Embedded SQL (C) with multithreaded applications - crashes

I am adapting some legacy code to make it multithreaded. The code always crashes in the Oracle library in a sub-thread when it is trying to connect to a database.

The first statement in the main thread is:

    EXEC SQL ENABLE THREADS; and the ProC preprocessor is called with "threads=yes".

The first thing the sub-threads do is to connect to the databases using this code:

    EXEC SQL CONTEXT ALLOCATE :ctx;
    EXEC SQL CONTEXT USE :ctx;
    EXEC SQL CONNECT :db_name IDENTIFIED BY :oracle_db_password;     EXEC SQL ALTER SESSION SET SESSION_CACHED_CURSORS = 10;     EXEC SQL ALTER SESSION SET NLS_DATE_LANGUAGE = 'ENGLISH';     EXEC SQL ALTER SESSION SET NLS_DATE_FORMAT = 'DD/MM/YY HH24:MI:SS';

    EXEC SQL ALTER SESSION SET NLS_TIMESTAMP_FORMAT = 'DD/MM/YY HH24:MI:SS';

    EXEC SQL COMMIT; The second time this code is called (to connect to a second database) the code crashes at the CONNECT statement with a segmentation fault.

Part of the traceback is:

Program terminated with signal 11, Segmentation fault.
#0 0x0610bc9e in kpummTLSGET1 ()

   from /oracle/instantclient/libclntsh.so.11.1 (gdb) bt
#0 0x0610bc9e in kpummTLSGET1 ()

   from /oracle/instantclient/libclntsh.so.11.1
#1 0x04a01ad3 in kpuhhini ()

   from /oracle/instantclient/libclntsh.so.11.1
#2 0x04a0e2c2 in kpughndl0 ()

   from /oracle/instantclient/libclntsh.so.11.1
#3 0x049ea7c4 in OCIHandleAlloc ()

   from /oracle/instantclient/libclntsh.so.11.1
#4 0x049d255e in sqlv8c ()

   from /oracle/instantclient/libclntsh.so.11.1
#5 0x049d216d in sqllam ()

   from /oracle/instantclient/libclntsh.so.11.1
#6 0x049d3b0f in sqllo3t ()

   from /oracle/instantclient/libclntsh.so.11.1
#7 0x049d36cc in sqlcnt ()

   from /oracle/instantclient/libclntsh.so.11.1
#8 0x05fdc889 in sqlexp ()

...

I know there is not much information in this post but I don't want to make it too long to read. I would be grateful for specific ideas on how to solve this. I know plenty about programming and multithreaded applications but I am new to Oracle.

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