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 .

CPP

//EXEC.c

#include<stdio.h>
#include<unistd.h>

int main()
{
    int i;

    printf("I am EXEC.c called by execvp() ");
    printf("\n");

    return 0;
}

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

gcc EXEC.c -o EXEC

CPP

//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);

        /*All statements are ignored after execvp() call as this whole
        process(execDemo.c) is replaced by another process (EXEC.c)
        */
        printf("Ending-----");

    return 0;
}

• 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 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 .
  

CPP

//EXEC.c

#include<stdio.h>
#include<unistd.h>

int main()
{
    int i;

    printf("I am EXEC.c called by execv() ");
    printf("\n");
    return 0;
}

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

gcc EXEC.c -o EXEC

CPP

//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);

        /*All statements are ignored after execvp() call as this whole
        process(execDemo.c) is replaced by another process (EXEC.c)
        */
        printf("Ending-----");

    return 0;
}

• 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 them are a bit different which is as shown below:

Syntax:

int execlp(const char *file, const char *arg,.../* (char  *) NULL */);
int execl(const char *path, const char *arg,.../* (char  *) NULL */);

• 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 syntaxes 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, .../*, (char *) NULL, 
char * const envp[] */);

• 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
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