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Built-in functions for atomic memory access

Using a semaphore requires an atomic operation, so that a deadlock cannot occur. The Intel Itanium Processor-specific Application Binary Interface specifies some built-ins that are atomic operations… There are built-ins for a few functionalities, but these are the most imporant:

• add, sub, or, and, xor, nand
• compare and swap

The first of the two can be called in gcc by using the following built-in functions:

• Type 1:
  • type __sync_fetch_and_add(type *ptr, type value, ...)
  • type __sync_fetch_and_sub (type *ptr, type value, ...)
  • type __sync_fetch_and_or (type *ptr, type value, ...)
  • type __sync_fetch_and_and (type *ptr, type value, ...)
  • type __sync_fetch_and_xor (type *ptr, type value, ...)
  • type __sync_fetch_and_nand (type *ptr, type value, ...)
• Type 2:
  • type __sync_add_and_fetch (type *ptr, type value, ...)
  • type __sync_sub_and_fetch (type *ptr, type value, ...)
  • type __sync_or_and_fetch (type *ptr, type value, ...)
  • type __sync_and_and_fetch (type *ptr, type value, ...)
  • type __sync_xor_and_fetch (type *ptr, type value, ...)
  • type __sync_nand_and_fetch (type *ptr, type value, ...)

Type 1 and 2 are only different in the sense, that Type 1 returns the value that had previously been in memory, and Type 2 returns the new value.

The second of the two are atomic operations for comparing and swapping

• Type 1:
  • bool __sync_bool_compare_and_swap (type *ptr, type oldval, type newval, ...)
• Type 2:
  • type __sync_val_compare_and_swap (type *ptr, type oldval, type newval, ...)

These functions take 3 arguments:

• a pointer ptr to the value that needs changing
• a value oldval which is the value before the swap
• and a value newval which is the new value after swapping

These functions compare the current value of ptr to oldval and if they are the same, newval is being written to ptr… The two types are different in the way that the first type returns a bool if the comparison is successful and newval is written into ptr and the second type returns the content of ptr before the operation.