使用C语言的fork()函数在Linux中创建进程的实例讲解

/* $begin csapp.h */
#ifndef __CSAPP_H__
#define __CSAPP_H__

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <ctype.h>
#include <setjmp.h>
#include <signal.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <errno.h>
#include <math.h>
#include <pthread.h>
#include <semaphore.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/in.h>
#include <arpa/inet.h>


/* Default file permissions are DEF_MODE & ~DEF_UMASK */
/* $begin createmasks */
#define DEF_MODE  S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH
#define DEF_UMASK S_IWGRP|S_IWOTH
/* $end createmasks */

/* Simplifies calls to bind(), connect(), and accept() */
/* $begin sockaddrdef */
typedef struct sockaddr SA;
/* $end sockaddrdef */

/* Persistent state for the robust I/O (Rio) package */
/* $begin rio_t */
#define RIO_BUFSIZE 8192
typedef struct {
  int rio_fd;        /* Descriptor for this internal buf */
  int rio_cnt;        /* Unread bytes in internal buf */
  char *rio_bufptr;     /* Next unread byte in internal buf */
  char rio_buf[RIO_BUFSIZE]; /* Internal buffer */
} rio_t;
/* $end rio_t */

/* External variables */
extern int h_errno;  /* Defined by BIND for DNS errors */ 
extern char **environ; /* Defined by libc */

/* Misc constants */
#define  MAXLINE   8192 /* Max text line length */
#define MAXBUF  8192 /* Max I/O buffer size */
#define LISTENQ 1024 /* Second argument to listen() */

/* Our own error-handling functions */
void unix_error(char *msg);
void posix_error(int code, char *msg);
void dns_error(char *msg);
void app_error(char *msg);

/* Process control wrappers */
pid_t Fork(void);
void Execve(const char *filename, char *const argv[], char *const envp[]);
pid_t Wait(int *status);
pid_t Waitpid(pid_t pid, int *iptr, int options);
void Kill(pid_t pid, int signum);
unsigned int Sleep(unsigned int secs);
void Pause(void);
unsigned int Alarm(unsigned int seconds);
void Setpgid(pid_t pid, pid_t pgid);
pid_t Getpgrp();

/* Signal wrappers */
typedef void handler_t(int);
handler_t *Signal(int signum, handler_t *handler);
void Sigprocmask(int how, const sigset_t *set, sigset_t *oldset);
void Sigemptyset(sigset_t *set);
void Sigfillset(sigset_t *set);
void Sigaddset(sigset_t *set, int signum);
void Sigdelset(sigset_t *set, int signum);
int Sigismember(const sigset_t *set, int signum);

/* Unix I/O wrappers */
int Open(const char *pathname, int flags, mode_t mode);
ssize_t Read(int fd, void *buf, size_t count);
ssize_t Write(int fd, const void *buf, size_t count);
off_t Lseek(int fildes, off_t offset, int whence);
void Close(int fd);
int Select(int n, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, 
    struct timeval *timeout);
int Dup2(int fd1, int fd2);
void Stat(const char *filename, struct stat *buf);
void Fstat(int fd, struct stat *buf) ;

/* Memory mapping wrappers */
void *Mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);
void Munmap(void *start, size_t length);

/* Standard I/O wrappers */
void Fclose(FILE *fp);
FILE *Fdopen(int fd, const char *type);
char *Fgets(char *ptr, int n, FILE *stream);
FILE *Fopen(const char *filename, const char *mode);
void Fputs(const char *ptr, FILE *stream);
size_t Fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
void Fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);

/* Dynamic storage allocation wrappers */
void *Malloc(size_t size);
void *Realloc(void *ptr, size_t size);
void *Calloc(size_t nmemb, size_t size);
void Free(void *ptr);

/* Sockets interface wrappers */
int Socket(int domain, int type, int protocol);
void Setsockopt(int s, int level, int optname, const void *optval, int optlen);
void Bind(int sockfd, struct sockaddr *my_addr, int addrlen);
void Listen(int s, int backlog);
int Accept(int s, struct sockaddr *addr, socklen_t *addrlen);
void Connect(int sockfd, struct sockaddr *serv_addr, int addrlen);

/* DNS wrappers */
struct hostent *Gethostbyname(const char *name);
struct hostent *Gethostbyaddr(const char *addr, int len, int type);

/* Pthreads thread control wrappers */
void Pthread_create(pthread_t *tidp, pthread_attr_t *attrp, 
      void * (*routine)(void *), void *argp);
void Pthread_join(pthread_t tid, void **thread_return);
void Pthread_cancel(pthread_t tid);
void Pthread_detach(pthread_t tid);
void Pthread_exit(void *retval);
pthread_t Pthread_self(void);
void Pthread_once(pthread_once_t *once_control, void (*init_function)());

/* POSIX semaphore wrappers */
void Sem_init(sem_t *sem, int pshared, unsigned int value);
void P(sem_t *sem);
void V(sem_t *sem);

/* Rio (Robust I/O) package */
ssize_t rio_readn(int fd, void *usrbuf, size_t n);
ssize_t rio_writen(int fd, void *usrbuf, size_t n);
void rio_readinitb(rio_t *rp, int fd); 
ssize_t  rio_readnb(rio_t *rp, void *usrbuf, size_t n);
ssize_t  rio_readlineb(rio_t *rp, void *usrbuf, size_t maxlen);

/* Wrappers for Rio package */
ssize_t Rio_readn(int fd, void *usrbuf, size_t n);
void Rio_writen(int fd, void *usrbuf, size_t n);
void Rio_readinitb(rio_t *rp, int fd); 
ssize_t Rio_readnb(rio_t *rp, void *usrbuf, size_t n);
ssize_t Rio_readlineb(rio_t *rp, void *usrbuf, size_t maxlen);

/* Client/server helper functions */
int open_clientfd(char *hostname, int portno);
int open_listenfd(int portno);

/* Wrappers for client/server helper functions */
int Open_clientfd(char *hostname, int port);
int Open_listenfd(int port); 

#endif /* __CSAPP_H__ */
/* $end csapp.h */

#include "csapp.h" 
int main()
{
 pid_t pid;
 int x=1;
 pid=fork();
 if(pid==0) {
  printf("child :x=%d\n",++x);
  exit(0);
 }
 printf("parent:x=%d\n",--x);
 exit(0);
}

#include "csapp.h"

int main()
{
 if(fork()==0) {
  printf("a");
 }
 else {
  printf("b");
  waitpid(-1,NULL,0);
 }
 printf("c");
 exit(0);
}

#include "csapp.h"
int main()
{
 int x=1;
 if(fork()==0)
  printf("printf1:x=%d\n",++x);
 printf("printf2:x=%d\n",--x);
 exit(0);
}

printf2:x=0
printf1:x=2
printf2:x=1

#include "csapp.h"
#define N 3
int main()
{
 int status,i;
 pid_t pid; 

 for(i=0;i<N;i++)
  if((pid=fork())==0) //新建子进程
   exit(100+i); 

 while((pid=waitpid(-1,&status,0))>0) { //如果子进程是正常终止的，就返回进程的进程号PID
  if(WIFEXITED(status)) //返回退出状态
   printf("child %d terminated normally with exit status =%d\n",pid,WEXITSTATUS(status));
  else
   printf("child %d erminated abnormally\n",pid);
 }

 if(errno!=ECHILD)
  printf("waitpid error\n");
 exit(0);
}

child 28693 terminated normally with exit status =100
child 28694 terminated normally with exit status =101
child 28695 terminated normally with exit status =102

#include "csapp.h"
#define N 2
int main()
{
 int status,i;
 pid_t pid[N],retpid;

 for(i=0;i<N;i++)
  if((pid[i]=fork())==0)
   exit(100+i);//退出并返回状态码
 i=0;
 while((retpid=waitpid(pid[i++],&status,0))>0) {
  if(WIFEXITED(status))
   printf("child %d terminated normally with exit status=%d\n",retpid,WEXITSTATUS(status));
  else
   printf("child %d terminated abnormally\n",retpid);
 }

 if(errno !=ECHILD)
  printf("waitpid error!");
 exit(0);
}

child 29846 terminated normally with exit status=100
child 29847 terminated normally with exit status=101

#include "csapp.h"

/*推测此程序会输出什么样的结果*/

int main()
{
 int status;
 pid_t pid;

 printf("Hello\n");
 pid=fork();
 printf("%d\n",!pid);
 if(pid!=0) {
  if(waitpid(-1,&status,0)>0) {
   if(WIFEXITED(status)!=0)
  printf("%d\n",WEXITSTATUS(status));
  }
 }
 printf("Bye\n");
 exit(2);
}

Hello
0
1
Bye
2
Bye

Hello
1
Bye
0
2
Bye