a) Write a brief description about the following system calls used in Unix/Linux

Question

a) Write a brief description about the following system calls used in Unix/Linux. Include information

such as parameters, resources used, expected results/return values, and possible error conditions.

(Hint: use the man pages)

? Open

? Close

? Read

? Write

? Fork

b) Explain which of the following systems calls should be privileged and why.

1. Manipulating data in physical memory

2. Set the value of the timer

3. Switch to kernel mode

4. Acess IO device

5. Check the current list of running processes

Explanation / Answer

OPEN :

SYNOPSIS

#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);

DESCRIPTION

Given a pathname for a file, open() returns a file descriptor, a small, non-negative integer for use in subsequent system calls (read(2), write(2), lseek(2), fcntl(2), etc.). The file descriptor returned by a successful call will be the lowest-numbered file descriptor not currently open for the process.

The new file descriptor is set to remain open across an execve(2) (i.e., the FD_CLOEXEC file descriptor flag described in fcntl(2) is initially disabled). The file offset is set to the beginning of the file (see lseek(2)).

A call to open() creates a new open file description, an entry in the system-wide table of open files. This entry records the file offset and the file status flags (modifiable via the fcntl()F_SETFL operation). A file descriptor is a reference to one of these entries; this reference is unaffected if pathname is subsequently removed or modified to refer to a different file. The new open file description is initially not shared with any other process, but sharing may arise viafork(2).

The parameter flags must include one of the following access modes: O_RDONLY, O_WRONLY, or O_RDWR. These request opening the file read-only, write-only, or read/write, respectively.

RETURN VALUE

open() and creat() return the new file descriptor, or -1 if an error occurred (in which case,errno is set appropriately).

ERRORS

Tag

Description

EACCES

The requested access to the file is not allowed, or search permission is denied for one of the directories in the path prefix of pathname, or the file did not exist yet and write access to the parent directory is not allowed. (See also path_resolution(2).)

EEXIST

pathname already exists and O_CREAT and O_EXCL were used.

EFAULT

pathname points outside your accessible address space.

EISDIR

pathname refers to a directory and the access requested involved writing (that is, O_WRONLY or O_RDWR is set).

ELOOP

Too many symbolic links were encountered in resolving pathname, orO_NOFOLLOW was specified but pathname was a symbolic link.

EMFILE

The process already has the maximum number of files open.

ENAMETOOLONG

pathname was too long.

ENFILE

The system limit on the total number of open files has been reached.

ENODEV

pathname refers to a device special file and no corresponding device exists. (This is a Linux kernel bug; in this situation ENXIO must be returned.)

ENOENT

O_CREAT is not set and the named file does not exist. Or, a directory component in pathname does not exist or is a dangling symbolic link.

ENOMEM

Insufficient kernel memory was available.

ENOSPC

pathname was to be created but the device containing pathname has no room for the new file.

ENOTDIR

A component used as a directory in pathname is not, in fact, a directory, or O_DIRECTORY was specified and pathname was not a directory.

ENXIO

O_NONBLOCK | O_WRONLY is set, the named file is a FIFO and no process has the file open for reading. Or, the file is a device special file and no corresponding device exists.

EOVERFLOW

pathname refers to a regular file, too large to be opened; see O_LARGEFILE above.

EPERM

The O_NOATIME flag was specified, but the effective user ID of the caller did not match the owner of the file and the caller was not privileged (CAP_FOWNER).

EROFS

pathname refers to a file on a read-only filesystem and write access was requested.

ETXTBSY

pathname refers to an executable image which is currently being executed and write access was requested.

EWOULDBLOCK

The O_NONBLOCK flag was specified, and an incompatible lease was held on the file (see fcntl(2)).

BUGS

"The thing that has always disturbed me about O_DIRECT is that the whole interface is just stupid, and was probably designed by a deranged monkey on some serious mind-controlling substances."

#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);

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