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Is errno thread local?

stackoverflow Is errno thread-safe?

In errno.h, this variable is declared as extern int errno; so my question is, is it safe to check errno value after some calls or use perror() in multi-threaded code. Is this a thread safe variable? If not, then whats the alternative ?

I am using linux with gcc on x86 architecture.

A

Yes, it is thread safe. On Linux, the global errno variable is thread-specific. POSIX requires that errno be threadsafe.

See http://www.unix.org/whitepapers/reentrant.html

In POSIX.1, errno is defined as an external global variable. But this definition is unacceptable in a multithreaded environment, because its use can result in nondeterministic results. The problem is that two or more threads can encounter errors, all causing the same errno to be set. Under these circumstances, a thread might end up checking errno after it has already been updated by another thread.

To circumvent the resulting nondeterminism, POSIX.1c redefines errno as a service that can access the per-thread error number as follows (ISO/IEC 9945:1-1996, §2.4):

Some functions may provide the error number in a variable accessed through the symbol errno. The symbol errno is defined by including the header , as specified by the C Standard ... For each thread of a process, the value of errno shall not be affected by function calls or assignments to errno by other threads.

Also see http://linux.die.net/man/3/errno

errno is thread-local; setting it in one thread does not affect its value in any other thread.

A

Yes

Errno isn't a simple variable anymore, it's something complex behind the scenes, specifically for it to be thread-safe.

See $ man 3 errno:

ERRNO(3)                   Linux Programmer’s Manual                  ERRNO(3)

NAME
       errno - number of last error

SYNOPSIS
       #include <errno.h>

DESCRIPTION

      ...
       errno is defined by the ISO C standard to be  a  modifiable  lvalue  of
       type  int,  and  must not be explicitly declared; errno may be a macro.
       errno is thread-local; setting it in one thread  does  not  affect  its
       value in any other thread.

We can double-check:

$ cat > test.c
#include <errno.h>
f() { g(errno); }
$ cc -E test.c | grep ^f
f() { g((*__errno_location ())); }
$