1 Resource Duplication Across Forks

子进程是父进程的复制,在fork瞬间复制所有信息
1. 复制父进程的代码
2. 复制父进程的状态,包含全部的内存空间,比如变量

/*shared_variable.c*/int main(int argc, char * argv[]){int status;pid_t cpid, pid;int i=0;while(1){ //loop and fork childrencpid = fork();if( cpid == 0 ){/* CHILD */pid = getpid();printf("Child: %d: i:%d\n", pid, i);//set i in child to something differnti *= 3;printf("Child: %d: i:%d\n", pid, i);_exit(0); //NO FORK BOMB!!!}else if ( cpid > 0){/* PARENT *///wait for childwait(&status);//print i after waitingprintf("Parent: i:%d\n", i);i++;if (i > 5){       break; //break loop after 5 iterations}}else{/* ERROR */perror("fork");return 1;}//pretty printprintf("--------------------\n");}return 0;
}

>./shared_variables
Child: 3833: i:0
Child: 3833: i:0
Parent: i:0
--------------------
Child: 3834: i:1
Child: 3834: i:3
Parent: i:1
--------------------
Child: 3835: i:2
Child: 3835: i:6
Parent: i:2
--------------------
Child: 3836: i:3
Child: 3836: i:9
Parent: i:3
--------------------
Child: 3837: i:4
Child: 3837: i:12
Parent: i:4
--------------------
Child: 3838: i:5
Child: 3838: i:15
Parent: i:5

1.1 File Descriptor’s across Forks

子复制父的文件描述符
父子同时对文件进行读写,共享offset

/*shared_file.c*/
int main(int argc, char * argv[]){int fd, status;pid_t cpid;char c;if ( argc < 2){fprintf(stderr, "ERROR: Require path\n");return 1;}//shared between all children and parentif( (fd = open(argv[1], O_RDONLY)) < 0){perror("open");return 1;}while (1){cpid = fork();if( cpid == 0 ){/* CHILD *///try and read 1 byte from fileif( read(fd, &c, 1) > 0){printf("c: %c\n", c); // print the char_exit(0); //exit with status 0 on sucess read}else{//no more to read_exit(1); //exit with status 1 on failed read}}else if ( cpid > 0){/* PARENT *///wait for child to read firstwait(&status);//if exit status 1, break the loop, no more to readif( WEXITSTATUS(status) ){break; }//now parent reads a byteif( read(fd, &c, 1) > 0){printf("p: %c\n", c); // print the char}}else{/* ERROR */perror("fork");return 1;}}//done reading the fileclose(fd);return 0;
}

2 Inter-Process Communication and Pipes

pipeline是进程之间外部通讯(一个进程,接收另一个进程执行的结果),pipe是进程内部通讯

int main(int argc, char * argv[]){//print hello world through a pipe!char hello[] = "Hello World!\n";char c;int pfd[2]; //pfd[0] reading end of pipe//pfd[1] writing end of pipe//open the pipeif( pipe(pfd) < 0){perror("pipe");return 1;}//write hello world to pipewrite(pfd[1], hello, strlen(hello));//close write end of pipeclose(pfd[1]);//read hello world from pipe, write to stdoutwhile( read(pfd[0], &c, 1)){write(1, &c, 1);}//close the read end of the pipeclose(pfd[0]);return 0;
}

2.2 Pipes Bursting! and Blocking!

O.S. and the C standard library provides some amount of buffering on reads and writes. (C库和操作系统读写都有buffer)
pipe就是kernel中的buffer
when the pipe is full, the write will block.
fcntl() or file descriptor control.通过fd来改变buffer的属性fcntl(pfd[1], F_SETFL, O_NONBLOCK);

/*pipe_block.c*/
int main(int argc, char * argv[]){char c = 'A';int i;int pfd[2]; //pfd[0] reading end of pipe//pdf[1] writing end of pipe//open the pipeif( pipe(pfd) < 0){perror("pipe");return 1;}//write A's to pipe until it's fulli = 0;while( write(pfd[1], &c, 1) > 0){printf("%d\n",i);i++;}perror("write");//close write end of pipeclose(pfd[1]);//read from pipe?!?while( read(pfd[0], &c, 1)){write(1, &c, 1);}close(pfd[0]);return 0;
}

2.3 Inter Process Pipes

1.子进程复制父进程,所以pipe对应的文件描述符也会被复制

/*pipe_hello_to_child.c*/
int main(int argc, char * argv[]){//print hello world through a pipe! To child!char hello[] = "Hello World!\n";char c;int pfd[2]; //pfd[0] reading end of pipe//pfd[1] writing end of pipepid_t cpid;int status;//open a pipe, pfd[0] for reading, pfd[1] for writingif ( pipe(pfd) < 0){perror("pipe");return 1;}cpid = fork();if( cpid == 0 ){/* CHILD *///close the writing end in childclose(pfd[1]);//try and read 1 byte from pipe, write byte stdoutwhile( read(pfd[0], &c, 1) > 0){write(1, &c,1); }//close pipeclose(pfd[0]);_exit(0);    }else if ( cpid > 0){/* PARENT *///close reading end in parentclose(pfd[0]);//write hello world to pipewrite(pfd[1], hello, strlen(hello));//close the pipeclose(pfd[1]);//wait for childwait(&status);}else{/* ERROR */perror("fork");return 1;}return 0;
}

3 Duplicating File Descriptor and Pipelines

1.pipe 是通过文件描述符来实现进程内部通讯
2.pipline则是通过stdout和stdin来进程进程外部通讯

3.1 Duplicating a File Descriptor

int dup2(int filedes, int filedes2);
duplicate the file descriptor fildes onto the file descriptor filedes2
read and write from fildes2 now, it would be the same as reading and writing from filedes

/*hello_dup.c*/
int main(int argc, char * argv[]){//print hello world to a file with dupint fd;//check argsif ( argc < 2){fprintf(stderr, "ERROR: Require destination path\n");return 1;}//open destination fileif( (fd = open(argv[1], O_WRONLY | O_TRUNC | O_CREAT , 0644)) < 0){perror("open");return 1;}//close standard outclose(1);//duplicate fd to stdoutdup2(fd, 1);//print to stdout, which is now duplicated to fdprintf("Hello World!\n");return 0;
}

3.2 Setting up a pipeline

   /*pipe_dup.c*/int main(int argc, char * argv[]){int status;int pfd[2];pid_t cpid;char c;//open a pipe, pfd[0] for reading, pfd[1] for writingif ( pipe(pfd) < 0){perror("pipe");return 1;}//Setup a pipe between child 1 and child 2, like:// parent | childcpid = fork();if( cpid == 0 ){/* CHILD 1*///close stdinclose(0);//duplicate reading end to stdindup2(pfd[0], 0);//close the writing endclose(pfd[1]);//try and read 1 byte from stding and write stdoutwhile( read(0, &c, 1) > 0){ //stdin now pipe!write(1, &c,1); //this is still stdout}exit(0);} else if ( cpid > 0){/* PARENT *///close stdoutclose(1);//duplicate writing end to stdoutdup2(pfd[1], 1);//close reading end close(pfd[0]);//read and read 1 byte from stdin, write byte to pipewhile( read(0,&c,1) > 0){write(1, &c, 1);}//close the pipe and stdoutclose(pfd[1]);close(1);//wait for childwait(&status);}else{/* ERROR */perror("fork");return 1;}return 0;
}

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18 File Duplication and Pipes

1 Resource Duplication Across Forks

1.1 File Descriptor’s across Forks

2 Inter-Process Communication and Pipes

2.2 Pipes Bursting! and Blocking!

2.3 Inter Process Pipes

3 Duplicating File Descriptor and Pipelines

3.1 Duplicating a File Descriptor

3.2 Setting up a pipeline

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