I'm facing an issue with libxml2 (version 2.7.8.13).
I'm attempting to dump a node while parsing an in-memory document with a xmlTextReaderPtr reader.
So upon parsing the given node, I use xmlNodeDump() to get its whole content, and then switch to the next node.
Here is how I proceed:
[...]
// get the xmlNodePtr from the text reader
node = xmlTextReaderCurrentNode(reader);
// allocate a buffer to dump into
buf = xmlBufferCreate();
// dump the node
xmlNodeDump(buf, node->doc, node, 0 /* level of indentation */, 0 /* disable formatting */);
result = strdup((char*)xmlBufferContent(buf));
This works in most cases, but sometimes the result is missing some children from the parsed node. For instance, the whole in-memory xml document contains
[...]
<aList>
<a>
<b>42</b>
<c>aaa</c>
<d/>
</a>
<a>
<b>43</b>
...
</aList>
and I get something like:
<aList>
<a>
<b>42</b>
</c>
</a>
</aList>
The result is well formed but it lacks some data ! A whole bunch of children has "disappeared". xmlNodeDump() should recursively dumps all children of .
It looks like some kind of size limitation.
I guess I do something wrong, but I can't figure out what.
Thank you for your answers.
I succeeded in implementing this correctly another way, still I do not understand what happened there. Thank you for having read my question.
FYI, instead of trying to tinker an existing parsing code based on xmlTextReader, I have just rewritten a small parsing module for my case (dump all the 1st level siblings into separate memory chunks).
I did so by using the parsing and tree modules of libxml2, so:
get the tree from the in-memory xml document with xmlReadMemory()
get the first node with xmlDocGetRootElement()
for each sibling (with xmlNextElementSibling() ), dump its content (all children recursively) with xmlNodeDump()
Et voilà, kinda straightforward actually. Sometimes it's easier to start from scratch...
I guess there was some side effect.
Related
currently im working in parsing xml by c and c++
im using pugixml library in c++ and libxml2 library in c for parsing xml
assume i have root element in xml as "configuration" and it have 4 child elements which are protocolversion,servername,daqlist and device.
Now i can get root element (configuration),by using this root i want to move its particular child (device) without moving one by one.
In C++ by using pugixml,we have following line to directly move from configuration to its child device,
doc.child("Configuration").child("device").
In c by using libxml i just move one by one child as,
if cur node is root (configuration),then im using,
cur= cur->children->next->next->next->next->next->next->next->next->next(for move to device from config)
i dont want to move by next next..
i want simple function to move from current to particular node in c by libxml..
could anyone please help to solve this issue..?
The ->next->next... approach assumes a fixed layout of nodes and is unreliable. I'd suggest that you:
iterate the children until you find one with
child->type == XML_ELEMENT_NODE and
strcmp((char *)child->name, "device") == 0
or xmlStrcmp(child->name, BAD_CAST "device") == 0
use XPath to find nodes matching /configuration/device
I think I'm missing something trivial but I'm losing a bunch of time on this, so its solution may be useful to others too:
I'm working with libxml2 2.9.8 (pure C, not C++ bindings) under linux.
I have an external (non-libxml) tree structure representing an XML file and I'm trying to write into a string representation using libxml2. All is trivial and working nice traversing it and writing using xmlTextWriter API (it is a struct with simple attributes, like
typedef struct _simplifiedNode {
char *tag,
char *content,
struct _simplifiedNode *parent,
struct _simplifiedNodeList *children,
} simplifiedNode;
), except at a certain point I encounter a string node that may contain the string representation of an xml document. I can parse it using the xmlReadMemory API, but then I need to nest it (and not its escaped string representation) into the on-going writer, including namespaces and attributes.
Is there a trivial way I am missing to do this recursively having the parsed doc/root element, without introspecting every sub-element?
e.g.
I'm producing the following document using xmlTextWriter API
<Title>
TitleValue
</Title>
<Date>
2018-11-26
</Date>
<Content>
The Content node in the non-libxml tree is a leaf node with tag Content containing a string like
char *content = "<SomeXmlComplexDocument ss:someattr=\"attrval\">Somecontent</SomeXmlComplexDocument>"
What I Want to achieve is, instead of having something like
<Content><SomeXmlComplexDocument> ... </Content>
after having parsed and validated the content with xmlReadMemory to re-inject the document obtaining
<Content>
<SomeXmlComplexDocument ss:someattr="attrval">Somecontent</SomeXmlComplexDocument>
</Content>
namespaces and attributes should be preserved.
To serialize the inner XML fragments unescaped, you can simply use xmlTextWriterWriteRaw. This won't check whether the XML is well-formed, though. If you need validation, you'll have to parse the XML fragments at some point. Depending on the content model, you might have to use xmlParseBalancedChunkMemory instead of xmlReadMemory. It should also be possible to parse the result document in one go after it was written, but you'll lose information like original line numbers.
I'm using libxml via C, to work with xml file creation and parsing. Until recently everything worked smoothly, but a case emerged where a single tree has a subnode, lets call it S, with approximately 200,000 children. This case works surprisingly slow and I suspect the function :
xmlNewChild(/**/);
which I'm using to build the tree, has to iterate over every child of S to add one more child. Since a function that also accepts a hint (a pointer to the last added function) doesn't seem to exist, is there a better way to build the tree (maybe a batch build method) ? In case such numbers are insignificant and I should search for deficiencies elsewhere, please let me know.
Yeah, rather than keeping the entire XML in memory with xmlTree, you may want to use a combination of libxml's xmlReader and xmlWriter APIs. They're both streaming, so it won't have to keep the entire document in memory and won't have any scaling problems based on the number of elements.
Examples of both xmlReader and xmlWriter can be found here:
http://www.xmlsoft.org/examples/index.html
Original question below, update regarding solution, if someone has a similar problem:
For a fast regex I found http://re2c.org/ ; for xml parsing http://expat.sourceforge.net/
Is there an xml library I can use to parse xml from memory (and not from file) in a streaming manner in c?
Currently I have:
libxml2 ; XMLReader seems to only be possible to use with a filehandle and not in-memory
rapidxml is c++ and does not seem to expose a c interface
Requirements:
I need to process the individual xml nodes without having the whole xml (400GB uncompressed, and "only" 29GB as original .bz2 file) in memory ( bzip'd file gets read in and decompressed piecewise, and I would pass those uncompressed pieces to be consumed by the xml parser )
It does not need to very fast, but I would prefer an efficient solution
I (most probably) don't need the path of an extracted node, so it would be fine to just discard them as soon as they have been processed by my callback (if I would need the path contrary to what I think right now, I could then still track it myself)
This is part of me trying to solve my own problem posted here (and no, it's not the same question): How to efficiently parse large bz2 xml file in C
Ideally I'd like to be able to feed the library a certain amount of bytes at a time and have a function called whenever a node is completed.
Thank you very much
Here's some pseudo c code (way shorter than actual c code) for a better understanding
// extracted data gets put here
strm.next_out = buffer_ptr;
while( bytes_processed_total < filesize ) {
// extracts up to amount of data set in strm.avail_in
BZ2_bzDecompress( strm );
bytes_processed = strm.next_out - buffer_ptr;
bytes_processed_total += bytes_processed;
// here I would like to pass bytes_processed of buffer_ptr to xmlreader
}
About the data I want to parse: http://wiki.openstreetmap.org/wiki/OSM_XML
At the moment I only need certain <node ...> nodes from this, which have subnode <tag k="place" v="country|county|city|town|village"> (the '|' means at least one of those in this context, in the file it's of course only "country" etc without the '|')
xmlReaderForMemory from libxml2 seems a good one to me (but haven't used it so, I may be wrong)
the char * buffer needs to point to a valid XML document (that can be a part of your entire XML file). This can be extracted reading in chuncks your file but obtaining a valid XML fragment.
What's the structure of your XML file ? A root containing subsequent similar nodes or a fully fledged tree ?
If I had an XML like this:
<root>
<node>...</node>
<node>...</node>
<node>...</node>
</root>
I'd read starting from the opening <node> till the closing </node> and then parse it with the xmlReaderForMemory function, do what I need to do, then go on with the next <node> node.
Ofc if your <node> content is too complex/long, you may have to go deep some levels:
<node>
<subnode>....</subnode>
<subnode>....</subnode>
<subnode>....</subnode>
<subnode>....</subnode>
</node>
And read from the file until you have the entire <subnode> node (but keeping track that you're in a <node>.
I know it's ugly, but is a viable way. Or you can try to use a sax parser (dunno if some C implementation exists).
Sax parsing fires events on each node start and node end, so you can do nothing untill you find your nodes and process just them.
Another viable way can be using some external tools to filter the whole XML (XQuery or XPath processors) in order to extract just your interesting nodes from the whole file, obtain a smaller doc and then work on it.
EDIT: Zorba was a good XQuery framework, with command line preprocessor, may be a good place to look at
EDIT2: well since you have this dimensions, one alternative solution can be manage the file as a text file, so read and uncompress in chunks and then matching something like:
<yourNode>.*</yourNode>
with regexp.
If you're on a Linux/Unix you should have POSIX regexp library. Check
this question on S.O. for further insights.
Given a valid, arbitrary xmlNodePtr, I would like the string representation of that node, including the tag, attributes, and children in the same form (recursive).
FWIW, my scenario is I am using PerformXPathQuery to get a block of data from within an existing document. I need to get the results of the query, which has nested XML elements in it, as the raw string, so I can insert it into a text field.
These seems like a simple task, however I cannot find an easy way. Writing an xmlDocPtr to file must do this, however, I cannot see a handy method that will do the same thing to an arbitrary node in the tree, and return it in memory.
I hope I am just going blind from the brown-on-brown documentation color scheme at xmlsoft.org
Is xmlNodeDump (or xmlNodeDumpOutput) what you are looking for?
My code I used to dump a node to a string. It's objectiv-c so just change your output as needed.
xmlBufferPtr buffer = xmlBufferCreate();
int size = xmlNodeDump(buffer, myXMLDoc, myXMLNode, 0, 1);
NSLog(#"%d", size);
NSLog(#"%s", buffer->content);
Don't forget to free your buffer again.
One way you could do it definitely is to create a new document, then use xmlDocCopyNode to copy the node into it and serialize it.