getenv("LOGNAME") is incorrect when a child create a shell which call parent

powah · 08-21-2007, 02:19 PM

The command line interface parent process (CLI) fork and its child create a shell which then create a new process for CLI (we call it CLI2).
Inside CLI2, the getenv("LOGNAME") value is incorrect. Is it because getenv is not reentrant? getenv_r is not supported on the linux I used.
How to ensure inside CLI2 the environment value of getenv("LOGNAME") value of CLI is right?
This is an old program which I maintained.

wjevans_7d1@yahoo.co · 08-22-2007, 07:36 AM

What output do you get when you run this program?

Code:

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

int main(int    argc,
         char **argv
        )
{
  int   child_process;
  int   status;

  char *env_logname;
  char *env_stophere;

  char  answer[4];

  env_stophere=getenv("STOPHERE");

  if(env_stophere)
  {
    printf("In child  process, ");
  }
  else
  {
    printf("In parent process, ");
  }

  env_logname=getenv("LOGNAME");

  if(env_logname)
  {
    printf("LOGNAME is %s.\n",
           env_logname
          );
  }
  else
  {
    printf("LOGNAME is undefined.\n");
  }

  env_stophere=getenv("STOPHERE");

  if(env_stophere)
  {
    return 0;
  }

  for(;;)
  {
    printf("About to spawn a new process.  Should I continue?  ");

    fgets(answer,
          sizeof(answer),
          stdin
         );

    if(answer[0]=='n')
    {
      return 0;
    }

    if(answer[0]=='y')
    {
      break;
    }
  }

  if(putenv("STOPHERE=1")<0)
  {
    perror("putenv(\"STOPHERE=1\")");

    return 1;
  }

  child_process=fork();

  if(child_process<0)
  {
    perror("fork()");

    exit(1);
  }

  if(child_process==0)
  {
    if(execl("/bin/bash",
             "/bin/bash",
             "-c",
             argv[0],
             NULL
            )
       <0
      )
    {
      perror("execl()");

      return 1;
    }

    return 0;
  }

  wait(&status);

  return 0;

} /* main() */

powah · 08-22-2007, 08:29 AM

Quote:

Originally Posted by wjevans_7d1@yahoo.co

What output do you get when you run this program?

Code:

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

int main(int    argc,
         char **argv
        )
{
  int   child_process;
  int   status;

  char *env_logname;
  char *env_stophere;

  char  answer[4];

  env_stophere=getenv("STOPHERE");

  if(env_stophere)
  {
    printf("In child  process, ");
  }
  else
  {
    printf("In parent process, ");
  }

  env_logname=getenv("LOGNAME");

  if(env_logname)
  {
    printf("LOGNAME is %s.\n",
           env_logname
          );
  }
  else
  {
    printf("LOGNAME is undefined.\n");
  }

  env_stophere=getenv("STOPHERE");

  if(env_stophere)
  {
    return 0;
  }

  for(;;)
  {
    printf("About to spawn a new process.  Should I continue?  ");

    fgets(answer,
          sizeof(answer),
          stdin
         );

    if(answer[0]=='n')
    {
      return 0;
    }

    if(answer[0]=='y')
    {
      break;
    }
  }

  if(putenv("STOPHERE=1")<0)
  {
    perror("putenv(\"STOPHERE=1\")");

    return 1;
  }

  child_process=fork();

  if(child_process<0)
  {
    perror("fork()");

    exit(1);
  }

  if(child_process==0)
  {
    if(execl("/bin/bash",
             "/bin/bash",
             "-c",
             argv[0],
             NULL
            )
       <0
      )
    {
      perror("execl()");

      return 1;
    }

    return 0;
  }

  wait(&status);

  return 0;

} /* main() */

Output is:
$ a.out
In parent process, LOGNAME is powah.
About to spawn a new process. Should I continue? y
In child process, LOGNAME is powah.

wjevans_7d1@yahoo.co · 08-23-2007, 12:29 AM

Ok. So my sample program does exactly what you say your program does, and my sample program shows that environment variable LOGNAME remains in the child process.

But your program doesn't behave that way.

So your job is to figure out why my program works in this regard and yours doesn't.

Good luck.