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#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
main ()
{
  pid_t pid; pid=fork();
  if (pid < 0)
    printf("error in fork!");
  else if (pid == 0)
    printf("i am the child process, my process id is %dn",getpid());
  else
    printf("i am the parent process, my process id is %dn",getpid());
}
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# ./a.out
i am the child process, my process id is 4286
i am the parent process, my process id is 4285
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main()
{
  pid_t pid;
  printf("\n[%d]not fork pid=%d\n",getpid(),pid);
  pid=fork();
  printf("\n[%d]forked pid=%d\n",getpid(),pid);
  if(pid<0)
  {
    printf("error in fork!\n");
    getchar();
    exit(1);
  }
  else if(pid==0)
    printf("\n[%d]in child process,p_id=%d\n",getpid(),getpid());
  else
  {
    printf("\n[%d]in parent process,my pid=%d\n",getpid(),pid);
    printf("\n[%d]in parent process,my getpid=%d\n",getpid(),getpid());
  }
}
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$ ./fork
[3819]not fork
[3820]forked pid=0
[3820]in child process,p_id=3820
[3819]forked pid=3820
[3819]in parent process,my pid=3820
[3819]in parent process,my getpid=3819
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