Re: errno, Code density

According to EricP  <ThatWouldBeTelling@thevillage.com>:

extern int *__errno_location (void) __THROW __attribute__ ((__const__));
 
#  if !defined _LIBC || defined _LIBC_REENTRANT
/* When using threads, errno is a per-thread value.  */
#   define errno (*__errno_location ())
#  endif
# endif /* !__ASSEMBLER__ */
#endif /* _ERRNO_H */

>
That replaces a memory reference with a function call.

Not really. On all of the Unix-like systems I know, errno is a macro
wrapped around a function call that fetches the most recent error in
the current thread, done that way to avoid breaking old programs
written back before threads when errno was an extern int. It's a
peculiar special case and I don't offhand know of anything else like
that.

Here's the ABI manual for amd64 systems:

http://refspecs.linux-foundation.org/elf/x86_64-abi-0.95.pdf

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