)?

Back 58

128 bytes

Front 59

An file descriptor set (used by select) is of type...?

Back 59

fd_set

Front 60

How can I clear all fd's from an fd_set called myset?

Back 60

FD_ZERO(&fd_set);

Front 61

How can we add a file descriptor to a given set?

Back 61

FD_SET(fd, &fd_set);

Front 62

How can we remove a file descriptor from a given set?

Back 62

FD_CLR(fd, &fd_set);

Front 63

How can we check if a file descriptor is set?

Back 63

FD_ISSET(fd, &fd_set));

Front 64

On success, select returns what?

Back 64

The number of fd's ready for I/O

Front 65

Why use select as a portable subsecond-resolution sleep?

Back 65

Implemented on lots of Unix systems, call would be select(0,NULL, NULL, NULL, &time);

Front 66

What's the diff between select() and pselect()?

Back 66

pselect does not reset the timeout param, so no need to reinitialize each time. And pselect uses the sigmask parameter.

Front 67

What the signature of poll look like?

Back 67

int poll (struct pollfd *fds, unsigned int nfds, int timeout);

Front 68

Instead of using 3 fd_set structures like select(), epoll uses an array of what?

Back 68

struct pollfd

Front 69

What does a struct pollfd look like?

Back 69

struct pollfd {
int fd; /* file descriptor */
short events; /* requested events to watch */
short revents; /* returned events witnessed */
};