系統調用exe函數族對當前進程進行替換，替換著為一個指定程序，其參數包括文件名filename，參數列表argv，以及環境變量envp

整個函數家族如下：

#include <unistd.h>

extern char **environ;
int execl(const char *path, const char *arg, ...);
int execlp(const char *file, const char *arg, ...);
int execle(const char *path, const char *arg, ..., char *const envp[]);
int execv(const char *path, char *const argv[]);
int execvp(const char *file, char *const argv[]);
int execve(const char *path, char *const argv[], char *const envp[]);

在名字中缺“e”的是從當前進程中拷貝環境，有字母“p”的是可以在當前路徑中查找文件，而其他函數必須在指定路徑中找，這里要注意執行路徑，有安全隱患，慎用。

?

練習1：用系統命令替換

[cpp]?view plaincopy

/*?exec.c?*/??

#include?<unistd.h>??

main()??

{?????

????/*定義參數*/??

????char?*envp[]={"PATH=/tmp",??"USER=lei",?"STATUS=testing",NULL};???

??????

????char?*argv_execv[]={"echo",?"excuted?by?execv",?NULL};????

??????

????char?*argv_execvp[]={"echo",?"executed?by?execvp",?NULL};??

??????

????char?*argv_execve[]={"env",?NULL};????

????/*全部在子進程運行*/??

????if(fork()==0)?????????

????????if(execl("/bin/echo",?"echo",?"executed?by?execl",?NULL)<0)??//調用echo命令，參數：executed?by?execl???

????????????perror("Err?on?execl");??

????????if(fork()==0)?????

????????if(execlp("echo",?"echo",?"executed?by?execlp",?NULL)<0)??

//調用echo命令，參數：executed?by?execlp??????

????????????????perror("Err?on?execlp");??????

????????????if(fork()==0)?????????

????????????????if(execle("/usr/bin/env",?"env",?NULL,?envp)<0)??//env命令，打印當前環境envp??

????????????????????perror("Err?on?execle");??????

????????????????if(fork()==0)?????????

????????????????????if(execv("/bin/echo",?argv_execv)<0)??//echo命令，參數argv_execv指向的內容??

????????????????????????perror("Err?on?execv");???

????????????????????if(fork()==0)?????

????????????????????if(execvp("echo",?argv_execvp)<0)????//echo命令，參數argv_execvp指向的內容??

????????????????????????????perror("Err?on?execvp");??

????????????????????????if(fork()==0)?????????

????????????????if(execve("/usr/bin/env",?argv_execve,?envp)<0)????????

//env命令??

????????????????????????????????perror("Err?on?execve");??

}??

程序里調用了2個Linux常用的系統命令，echo和env。echo會把后面跟的命令行參數原封不動的打印出來，env用來列出所有環境變量。

?

編譯，測試：

$ ./exec
executed by execl

executed by execlp
PATH=/tmp
USER=lei
STATUS=testing
executed by execlp
excuted by execv
executed by execvp
PATH=/tmp
USER=lei
STATUS=testing

?

練習2：子進程執行用戶自己編寫的程序

先寫父進程的程序：

[cpp]?view plaincopy

/*parent.c*/??

#include?<sys/types.h>??

#include?<unistd.h>??

#include?<stdio.h>??

??

int?main(void)??

{??

????pid_t?pid;??

????const?char?*usr_envp[]?=?{"MYDEFINE=unknown","PATH=/home",NULL};//路徑??

????printf("Begin?Fork()/n");??

????pid?=?fork();??

????switch(pid)??

????{??

????case?-1:??

????????perror("fork?failed!/n");??

????????break;??

????case?0:??

????????/*準備執行子進程的程序*/??

????????if(execle("/home/child","myarg1","myarg2",(char?*)0,usr_envp))?//路徑/home/child,參數...??

????????perror("execle?failed!/n");??

????????break;??

????default:??

????????break;??

????}??

????//等待子進程退出??

????if(waitpid(pid,NULL,0)<0)??

????????perror("waitpid?failed!/n");??

????printf("parent?excting!");??

????return?0;??

}??

然后我們編寫子進程要執行的程序：

[cpp]?view plaincopy

/*child.c*/??

#include?<sys/types.h>??

#include?<unistd.h>??

#include?<stdio.h>??

int?main(int?argc,char?*argv[])??

{??

????int?i;??

????char?**pStr;??

????extern?char?**environ;??

????printf("child?starting!/n");??

??????

????//打印出參數名??

????for(i?=?0;i?<?argc;i++)??

????{??

????????printf("argv[%d]:%s/n",i,argv[i]);??

????}??

????//打印出路徑名??

????for(pStr?=?environ;?*pStr?!=?0;pStr++)??

????{??

????????printf("%s/n",i,*pStr);??

????}??

????printf("child?exiting!/n");??

????return?0;??

}??

然后編譯，執行parent

$ ./parent

Begin Fork()

child starting!

argv[0]:myargv1

argv[1]:myargv2

child exiting!

parent excting!

總結

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