操作系统（OS）是一种程序，它在最初被启动程序加载到计算机中后，管理着计算机中的所有其他应用程序。应用程序通过定义的应用程序接口（API）对服务提出请求，从而使用操作系统。此外，用户可以通过用户界面直接与操作系统互动，如命令行界面（CLI）或图形用户界面（GUI）。
为什么要使用操作系统？
操作系统给计算机软件和软件开发带来了强大的好处。如果没有操作系统，每个应用程序都需要包括自己的用户界面，以及处理底层计算机所有低级功能所需的综合代码，如磁盘存储、网络接口等。考虑到现有的大量底层硬件，这将大大增加每个应用程序的规模，使软件开发变得不切实际。
相反，许多常见的任务，如发送网络数据包或在标准输出设备（如显示器）上显示文本，可以卸载到作为应用程序和硬件之间中介的系统软件中。系统软件为应用程序与硬件的交互提供了一致的、可重复的方式，而应用程序不需要知道关于硬件的任何细节。
只要每个应用程序以相同的方式访问相同的资源和服务，该系统软件–操作系统–可以为几乎所有数量的应用程序提供服务。这大大减少了开发和调试应用程序所需的时间和编码量，同时确保用户可以通过一个通用的、被充分理解的界面来控制、配置和管理系统硬件。
操作系统的学习也是较为复杂，当基础知识不扎实的时候，面对作业难题时难以解决。当你在学习操作系统课程的过程中遇到任何的难题，请联系TopMask代写，24小时在线为您服务，为您提供最专业的OS操作系统代写服务。
下面是一个操作系统代写的实操案例，关于函数返回值的应用：
fork()
As we have already seen in class, the fork() command makes a complete copy of the running process and the only way to differentiate the two is by looking at the returned value:
- fork() returns the process identifier (pid) of the child process in the parent, and
- fork() returns 0 in the child.
For example, the following program performs a simple fork. The return value of fork() is pid_t (defined in the library header file <sys/types.h>; however, below it is simply assigned and implicitly cast to an int.
#include <stdio.h>
/* This program forks and and the prints whether the process is
-the child (the return value of fork() is 0), or
-the parent (return value of fork() is not zero)
When this was run 100 times on the computer the author is on, only twice did the
parent process execute before the child process executed.
*/
int main( void ) {
int pid = fork();
if ( pid == 0 ) {
printf( "This is being printed from the child process\n" );
} else {
printf( "This is being printed in the parent process:\n"
" - the process identifier (pid) of the child is %d\n", pid);
}
return 0;
}
This program, when compiled and executed on this computer prints
% gcc fork.c
% ./a.out
This is being printed from the child process
This is being printed in the parent process:
- the process identifier (pid) of the child is 24338
%getpid()
Every process can query its own process identifier using the getpid().
#include <stdio.h>
/* This program demonstrates that the use of the getpid() function. The return value
of fork() is actually pid_t ('pid' type). When assigned to 'int pid', if the type is
different, there’s an implicit cast; however, when we print the return value of
getpid(), it is necessary to explicitly cast it as an integer. The type 'pid_t' is
defined in the library header <sys/types.h>
*/
int main( void ) {
printf( "The process identifier (pid) of the parent process is %d\n",
(int)getpid() );
int pid = fork();
if ( pid == 0 ) {
printf( "After the fork, the process identifier (pid) "
"of the child is %d\n", (int)getpid() );
} else {
printf( "After the fork, the process identifier (pid) "
"of the parent is still %d\n - fork() returned %d\n",
(int)getpid(), pid );
}
return 0;
}When executed, this prints
% gcc getpid.c
% ./a.out
The process identifier (pid) of the parent process is 25201
After the fork, the process identifier (pid) of the child is 25202
After the fork, the process identifier (pid) of the parent is still 25201
- fork() returned 25202
% As you may guess, approximately 1000 processes were created by the operating system between the time that the source code for fork.c was executed and the time that getpid.c was executed.
