• Chris Katko (13/16) Jan 11 2020 Saving and restoring a global variable is is __not__ the same as
• Rob Rau (5/21) Jan 11 2020 To be posix compliant errno must be thread safe. In linux I

Chris Katko <ckatko gmail.com> writes:

Pure implies (*requires) no side effects, right?

UNIX 98 requires that errno be set to ENOMEM upon failure. 
Purity is achieved by saving and restoring the value of errno, 
thus behaving as if it were never changed.

Saving and restoring a global variable is is __not__ the same as 
no side effects. without a mutex, two threads could easily create 
a date race.

void* pureMalloc()(size_t size)  trusted pure  nogc nothrow
{
     const errnosave = fakePureErrno;
     void* ret = fakePureMalloc(size);
     fakePureErrno = errnosave;
     return ret;
}

I mean, on a practical level, malloc almost never fails so this 
probably hasn't shown up as a bug in 99% of code.

Rob Rau <rfr2 mac.com> writes:

On Saturday, 11 January 2020 at 19:05:48 UTC, Chris Katko wrote:
 Pure implies (*requires) no side effects, right?

UNIX 98 requires that errno be set to ENOMEM upon failure. 
Purity is achieved by saving and restoring the value of errno, 
thus behaving as if it were never changed.

 Saving and restoring a global variable is is __not__ the same 
 as no side effects. without a mutex, two threads could easily 
 create a date race.

 void* pureMalloc()(size_t size)  trusted pure  nogc nothrow
 {
     const errnosave = fakePureErrno;
     void* ret = fakePureMalloc(size);
     fakePureErrno = errnosave;
     return ret;
 }

 I mean, on a practical level, malloc almost never fails so this 
 probably hasn't shown up as a bug in 99% of code.

To be posix compliant errno must be thread safe. In linux I 
believe this is done by each thread having its own thread local 
errno.

-Rob