LWP Code Implementation

lwp.c

#include <malloc.h> 
#include <sys/types.h> 
#include <sys/wait.h> 
#include <signal.h> 
#include <sched.h> 
#include <stdio.h> 
#include <fcntl.h> 

// 64kB stack 
#define STACK 1024*64 

// The child thread will execute this function 
int threadFunction( void* argument ) {
     printf( "child thread entering\n" ); 
     close((int*)argument); 
     printf( "child thread exiting\n" ); 
     return 0; 
}

int main() {
     void* stack; 
     pid_t pid; 
     int fd; 

     fd = open("/dev/null", O_RDWR); 
     if (fd < 0) { 
         perror("/dev/null"); 
         exit(1); 
     } 

     // Allocate the stack 
     stack = malloc(STACK); 
     if (stack == 0) {
         perror("malloc: could not allocate stack"); 
         exit(1); 
     }
     printf("Creating child thread\n"); 

     // Call the clone system call to create the child thread 
     pid = clone(&threadFunction, 
                 (char*) stack + STACK, 
                 SIGCHLD | CLONE_FS | CLONE_FILES |\ 
                  CLONE_SIGHAND | CLONE_VM, 
                 (void*)fd); 

     if (pid == -1) { 
          perror("clone"); 
          exit(2); 
     } 

     // Wait for the child thread to exit 
     pid = waitpid(pid, 0, 0); 
     if (pid == -1) { 
         perror("waitpid"); 
         exit(3); 
     } 

     // Attempt to write to file should fail, since our thread has 
     // closed the file. 
     if (write(fd, "c", 1) < 0) { 
         printf("Parent:\t child closed our file descriptor\n"); 
     } 

     // Free the stack 
     free(stack); 

     return 0; 
}