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04 Processes 3

execlp()

replaces the data & text region of the calling process with the new data of program

execlp(program path, exec_name, arg_1, arg_2, ... , NULL);

NULL is always the last parameter of execlp()

execlp() will return to the child process only in case of error. It will go back to the parent process regardless.

Hence, in the following example, “blah blah” will not execute.

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

void main()
{
  int rv;
  rv = fork();

  if(rv == 0)
  {
    printf("I am a child process\n");
    execlp("ls", "ls", NULL);
    printf("blah blah"); 
  }
  else if(rv > 0)
  {
    wait(NULL);
    printf("Hi. I am the parent\n");
  }
  else
  {
    printf("Unsuccessful");
  }
}

I am a child process
a.out  main.c
Hi. I am the parent

Code 2: Long Listing

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

void main()
{
  int rv;
  rv = fork();

  if(rv == 0)
  {
    printf("I am a child process\n");
    execlp("ls", "ls", "-l", NULL);
    printf("blah blah");
  }
  else if(rv > 0)
  {
    wait(NULL);
    printf("Hi. I am the parent\n");
  }
  else
  {
    printf("Unsuccessful");
  }
}

I am a child process
total 24
-rwxr-xr-x 1 runner3 runner3 16864 Oct 20 05:55 a.out
-rwxrwxrwx 1 root    root      371 Oct 20 05:55 main.c
Hi. I am the parent

Code 3: Without NULL

Gives error: missing sentinel in functional call

This causes blah blah to be displayed

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

void main()
{
  int rv;
  rv = fork();

  if(rv == 0)
  {
    printf("I am a child process\n");
    execlp("ls", "ls", "-l");
    printf("blah blah");
  }
  else if(rv > 0)
  {
    wait(NULL);
    printf("Hi. I am the parent\n");
  }
  else
  {
    printf("Unsuccessful");
  }
}

main.c: In function ‘main’:
main.c:14:5: warning: missing sentinel in function call [-Wformat=]
   14 |     execlp("ls", "ls", "-l");
      |     ^~~~~~
I am a child process
blah blahHi. I am the parent

Code 4: Executing another program

sample.c

#include <stdio.h>
void main()
{
  printf("Hi there!");
  something
}

cc sample.c a.out
cc sample.c -o sample.o
Last Updated: 2023-01-25 ; Contributors: AhmedThahir

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