src/third_party/openssl/openssl/doc/crypto/threads.pod

=pod

=head1 NAME

CRYPTO_THREADID_set_callback, CRYPTO_THREADID_get_callback,
CRYPTO_THREADID_current, CRYPTO_THREADID_cmp, CRYPTO_THREADID_cpy,
CRYPTO_THREADID_hash, CRYPTO_set_locking_callback, CRYPTO_num_locks,
CRYPTO_set_dynlock_create_callback, CRYPTO_set_dynlock_lock_callback,
CRYPTO_set_dynlock_destroy_callback, CRYPTO_get_new_dynlockid,
CRYPTO_destroy_dynlockid, CRYPTO_lock - OpenSSL thread support

=head1 SYNOPSIS

 #include <openssl/crypto.h>

 /* Don't use this structure directly. */
 typedef struct crypto_threadid_st
         {
         void *ptr;
         unsigned long val;
         } CRYPTO_THREADID;
 /* Only use CRYPTO_THREADID_set_[numeric|pointer]() within callbacks */
 void CRYPTO_THREADID_set_numeric(CRYPTO_THREADID *id, unsigned long val);
 void CRYPTO_THREADID_set_pointer(CRYPTO_THREADID *id, void *ptr);
 int CRYPTO_THREADID_set_callback(void (*threadid_func)(CRYPTO_THREADID *));
 void (*CRYPTO_THREADID_get_callback(void))(CRYPTO_THREADID *);
 void CRYPTO_THREADID_current(CRYPTO_THREADID *id);
 int CRYPTO_THREADID_cmp(const CRYPTO_THREADID *a,
                         const CRYPTO_THREADID *b);
 void CRYPTO_THREADID_cpy(CRYPTO_THREADID *dest,
                          const CRYPTO_THREADID *src);
 unsigned long CRYPTO_THREADID_hash(const CRYPTO_THREADID *id);

 int CRYPTO_num_locks(void);

 /* struct CRYPTO_dynlock_value needs to be defined by the user */
 struct CRYPTO_dynlock_value;

 void CRYPTO_set_dynlock_create_callback(struct CRYPTO_dynlock_value *
	(*dyn_create_function)(char *file, int line));
 void CRYPTO_set_dynlock_lock_callback(void (*dyn_lock_function)
	(int mode, struct CRYPTO_dynlock_value *l,
	const char *file, int line));
 void CRYPTO_set_dynlock_destroy_callback(void (*dyn_destroy_function)
	(struct CRYPTO_dynlock_value *l, const char *file, int line));

 int CRYPTO_get_new_dynlockid(void);

 void CRYPTO_destroy_dynlockid(int i);

 void CRYPTO_lock(int mode, int n, const char *file, int line);

 #define CRYPTO_w_lock(type)	\
	CRYPTO_lock(CRYPTO_LOCK|CRYPTO_WRITE,type,__FILE__,__LINE__)
 #define CRYPTO_w_unlock(type)	\
	CRYPTO_lock(CRYPTO_UNLOCK|CRYPTO_WRITE,type,__FILE__,__LINE__)
 #define CRYPTO_r_lock(type)	\
	CRYPTO_lock(CRYPTO_LOCK|CRYPTO_READ,type,__FILE__,__LINE__)
 #define CRYPTO_r_unlock(type)	\
	CRYPTO_lock(CRYPTO_UNLOCK|CRYPTO_READ,type,__FILE__,__LINE__)
 #define CRYPTO_add(addr,amount,type)	\
	CRYPTO_add_lock(addr,amount,type,__FILE__,__LINE__)

=head1 DESCRIPTION

OpenSSL can safely be used in multi-threaded applications provided
that at least two callback functions are set, locking_function and
threadid_func.

locking_function(int mode, int n, const char *file, int line) is
needed to perform locking on shared data structures. 
(Note that OpenSSL uses a number of global data structures that
will be implicitly shared whenever multiple threads use OpenSSL.)
Multi-threaded applications will crash at random if it is not set.

locking_function() must be able to handle up to CRYPTO_num_locks()
different mutex locks. It sets the B<n>-th lock if B<mode> &
B<CRYPTO_LOCK>, and releases it otherwise.

B<file> and B<line> are the file number of the function setting the
lock. They can be useful for debugging.

threadid_func(CRYPTO_THREADID *id) is needed to record the currently-executing
thread's identifier into B<id>. The implementation of this callback should not
fill in B<id> directly, but should use CRYPTO_THREADID_set_numeric() if thread
IDs are numeric, or CRYPTO_THREADID_set_pointer() if they are pointer-based.
If the application does not register such a callback using
CRYPTO_THREADID_set_callback(), then a default implementation is used - on
Windows and BeOS this uses the system's default thread identifying APIs, and on
all other platforms it uses the address of B<errno>. The latter is satisfactory
for thread-safety if and only if the platform has a thread-local error number
facility.

Once threadid_func() is registered, or if the built-in default implementation is
to be used;

=over 4

=item *
CRYPTO_THREADID_current() records the currently-executing thread ID into the
given B<id> object.

=item *
CRYPTO_THREADID_cmp() compares two thread IDs (returning zero for equality, ie.
the same semantics as memcmp()).

=item *
CRYPTO_THREADID_cpy() duplicates a thread ID value,

=item *
CRYPTO_THREADID_hash() returns a numeric value usable as a hash-table key. This
is usually the exact numeric or pointer-based thread ID used internally, however
this also handles the unusual case where pointers are larger than 'long'
variables and the platform's thread IDs are pointer-based - in this case, mixing
is done to attempt to produce a unique numeric value even though it is not as
wide as the platform's true thread IDs.

=back

Additionally, OpenSSL supports dynamic locks, and sometimes, some parts
of OpenSSL need it for better performance.  To enable this, the following
is required:

=over 4

=item *
Three additional callback function, dyn_create_function, dyn_lock_function
and dyn_destroy_function.

=item *
A structure defined with the data that each lock needs to handle.

=back

struct CRYPTO_dynlock_value has to be defined to contain whatever structure
is needed to handle locks.

dyn_create_function(const char *file, int line) is needed to create a
lock.  Multi-threaded applications might crash at random if it is not set.

dyn_lock_function(int mode, CRYPTO_dynlock *l, const char *file, int line)
is needed to perform locking off dynamic lock numbered n. Multi-threaded
applications might crash at random if it is not set.

dyn_destroy_function(CRYPTO_dynlock *l, const char *file, int line) is
needed to destroy the lock l. Multi-threaded applications might crash at
random if it is not set.

CRYPTO_get_new_dynlockid() is used to create locks.  It will call
dyn_create_function for the actual creation.

CRYPTO_destroy_dynlockid() is used to destroy locks.  It will call
dyn_destroy_function for the actual destruction.

CRYPTO_lock() is used to lock and unlock the locks.  mode is a bitfield
describing what should be done with the lock.  n is the number of the
lock as returned from CRYPTO_get_new_dynlockid().  mode can be combined
from the following values.  These values are pairwise exclusive, with
undefined behaviour if misused (for example, CRYPTO_READ and CRYPTO_WRITE
should not be used together):

	CRYPTO_LOCK	0x01
	CRYPTO_UNLOCK	0x02
	CRYPTO_READ	0x04
	CRYPTO_WRITE	0x08

=head1 RETURN VALUES

CRYPTO_num_locks() returns the required number of locks.

CRYPTO_get_new_dynlockid() returns the index to the newly created lock.

The other functions return no values.

=head1 NOTES

You can find out if OpenSSL was configured with thread support:

 #define OPENSSL_THREAD_DEFINES
 #include <openssl/opensslconf.h>
 #if defined(OPENSSL_THREADS)
   // thread support enabled
 #else
   // no thread support
 #endif

Also, dynamic locks are currently not used internally by OpenSSL, but
may do so in the future.

=head1 EXAMPLES

B<crypto/threads/mttest.c> shows examples of the callback functions on
Solaris, Irix and Win32.

=head1 HISTORY

CRYPTO_set_locking_callback() is
available in all versions of SSLeay and OpenSSL.
CRYPTO_num_locks() was added in OpenSSL 0.9.4.
All functions dealing with dynamic locks were added in OpenSSL 0.9.5b-dev.
B<CRYPTO_THREADID> and associated functions were introduced in OpenSSL 1.0.0
to replace (actually, deprecate) the previous CRYPTO_set_id_callback(),
CRYPTO_get_id_callback(), and CRYPTO_thread_id() functions which assumed
thread IDs to always be represented by 'unsigned long'.

=head1 SEE ALSO

L<crypto(3)|crypto(3)>

=cut