Linux/Unix System Programming

The sysctl() system call is used to query and modify kernel parameters, including system memory information. Options A and C refer to non-standard functions, while malloc() is for memory allocation, not querying system memory. On modern Linux, /proc/meminfo is also commonly used.

A socket is a software abstraction that acts as an endpoint for sending and receiving data across processes over a network or locally.

A semaphore is a synchronization variable used to control access to shared, non-shareable resources by multiple processes in concurrent systems.

A message queue is an IPC mechanism where reading a message removes it from the queue (destructive read). Once a process reads a message, other processes cannot read the same message. This contrasts with shared memory where multiple processes can read the same data repeatedly (non-destructive read).

The key difference is in read semantics: shared memory allows non-destructive reads (multiple processes read the same data), while message queues have destructive reads (messages are removed after being read). Option D has the direction reversed.

The sed command 's/ *$//' removes trailing spaces from each line - it matches zero or more spaces at the end of the line and replaces them with nothing. The $ anchor matches the end of line, and * matches zero or more spaces. This effectively strips trailing whitespace.

The dup() system call duplicates an open file descriptor, returning a new descriptor that refers to the same open file description.

The default shell for a user is stored in /etc/passwd file. The last field of each line specifies the login shell. Options A, B, and C are incorrect - there's no chshell command, and the paths are wrong (/var/passwd and /etc/users don't exist for this purpose). The chsh command is actually the preferred method to interactively change shells.

The at utility is designed to execute a job once at a specified time, taking input from the user/stdin, whereas cron is used for recurring scheduled tasks defined in configuration files (crontabs).

The standard mount command is used to mount filesystems. The -t option specifies the filesystem type, which is often required for temporary manual mounts.

The lprm command is the standard Unix/Linux utility used to remove active print jobs from the print spool queue.

Runlevel 6 is system reboot. At this runlevel, the system is shutting down and restarting, so /etc/fstab shouldn't be altered as the system is in an unstable state. While technically possible to edit, it's not safe practice as filesystems are being unmounted and the system is terminating.

fork() creates a new child process by duplicating the calling process, while exec() replaces the current process image with a new program. They perform entirely different tasks.

The system() function returns the termination status of the shell command it executed, which can be processed using standard macros like WIFEXITED and WEXITSTATUS.

The system() function executes a shell command and can be run by any user; it does not require root privileges, so the statement is false.

The lpc (line printer control) command is used to manage print queues and printers. It allows administrators to control printer queues, start/stop printing, check queue status, and manage print jobs. Options B and C are incorrect as lpc doesn't handle modems or flash drives. Option D is wrong as lpc is a valid Unix command.

/proc/kcore is a file in the /proc filesystem that represents the system's physical RAM in core dump format. It provides a virtual view of the entire system memory, similar to how a core dump captures process memory. Options A and D are incorrect because /proc/kcore is not related to the init process or individual program core dumps.

The error indicates the dynamic linker cannot find a required shared library at runtime. The LD_LIBRARY_PATH environment variable tells the dynamic linker where to search for shared libraries beyond the default paths. Options A, B, and D are incorrect: PATH is for executables, SHELLPATH doesn't exist for this purpose, and LIBRARY_PATH is used at compile time, not runtime.

Linux's setuid implementation has security restrictions that differ from traditional Unix systems. Modern Linux kernels restrict setuid on scripts and have additional security mechanisms like file capabilities that can affect setuid behavior. The statement oversimplifies the compatibility between Linux setuid and traditional Unix setuid.