为什么要用exec族函数，有什么作用

为什么要用exec族函数，有什么作用

作用：直接调用其他可执行程序 ，代码简洁

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
//函数原型：int execl(const char *path, const char *arg, ...);int main(void)
{pid_t pid;int data;while(1){printf("input a data:n");scanf("%d",&data);if(data == 1){pid = fork();if(pid > 0){wait();//父进程等待子进程退出}if(pid==0){    //子进程干活（修改文件）int fdSrc;char *readBuf = NULL;fdSrc = open(&#",O_RDWR);int size = lseek(fdSrc,0,SEEK_END);lseek(fdSrc,0,SEEK_SET);readBuf =(char *) malloc(sizeof(char)*size + 8);int n_read = read(fdSrc,readBuf,size);char *p = strstr(readBuf,"LENG=5");if(p==NULL){printf("nou foundn");exit(-1);}p = p + strlen("LENG=");*p = '5';lseek(fdSrc,0,SEEK_SET);int n_write = write(fdSrc,readBuf,strlen(readBuf));close(fdSrc);}else{printf("wait, do nothingn");}}}return 0;
}

 使用exec函数

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
//函数原型：int execl(const char *path, const char *arg, ...);int main(void)
{pid_t pid;int data;while(1){printf("input a data:n");scanf("%d",&data);if(data == 1){pid = fork();if(pid > 0){wait();}if(pid==0){execl("./changData","changData",&#",NULL);exit(0);}}else{printf("wait, do nothingn");}}return 0;
}

函数族：
　　exec函数族分别是：execl, execlp, execle, execv, execvp, execvpe
函数原型：

#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 execvpe(const char *file, char *const argv[],char *const envp[]);

返回值：
　　exec函数族的函数执行成功后不会返回，调用失败时，会设置errno并返回-1，然后从原程序的调用点接着往下执行。
参数说明：
path：可执行文件的路径名字
arg：可执行程序所带的参数，第一个参数为可执行文件名字，没有带路径且arg必须以NULL结束
file：如果参数file中包含/，则就将其视为路径名，否则就按 PATH环境变量，在它所指定的各目录中搜寻可执行文件。

exec族函数参数极难记忆和分辨，函数名中的字符会给我们一些帮助：
l : 使用参数列表
p：使用文件名，并从PATH环境进行寻找可执行文件
v：应先构造一个指向各参数的指针数组，然后将该数组的地址作为这些函数的参数。
e：多了envp[]数组，使用新的环境变量代替调用进程的环境变量

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
//函数原型：int execl(const char *path, const char *arg, ...);int main(void)
{printf("before execln");if(execl("./bin/echoarg","echoarg","abc",NULL) == -1){printf("execl failed!n");}printf("after execln");return 0;
}

CLC@Embed_Learn:~/2_jingCheng$ ./a.out 
before execl
execl failed!
after execl

#include <stdio.h>
//函数原型：int execl(const char *path, const char *arg, ...);int main(int argc,char *argv[])
{int i = 0;for(i = 0; i < argc; i++){printf("argv[%d]: %sn",i,argv[i]);}return 0;
}

CLC@Embed_Learn:~/2_jingCheng$ ./execpro file file2
argv[0]: ./execpro
argv[1]: file
argv[2]: file2

ls 

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
//函数原型：int execl(const char *path, const char *arg, ...);int main(void)
{printf("before execln");//execl成功后不会返回，失败后返回-1if(execl("/bin/ls","ls",NULL,NULL) == -1){printf("execl failed!n");//错误原因，perror函数perror("why");}printf("after execln");return 0;
}

CLC@Embed_Learn:~/2_jingCheng$ ./a.out 
before execl
execl failed!
why: No such file or directory
after execl

CLC@Embed_Learn:~/2_jingCheng$ gcc exec2.c
CLC@Embed_Learn:~/2_jingCheng$ ./a.out 
before execl
a.out	  demo11.c  demo13.c  demo15.c	demo1.c  demo3.c  demo5.c  demo7.c  demo9.c  exec2.c  execpro  newpro
demo10.c  demo12.c  demo14.c  demo16.c	demo2.c  demo4.c  demo6.c  demo8.c  exec1.c  exec.c   forkPro
CLC@Embed_Learn:~/2_jingCheng$ ls
a.out     demo11.c  demo13.c  demo15.c  demo1.c  demo3.c  demo5.c  demo7.c  demo9.c  exec2.c  execpro  newpro
demo10.c  demo12.c  demo14.c  demo16.c  demo2.c  demo4.c  demo6.c  demo8.c  exec1.c  exec.c   forkPro
CLC@Embed_Learn:~/2_jingCheng$ 

ls -l 

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
//函数原型：int execl(const char *path, const char *arg, ...);int main(void)
{printf("before execln");if(execl("/bin/ls","ls","-l",NULL) == -1){printf("execl failed!n");perror("why");}printf("after execln");return 0;
}

where is date?时间 在/bin路径找到date

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
//函数原型：int execl(const char *path, const char *arg, ...);int main(void)
{printf("this pro get system date:n");if(execl("/bin/date","date",NULL,NULL) == -1){printf("execl failed!n");perror("why");}printf("after execln");return 0;
}

CLC@Embed_Learn:~/2_jingCheng$ vi exec4.c
CLC@Embed_Learn:~/2_jingCheng$ gcc exec4.c
CLC@Embed_Learn:~/2_jingCheng$ ./a.out 
this pro get system date:
Sun Mar  6 13:50:41 CST 2022

老是要找路径好烦，咋办？用execlp!!!  p：使用文件名，并从PATH环境进行寻找可执行文件

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
//函数原型：int execl(const char *path, const char *arg, ...);int main(void)
{printf("this pro ps:n");if(execlp("ps","ps",NULL,NULL) == -1){printf("execl failed!n");perror("why");}printf("after execln");return 0;
}

this pro ps:PID TTY          TIME CMD
30589 pts/27   00:00:01 bash
31772 pts/27   00:00:00 ps

如何让程序在其他路径也可以运行可执行程序呢？

配置环境变量！！！

CLC@Embed_Learn:~/2_jingCheng$ echo $PATH
/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/opt/FriendlyARM/toolschain/4.5.1/bin
CLC@Embed_Learn:~/2_jingCheng$ 
CLC@Embed_Learn:~/2_jingCheng$ export PATH=$PATH:/home/CLC/2_jingCheng//添加环境变量路径
CLC@Embed_Learn:~/2_jingCheng$ 
CLC@Embed_Learn:~/2_jingCheng$ echo $PATH
/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/opt/FriendlyARM/toolschain/4.5.1/bin:/home/CLC/2_jingCheng

配置好后，在其他路径运行不需要加./ 

CLC@Embed_Learn:~$ a.out
this pro ps:PID TTY          TIME CMD
30589 pts/27   00:00:01 bash
31846 pts/27   00:00:00 ps
CLC@Embed_Learn:~$ 
CLC@Embed_Learn:~$ 
CLC@Embed_Learn:~$ newpro
son print , pid = 31850
son print , pid = 31850
son print , pid = 31850
son print , pid = 31850
son print , pid = 31850
father print , pid = 31849
cnt = 5
father print , pid = 31849
cnt = 5

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
//函数原型：int execl(const char *path, const char *arg, ...);int main(void)
{printf("before execlp****n");char *argv[] = {"ps","-l",NULL};if(execvp("ps",argv) == -1){printf("execvp failed!n");}printf("after execlp*****n");return 0;
}

CLC@Embed_Learn:~/2_jingCheng$ ./a.out 
before execlp****
F S   UID   PID  PPID  C PRI  NI ADDR SZ WCHAN  TTY          TIME CMD
0 S  1000 30589  3796  0  80   0 -  6994 wait   pts/27   00:00:01 bash
0 R  1000 31924 30589  0  80   0 -  3482 -      pts/27   00:00:00 ps
CLC@Embed_Learn:~/2_jingCheng$ vi exec6.c