Per thread logging in C using POSIX threads

This is a simple implementation of per thread logging in unix/linux.
It creates separate log files per thread and independently writes log to each of the thread specific log files.

#include <malloc.h>
#include <pthread.h>
#include <stdio.h>

/* The key used to associate a log file pointer with each thread. */
static pthread_key_t thread_log_key;


void write_to_thread_log (const char* message)
{
  FILE* thread_log = (FILE*) pthread_getspecific (thread_log_key);
  fprintf (thread_log, "%s\n", message);
}

void close_thread_log (void* thread_log)
{
  fclose ((FILE*) thread_log);
}
void* thread_function (void* args)
{
  char thread_log_filename[20];
  FILE* thread_log;

  /* Generate the filename for this thread.s log file. */
  sprintf (thread_log_filename, "thread%d.log", (int) pthread_self ());


  thread_log = fopen (thread_log_filename, "w");

  /* Store the file pointer in thread-specific data under thread_log_key. */
  pthread_setspecific (thread_log_key, thread_log);
  write_to_thread_log ("Thread starting.");
 
 /* Do work here... */

  return NULL;
}
int main ()
{
  int i;
  pthread_t threads[5];
 

 /* Create a key to associate thread log file pointers in
   * thread-specific data. Use close_thread_log to clean up the file
   * pointers. */
  pthread_key_create (&thread_log_key, close_thread_log);

  /* Create threads . */
  for (i = 0; i < 5; ++i)
    pthread_create (&(threads[i]), NULL, thread_function, NULL);


  for (i = 0; i < 5; ++i)
    pthread_join (threads[i], NULL);
  return 0;
}