Fix grammar error in verify pod. PR#3355

[oweals/openssl.git] / doc / crypto / threads.pod

diff --git a/doc/crypto/threads.pod b/doc/crypto/threads.pod
index bcaa94d1bb8a8b135a7d271e63b4262a295f80bd..3df4ecd7768e2761b47154cc166bbba86d48f4f8 100644 (file)
--- a/doc/crypto/threads.pod
+++ b/doc/crypto/threads.pod
@@ -53,8 +53,10 @@ OpenSSL can safely be used in multi-threaded applications provided
 that at least two callback functions are set.
 
 locking_function(int mode, int n, const char *file, int line) is
-needed to perform locking on shared data stuctures. Multi-threaded
-applications will crash at random if it is not set.
+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> &
@@ -63,9 +65,10 @@ 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.
 
-id_function(void) is a function that returns a thread ID. It is not
+id_function(void) is a function that returns a thread ID, for example
+pthread_self() if it returns an integer (see NOTES below).  It isn't
 needed on Windows nor on platforms where getpid() returns a different
-ID for each thread (most notably Linux).
+ID for each thread (see NOTES below).
 
 Additionally, OpenSSL supports dynamic locks, and sometimes, some parts
 of OpenSSL need it for better performance.  To enable this, the following
@@ -89,7 +92,7 @@ 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 nunmbered n. Multi-threaded
+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
@@ -106,7 +109,7 @@ 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 behavior if misused (for example, CRYPTO_READ and CRYPTO_WRITE
+undefined behaviour if misused (for example, CRYPTO_READ and CRYPTO_WRITE
 should not be used together):
 
        CRYPTO_LOCK     0x01
@@ -122,13 +125,13 @@ CRYPTO_get_new_dynlockid() returns the index to the newly created lock.
 
 The other functions return no values.
 
-=head1 NOTE
+=head1 NOTES
 
 You can find out if OpenSSL was configured with thread support:
 
  #define OPENSSL_THREAD_DEFINES
  #include <openssl/opensslconf.h>
- #if defined(THREADS)
+ #if defined(OPENSSL_THREADS)
    // thread support enabled
  #else
    // no thread support
@@ -137,6 +140,22 @@ You can find out if OpenSSL was configured with thread support:
 Also, dynamic locks are currently not used internally by OpenSSL, but
 may do so in the future.
 
+Defining id_function(void) has it's own issues.  Generally speaking,
+pthread_self() should be used, even on platforms where getpid() gives
+different answers in each thread, since that may depend on the machine
+the program is run on, not the machine where the program is being
+compiled.  For instance, Red Hat 8 Linux and earlier used
+LinuxThreads, whose getpid() returns a different value for each
+thread.  Red Hat 9 Linux and later use NPTL, which is
+Posix-conformant, and has a getpid() that returns the same value for
+all threads in a process.  A program compiled on Red Hat 8 and run on
+Red Hat 9 will therefore see getpid() returning the same value for
+all threads.
+
+There is still the issue of platforms where pthread_self() returns
+something other than an integer.  This is a bit unusual, and this
+manual has no cookbook solution for that case.
+
 =head1 EXAMPLES
 
 B<crypto/threads/mttest.c> shows examples of the callback functions on