和菜鸟一起深入学习国嵌实验之进程创建，exec函数，进程等待

1、 fork进程创建

代码：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>
#include <math.h>
 
int main(int argc, char *argv[])
{
   pid_t child;
 
   //create child process
   if((child = fork()) == -1)
    {
       printf("Fork Error: %s\n", strerror(errno));
       exit(1);
    }
   else
    {
       if(child == 0)
       {
           printf("I am the child : %d\n", getpid());
           exit(0);
        }
       else
       {
           printf("I am the father : %d\n", getpid());
           return 0;
       }
    }
 
   return 0;
}

Makefile：

CC = gcc
 
CURTDIR = $(shell pwd)
TARGET = myfork
 
%.o:%.c
       $(CC)-c $(EXTRAFLAGS) $< -o $@
%.o:%.S
       $(CC)-c $(EXTRAFLAGS) $< -o $@
 
.PHONY: all clean
 
$(TARGET): $(TARGET).o
       $(CC)  -o $@ $^
 
clean:
       rm-rf $(TARGET) $(TARGET).o

运行结果：

eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.8$make
gcc -c myfork.c -o myfork.o
gcc -o myfork myfork.o
 
eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.8$ls
Makefile myfork  myfork.c  myfork.o
 
eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.8$./myfork
I am the father : 8694
eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.8$I am the child : 8695

总结：fork函数创建子进程后，父子进程是独立、同时运行的，并没有先后顺序之分。

2、vfork进程创建

代码：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>
#include <math.h>
 
int main(int argc, char *argv[])
{
   pid_t child;
 
   //create child process
   if((child = vfork()) == -1)
    {
       printf("Fork Error: %s\n", strerror(errno));
       exit(1);
    }
   else
    {
       if(child == 0)
       {
           sleep(1);
           printf("I am the child :%d\n", getpid());
           exit(0);
       }
       else
       {
           printf("I am the father : %d\n", getpid());
           return 0;
       }
    }
 
   return 0;
}

Makefile：

CC = gcc
 
CURTDIR = $(shell pwd)
TARGET = myvfork
 
%.o:%.c
       $(CC)-c $(EXTRAFLAGS) $< -o $@
%.o:%.S
       $(CC)-c $(EXTRAFLAGS) $< -o $@
 
.PHONY: all clean
 
$(TARGET): $(TARGET).o
       $(CC)  -o $@ $^
 
clean:
       rm-rf $(TARGET) $(TARGET).o

运行结果：

eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.9$make
gcc -c myvfork.c -o myvfork.o
gcc -o myvfork myvfork.o
 
eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.9$ls
Makefile myvfork  myvfork.c  myvfork.o
 
eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.9$./myvfork
I am the child : 8870
I am the father : 8869

总结：调用fork函数产生的父子进程的运行顺序是不定的，而调用vfork函数产生的父子进程必定是子进程先运行完，父进程再运行的。

3、 exec函数族

代码：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>
#include <math.h>
 
int main(int argc, char *argv[])
{
   if(argc < 2)
    {
       perror("you haven't input the filename, please try again!\n");
       exit(EXIT_FAILURE);
    }
  
   if(execl("./file_create", "file_creat", argv[1],NULL) < 0)
       perror("execl error!\n");
 
   return 0;
}

Makefile：

CC = gcc
 
CURTDIR = $(shell pwd)
TARGET = myexec
 
%.o:%.c
       $(CC)-c $(EXTRAFLAGS) $< -o $@
%.o:%.S
       $(CC)-c $(EXTRAFLAGS) $< -o $@
 
.PHONY: all clean
 
$(TARGET): $(TARGET).o
       $(CC)  -o $@ $^
 
clean:
       rm-rf $(TARGET) $(TARGET).o

运行结果：

eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.10$make
gcc -c myexec.c -o myexec.o
gcc -o myexec myexec.o
 
eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.10$ls
file_create Makefile  myexec  myexec.c myexec.o
 
 
eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.10$./myexec file
create file file success!
 
eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.10$ls
file file_create  Makefile  myexec myexec.c  myexec.o

总结：exec函数族会在一个进程中启动另一个进程执行。并用它来取代原调用进程的数据段、代码段和堆栈段。在执行完exec函数调用后，原调用进程的内容除了进程号之外，其他全部都被新的进程替换了。

4、等待进程

代码：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>
#include <math.h>
 
int main(int argc, char *argv[])
{
   pid_t child;
 
   //create child process
   if((child = vfork()) == -1)
    {
       printf("Fork Error: %s\n", strerror(errno));
       exit(1);
    }
   else
    {
       if(child == 0)
       {
           printf("the child process is run\n");
           sleep(1);
           printf("I am the child : %d\n", getpid());
           exit(0);
       }
       else
       {
           wait(NULL);
           printf("the father process is run\n");
           printf("I am the father : %d\n", getpid());
            return 0;
       }
    }
 
   return 0;
}

Makefile：

CC = gcc
 
CURTDIR = $(shell pwd)
TARGET = mywait
 
%.o:%.c
       $(CC)-c $(EXTRAFLAGS) $< -o $@
%.o:%.S
       $(CC)-c $(EXTRAFLAGS) $< -o $@
 
.PHONY: all clean
 
$(TARGET): $(TARGET).o
       $(CC)  -o $@ $^
 
clean:
       rm-rf $(TARGET) $(TARGET).o

运行结果：

eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.11$make
gcc -c mywait.c -o mywait.o
gcc -o mywait mywait.o
 
eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.11$ls
Makefile mywait  mywait.c  mywait.o
 
eastmoon@eastmoon-virtual-machine:~/work/guoqian/1/1.11$./mywait
the child process is run
I am the child : 9275
the father process is run
I am the father : 9274

结论：用fork创建子进程后，父子进程的执行顺序fork函数不能确定。我们可以用wait函数和waitpid函数使父进程阻塞等待子进程退出的性质，来确保子进程先结束。