$ gdb python
GNU gdb (GDB) 7.5.91.20130417-cvs-ubuntu
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
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For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /usr/bin/python2.7...Reading symbols from /usr/lib/debug/usr/bin/python2.7...done.
done.
(gdb) break socket
Haltepunkt 1 at 0x4170e0
(gdb) run
Starting program: /usr/bin/python2.7
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Python 2.7.4 (default, Apr 19 2013, 18:28:01)
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from lxml import etree
>>> etree.LXML_VERSION
(3, 1, 0, 0)
>>> etree.LIBXML_VERSION
(2, 9, 0)
>>> etree.XMLSchema(file="premis.xsd")

Breakpoint 1, socket () at ../sysdeps/unix/syscall-template.S:81
81      ../sysdeps/unix/syscall-template.S: Datei oder Verzeichnis nicht gefunden.
(gdb) bt
#0  socket () at ../sysdeps/unix/syscall-template.S:81
#1  0x00007ffff59b0755 in have_ipv6 () at ../../nanohttp.c:196
#2  0x00007ffff59b0af8 in xmlNanoHTTPConnectHost (host=host@entry=0xb19190 "www.loc.gov", port=80) at ../../nanohttp.c:1057
#3  0x00007ffff59b2069 in xmlNanoHTTPMethodRedir__internal_alias (URL=0xb184c0 "http://www.loc.gov/standards/xlink/xlink.xsd",
    method=0x7ffff5a345f0 "GET", input=0x0, contentType=0x0, redir=redir@entry=0x0, headers=0x0, ilen=0)
    at ../../nanohttp.c:1385
#4  0x00007ffff59b2403 in xmlNanoHTTPMethod__internal_alias (URL=<optimized out>, method=<optimized out>,
    input=<optimized out>, contentType=<optimized out>, headers=<optimized out>, ilen=<optimized out>) at ../../nanohttp.c:1594
#5  0x00007ffff5973137 in __xmlParserInputBufferCreateFilename (URI=0xb184c0 "http://www.loc.gov/standards/xlink/xlink.xsd",
    enc=XML_CHAR_ENCODING_NONE) at ../../xmlIO.c:2633
#6  0x00007ffff5947380 in xmlNewInputFromFile__internal_alias (ctxt=ctxt@entry=0xa84990,
    filename=filename@entry=0xb184c0 "http://www.loc.gov/standards/xlink/xlink.xsd") at ../../parserInternals.c:1511
#7  0x00007ffff5975605 in xmlDefaultExternalEntityLoader (URL=0xb1bf60 "http://www.loc.gov/standards/xlink/xlink.xsd", ID=0x0,
    ctxt=0xa84990) at ../../xmlIO.c:4044
#8  0x00007ffff6133e81 in ?? () from /usr/lib/python2.7/dist-packages/lxml/etree.so
#9  0x00007ffff597546f in xmlLoadExternalEntity__internal_alias (URL=<optimized out>, ID=0x0, ctxt=0xa84990)
    at ../../xmlIO.c:4100
#10 0x00007ffff5961c60 in xmlCtxtReadFile__internal_alias (ctxt=0xa84990,
    filename=filename@entry=0x9b0811 "http://www.loc.gov/standards/xlink/xlink.xsd", encoding=encoding@entry=0x0,
    options=options@entry=2) at ../../parser.c:15396
#11 0x00007ffff59e2f2a in xmlSchemaAddSchemaDoc (pctxt=pctxt@entry=0x959d70, type=type@entry=1,
    schemaLocation=0x9b0811 "http://www.loc.gov/standards/xlink/xlink.xsd", schemaDoc=schemaDoc@entry=0x0,
    schemaBuffer=schemaBuffer@entry=0x0, schemaBufferLen=schemaBufferLen@entry=0, invokingNode=invokingNode@entry=0xa8fbd0,
    sourceTargetNamespace=sourceTargetNamespace@entry=0x9b0640 "info:lc/xmlns/premis-v2",
    importNamespace=0x9b0623 "http://www.w3.org/1999/xlink", bucket=bucket@entry=0x7fffffffd6e8) at ../../xmlschemas.c:10547
#12 0x00007ffff59ebb9a in xmlSchemaParseImport (node=0xa8fbd0, schema=0xa84850, pctxt=0x959d70) at ../../xmlschemas.c:10823
#13 xmlSchemaParseSchemaTopLevel (nodes=<optimized out>, schema=<optimized out>, ctxt=<optimized out>)
    at ../../xmlschemas.c:9770
#14 xmlSchemaParseNewDocWithContext (pctxt=pctxt@entry=0x959d70, schema=schema@entry=0xa84850, bucket=<optimized out>)
    at ../../xmlschemas.c:10142
#15 0x00007ffff59eec21 in xmlSchemaParse__internal_alias (ctxt=0x959d70) at ../../xmlschemas.c:21355
#16 0x00007ffff618f1f4 in ?? () from /usr/lib/python2.7/dist-packages/lxml/etree.so
#17 0x00000000004b1a1e in type_call.25713 (type=0x7ffff63fc420, args=(), kwds={'file': 'premis.xsd'})
    at ../Objects/typeobject.c:741
#18 0x000000000047c19d in PyObject_Call (kw={'file': 'premis.xsd'}, arg=<optimized out>, func=<type at remote 0x7ffff63fc420>)
    at ../Objects/abstract.c:2529