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exec family of functions in C

The exec family of functions replaces the current running process with a new process. It can be used to run a C program by using another C program. It comes under the header file unistd.h. There are many members in the exec family which are shown below with examples.

  • execvp : Using this command, the created child process does not have to run the same program as the parent process does. The exec type system calls allow a process to run any program files, which include a binary executable or a shell script . Syntax:
    int execvp (const char *file, char *const argv[]);

    file: points to the file name associated with the file being executed.
    argv:  is a null terminated array of character pointers.

    Let us see a small example to show how to use execvp() function in C. We will have two .C files , EXEC.c and execDemo.c and we will replace the execDemo.c with EXEC.c by calling execvp() function in execDemo.c .

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    //EXEC.c
      
    #include<stdio.h>
    #include<unistd.h>
      
    int main()
        int i;
          
        printf("I am EXEC.c called by execvp() ");
        printf("\n");
          
        return 0;

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    Now,create an executable file of EXEC.c using command

    gcc EXEC.c -o EXEC
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    //execDemo.c
      
    #include<stdio.h>
    #include<stdlib.h>
    #include<unistd.h>
    int main()
            //A null terminated array of character 
            //pointers
            char *args[]="./EXEC",NULL;
            execvp(args[0],args);
          
            
            printf("Ending-----");
          
        return 0;

    chevron_right

    Now, create an executable file of execDemo.c using command

    gcc execDemo.c -o execDemo

    After running the executable file of execDemo.cby using command ./excDemo, we get the following output:

    I AM EXEC.c called by execvp()

    When the file execDemo.c is compiled, as soon as the statement execvp(args[0],args) is executed, this very program is replaced by the program EXEC.c. “Ending—–” is not printed because because as soon as the execvp() function is called, this program is replaced by the program EXEC.c.

  • execv : This is very similar to execvp() function in terms of syntax as well. The syntax of execv() is as shown below:Syntax:
    int execv(const char *path, char *const argv[]);

    path: should point to the path of the file being executed.
    argv[]: is a null terminated array of character pointers.

    Let us see a small example to show how to use execv() function in C. This example is similar to the example shown above for execvp() . We will have two .C files , EXEC.c and execDemo.c and we will replace the execDemo.c with EXEC.c by calling execv() function in execDemo.c .

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    //EXEC.c
      
    #include<stdio.h>
    #include<unistd.h>
      
    int main()
        int i;
          
        printf("I am EXEC.c called by execv() ");
        printf("\n");
        return 0;

    chevron_right

    Now,create an executable file of EXEC.c using command

    gcc EXEC.c -o EXEC
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    //execDemo.c
      
    #include<stdio.h>
    #include<stdlib.h>
    #include<unistd.h>
    int main()
            //A null terminated array of character 
            //pointers
            char *args[]="./EXEC",NULL;
            execv(args[0],args);
          
            
            printf("Ending-----");
          
        return 0;

    chevron_right

    Now, create an executable file of execDemo.c using command

    gcc execDemo.c -o execDemo

    After running the executable file of execDemo.c by using command ./excDemo, we get the following output:

    I AM EXEC.c called by execv()
  • execlp and execl : These two also serve the same purpose but the syntax of of them are a bit different which is as shown below:Syntax:
    int execlp(const char *file, const char *arg,... );
    int execl(const char *path, const char *arg,... );

    file:  file name associated with the file being executed
    const char *arg and ellipses : describe a list of one or more pointers to null-terminated strings that represent the argument list available to the executed program.

    The same C programs shown above can be executed with execlp() or execl() functions and they will perform the same task i.e. replacing the current process the with a new process.

  • execvpe and execle : These two also serve the same purpose but the syntax of them are a bit different from all the above members of exec family. The synatxes of both of them are shown below :
    Syntax:

    int execvpe(const char *file, char *const argv[],char *const envp[]);Syntax:int execle(const char *path, const char *arg, ... );

    The syntaxes above shown has one different argument from all the above exec members, i.e.
    char * const envp[]: allow the caller to specify the environment of the executed program via the argument envp.
    envp:This argument is an array of pointers to null-terminated strings and must be terminated by a null pointer. The other functions take the environment for the new process image from the external variable environ in the calling process.

Reference: exec(3) man page

This article is contributed by MAZHAR IMAM KHAN. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to [email protected] See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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How to use execv system call in linux?

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up vote
3
down vote

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1

I am writing a program using execl to execute my exe file which is testing and it’s work very well and display the output in the Linux CLI. But I have not idea how to change the execl to execv, although I know both of the system call will give the same value. I am confused with the array argument for execv system call

This is my execl sample program

int main(void) int childpid; if((childpid = fork()) == -1 ) perror("can't fork"); exit(1); else if(childpid == 0) execl("./testing","","",(char *)0); exit(0);
else
printf("finish");
exit(0);

can I know how to change the execl to execv. What I read from online, we must set the file path for my exe file and the argument of array . What type of argument need to set for the array in order to ask the program to execute the testing exe file ?
https://support.sas.com/documentation/onlinedoc/sasc/doc/lr2/execv.htm

Is it the link consist of the thing I want ? But what I read from it ,the command is request the list the file,not execute the file. Correct me I make any mistake

c linux unix exec

share | improve this question

edited Mar 25 at 18:33

jww

52.2k37215479

asked Aug 21 ’15 at 13:55

doe

48114

  • 2

    Why do you want to use execv? What are you actually trying to accomplish?
    –  Kevin
    Aug 21 ’15 at 13:57

  • 1

    Did you try the example on the page you link? Here the offical Linux man-page: man7.org/linux/man-pages/man3/exec.3.html
    –  alk
    Aug 21 ’15 at 14:04


  • linux.die.net/man/3/execv — the difference is that execv wants a single pointer to a char * array, whereas execl accepts a variadic list of char * arguments.
    –  tripleee
    Aug 21 ’15 at 14:06


  • Related if not a duplicate: stackoverflow.com/q/10790719/694576
    –  alk
    Aug 21 ’15 at 14:13

  • 1

    Like Kevin said, why change it? With the exception of execvpe, which AFAIK is essentially an alias for the "real" execve, it’s basically all syntactic sugar. So use the one that already works.
    –  Brian McFarland
    Aug 21 ’15 at 14:28

 | 
show 1 more comment

2 Answers
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up vote
12
down vote

In order to see the difference between execl and execv, here is a line of code executing a

ls -l -R -a

with execl :

execl("/bin/ls", "ls", "-l", "-R", "-a", NULL);

with execv :

char* arr[] = "ls", "-l", "-R", "-a", NULL;
execv("/bin/ls", arr);

The array of char* sent to execv will be passed to /bin/ls as argv (in int main(int argc, char **argv))

Here is the execl(3) Linux manual page for more detail.

share | improve this answer

edited Feb 6 at 19:42

answered Aug 21 ’15 at 14:26

4rzael

452416

  • Thanks, I corrected my answer.
    –  4rzael
    Aug 21 ’15 at 20:12


  • Does C support inline arrays like execv("/bin/ls", ["ls", "-l", "-R", "-a", NULL]);? When compiling on a linux machine with gcc --std=c99 file.c I get an error: error: expected expression before '[' token. Couldn’t find anything about this elsewhere… Any ideas?
    –  Bruno Ely
    Feb 5 at 22:22

  • 1

    No, my bad, it was just to explain the difference. I will edit
    –  4rzael
    Feb 6 at 19:35

  • Oh thank you haha I was going nuts trying to look for a way to make that work.
    –  Bruno Ely
    Feb 8 at 22:30

add a comment  | 


up vote
8
down vote

According to the man page the use of execv is quite simple. The first argument is the path as a string to the program you want to execute. The second is an array of string that will be used as the arguments of the program you want to execute. It is the kind of array you get if you get the argv array in your main function.

So the array you will pass as a parameter will be the array received in the main function of the program you execute with execv.

By convention, the first argument should be the program name (the one you try to execute) but it is not mandatory (but strongly recommended since it is the behaviour a lot of programs are expecting). Each other string in the array should be an individual argument.

And of course, the array should be terminated with a NULL pointer to mark the end.

Array example: ["prog_name", "arg1", "arg2", "arg3", NULL]

[] is your array, each string separated with a coma is a frame of your array and at the end you have the null frame.

I hope I am clear enough!

share | improve this answer

edited Aug 21 ’15 at 20:30

answered Aug 21 ’15 at 14:04

Roger

432214

  • thank you ,your guide absolute help me but what is the argument type I need to specify in the array ?Is it similar with the 4rzael answer ?
    –  doe
    Aug 21 ’15 at 18:14

  • Yes it is similar. Your array is a string (char *) array. So it is more like ["prog_name", "arg1", "arg2", "arg3", NULL]
    –  Roger
    Aug 21 ’15 at 20:29


  • if the command has syntax of arg1 val1 arg2 val2 (such as iptables -A INPUT -p udp) is it that the valX is treated as argument ?
    –  ransh
    Nov 21 at 17:16

add a comment  | 

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