闲来无事,自己实现了popen函数mypopen,后来查看了popen函数的源码发现自己实现的与其相差无几,本函数与linux中的实现最大的不同是不需要用专门的pclose()函数来关闭文件指针,用普通的fclose()即可,linux实现的代码也会给出在下文,可以对比一下其中差异。
主要通过pipe管道实现,具体思路如下:
1、使用pipe()建立管道
2、使用fork()创建子进程
3、在子进程中调用exec族函数执行命令,通过管道将结果传送至父进程
4、在主进程中等待子进程执行,子进程执行完成后将接收其结果,返回结果的文件指针
下面是mypopen源代码,重点部位已通过注释进行标注:
- FILE *mypopen(char *cmd,char type)
- {
- int pipefd[2];
- int pid_t;
-
- if(type !='r' && type != 'w')
- {
- printf("myopen() flag error/n");
- return NULL;
- }
-
- if(pipe(pipefd)<0)
- {
- printf("myopen() pipe create error/n");
- return NULL;
- }
-
- pid_t=fork();
-
- if(pid_t < 0)
- return NULL;
-
- if(0 == pid_t)
- {
- if(type == 'r')
- {
- close(pipefd[0]);
- dup2(pipefd[1],STDOUT_FILENO);
- close(pipefd[1]);
-
- }
- else{
- close(pipefd[1]);
- dup2(pipefd[0],STDIN_FILENO);
- close(pipefd[0]);
- }
- char *argv[] = {cmd,NULL};
- if(execvp(cmd,argv)<0)
- return NULL;
- }
-
- wait(0);
-
- if(type=='r'){
- close(pipefd[1]);
- return fdopen(pipefd[0],"r");
- }else{
- close(pipefd[0]);
- return fdopen(pipefd[1],"w");
- }
- }
下面是popen()在linux中的实现:
-
-
-
-
- #include <sys/wait.h>
- #include <errno.h>
- #include <fcntl.h>
- #include "ourhdr.h"
-
- static pid_t *childpid = NULL;
-
- static int maxfd;
-
- #define SHELL "/bin/sh"
-
- FILE *
- popen(const char *cmdstring, const char *type)
- {
- int i, pfd[2];
- pid_t pid;
- FILE *fp;
-
-
- if ((type[0] != 'r' && type[0] != 'w') || type[1] != 0) {
- errno = EINVAL;
- return(NULL);
- }
-
- if (childpid == NULL) {
-
- maxfd = open_max();
- if ( (childpid = calloc(maxfd, sizeof(pid_t))) == NULL)
- return(NULL);
- }
-
- if (pipe(pfd) < 0)
- return(NULL);
-
- if ( (pid = fork()) < 0)
- return(NULL);
- else if (pid == 0) {
- if (*type == 'r') {
- close(pfd[0]);
- if (pfd[1] != STDOUT_FILENO) {
- dup2(pfd[1], STDOUT_FILENO);
- close(pfd[1]);
- }
- } else {
- close(pfd[1]);
- if (pfd[0] != STDIN_FILENO) {
- dup2(pfd[0], STDIN_FILENO);
- close(pfd[0]);
- }
- }
-
- for (i = 0; i < maxfd; i++)
- if (childpid[ i ] > 0)
- close(i);
-
- execl(SHELL, "sh", "-c", cmdstring, (char *) 0);
- _exit(127);
- }
-
- if (*type == 'r') {
- close(pfd[1]);
- if ( (fp = fdopen(pfd[0], type)) == NULL)
- return(NULL);
- } else {
- close(pfd[0]);
- if ( (fp = fdopen(pfd[1], type)) == NULL)
- return(NULL);
- }
- childpid[fileno(fp)] = pid;
- return(fp);
- }
-
- int
- pclose(FILE *fp)
- {
-
- int fd, stat;
- pid_t pid;
-
- if (childpid == NULL)
- return(-1);
-
- fd = fileno(fp);
- if ( (pid = childpid[fd]) == 0)
- return(-1);
-
- childpid[fd] = 0;
- if (fclose(fp) == EOF)
- return(-1);
-
- while (waitpid(pid, &stat, 0) < 0)
- if (errno != EINTR)
- return(-1);
-
- return(stat);
- }
参考资料,真诚致谢:
http://www.linuxforum.net/forum/showflat.php?Cat=&Board=program&Number=266385