Linux 中部分系统的调用的简单笔记。

# 系统调用简表

该表来自 xv-6 讲义 Figure 1-2.

System call	Description
`int fork()`	Create a process, return child’s PID.
int exit(int status)	Terminate the current process; status reported to wait(). No return.
`int wait(int *status)`	Wait for a child to exit; exit status in *status; returns child PID.
int kill(int pid)	Terminate process PID. Returns 0, or -1 for error.
int getpid()	Return the current process’s PID.
int sleep(int n)	Pause for n clock ticks.
int exec(char file, char argv[])	Load a file and execute it with arguments; only returns if error.
char *sbrk(int n)	Grow process’s memory by n bytes. Returns start of new memory.
`int open(char *file, int flags)`	Open a file; flags indicate read/write; returns an fd (file descriptor).
`int write(int fd, char *buf, int n)`	Write n bytes from buf to file descriptor fd; returns n.
`int read(int fd, char *buf, int n)`	Read n bytes into buf; returns number read; or 0 if end of file.
int close(int fd)	Release open file fd.
`int dup(int fd)`	Return a new file descriptor referring to the same file as fd.
`int pipe(int p[])`	Create a pipe, put read/write file descriptors in p[0] and p[1].
int chdir(char *dir)	Change the current directory.
int mkdir(char *dir)	Create a new directory.
int mknod(char *file, int, int)	Create a device file.
`int fstat(int fd, struct stat *st)`	Place info about an open file into *st.
`int stat(char file, struct stat st)`	Place info about a named file into *st.
int link(char file1, char file2)	Create another name (file2) for the file file1.
int unlink(char *file)	Remove a file.

# fork

fork() 系统调用通过复制当前进程创建新进程。子进程从父进程的初始状态开始运行，在父进程 fork() 处停止运行并返回 0 . 父进程 fork() 返回子进程的 PID. 下面是一个样例：

	#include <stdio.h>
	#include <stdlib.h>
	#include <unistd.h>
	int
	main(int argc, char *argv[])
	{
	printf("hello world (pid:%d)\n", (int) getpid());
	int rc = fork();
	if (rc < 0) { // fork failed; exit
	fprintf(stderr, "fork failed\n");
	exit(1);
	} else if (rc == 0) { // child (new process)
	printf("hello, I am child (pid:%d)\n", (int) getpid());
	} else { // parent goes down this path (main)
	printf("hello, I am parent of %d (pid:%d)\n",
	rc, (int) getpid());
	}
	return 0;
	}

该程序的输出可能为

hello world (pid:29146)
hello, I am parent of 29147 (pid:29146)
hello, I am child (pid:29147)

也可能为

hello world (pid:29146)
hello, I am child (pid:29147)
hello, I am parent of 29147 (pid:29146)

存在多种可能输出的原因是，父进程和子进程是并发执行的。

exec(char* file, char *argv[]) 的作用是执行一个文件中的程序，其传入参数 file 为文件名， argv 为新程序的指令。当 exec 成功被调用时，它将没有返回。仅当其运行出现错误时返回。

参考一只小香羊: linux 管道 pipe 使用。

pipe()