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LINUX System Programming Process

Operating System Responsibilities: Manage all resources and associate different devices and programs.

System calls are a set of special function interfaces provided by the operating system to user programs, through which user programs can obtain services provided by the system kernel, such as opening files, closing files, reading and writing files.

POSIX(Portable Operation System Interface)

In Linux, application programming interfaces (APIs) follow the POSIX standard and are used to ensure that applications can be ported across multiple operating systems at the source code level.

According to functional logic, system calls can be divided into: process control, inter-process communication, file system control, system control, memory management, network management, socket control, user management.

A negative return value is usually used to indicate an error, a value of 0 indicates success, and perror can be used to print error messages.

V. PROCESS

Program: executable file (static), Process: execution instance of program (dynamic)

Status of process: created, scheduled, dead

Processes have their own environments and resources

The exec function is used to read a program from the kernel into memory so that it executes as a process

VI. LIFE CYCLE OF THE PROCESS

create

scheduling

Ready state: execution conditions are met, waiting for allocation of CPU time

Execution status: CPU occupied

Waiting state: some execution conditions are not met, and execution cannot continue.

demise

PCB (Process Control Block)

OS controls and manages concurrent processes according to PCB. When creating a process, the system will open up a memory space to store PCB data structures related to this process.

The PCB records all the information needed to describe the progress of the process and control its operation.

PCB is the only sign of process existence. In linux, PCB is stored in task_struct structure. Open/include/linux/sched.h to find the definition of task_struct.

VIII. Process control

Process No. 0~32767 Scheduling Process No. 0 Init Process No. 1

All processes except the scheduling process are created directly or indirectly by the init process

PID process number getpid() PPID parent process number getppid() PGID process group number getpgid()

IX. File descriptors: assign 0-1023 the smallest available descriptors in units of process

X. Process state transition

CPU scheduling algorithm: priority, first come first scheduling, short time priority, time slice rotation

umask mask, masking file permissions

ps command to view the current process

Fork() vfork()

1. fork () returns 0 in the child process, returns a value greater than 0 in the parent process, after calling fork(), the source program is copied, only fork() returns different values in the two programs, the two programs are executed at the same time, the order is uncertain, the physical address of the parent-child process is different, and the virtual address is the same.

Q: If a program executes three forks simultaneously, it creates several processes

fork();1(0)

fork();2(0) 3(1)

fork();4(0) 5(1) 6(2) 7(3)

First call, process 0 creates process 1

On the second call, process 0 creates process 2 and process 1 creates process 3.

On the third call, process 0 creates process 4, process 1 creates process 5, process 2 creates process 6, and process 3 creates process 7.

Two to the nth minus one process was created.

2.vfork() is generally used in conjunction with exec. Child processes share the address space of the parent process, ensuring that the child process runs first, and the parent process runs after calling exec or exit.

The exec function family is a process replacement function that assigns a new address space to the original process, the process number remains unchanged, and the new program runs from the main.

exec returns only if it fails. The code after exec is not executed.

But note that exec is a family of functions with execlp,execvp,execle,execve, etc.

The child process can only be terminated with exit(), while return is the first address to return to the function execution

aexit (void (*function)(void)) Register exit handler function

XII. Input and output buffers

system call unbuffered

Standard IO library function inputs and outputs are line buffers

Standard IO library operation files are fully buffered

XIII.

Zombie process: child process ends without parent process calling wait() or waitpid() to reclaim its resources

Orphan process: parent process ends but child process does not end

Daemons: Special orphan processes that run in the background outside the terminal

wait()/waitpid(): Wait for the child process to end and reclaim its resources

Use ps to view the process, kill+pid or pkill+ process name to end the process

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