• Threads:
  • A mechanism that permits a single process to perform multiple tasks concurrently
• Shared Variables:
  • Threads can "share" variables in the initialized data, uninitialized data, and heap segments

• Purpose:
  • Allows a function to maintain a separate copy of a variable for each thread that calls the function
• Comparison to Thread-Specific Storage:
  • Similar but much simpler
  • Not part of the POSIX/SUS standard

• Declaration:
  • Include the __thread specifier immediately after the static or extern specifier (note that there are two underscores)
• Deallocation:
  • The memory is freed by the system when the thread terminates

unixexamples/threadspecificdata/grader_threadlocal.c

#include "grader.h"
#include <pthread.h>
#include <string.h>


static __thread char letter[2];

char
*letter_grade(float number_grade)
{
  if      (number_grade >= 90.0) strncpy(letter, "A", 1);
  else if (number_grade >= 80.0) strncpy(letter, "B", 1);
  else if (number_grade >= 70.0) strncpy(letter, "C", 1);
  else if (number_grade >= 60.0) strncpy(letter, "D", 1);
  else                           strncpy(letter, "F", 1);
  letter[1] = '\0';
  
  return letter;
}