I've tried using gSOAP for accessing a web service (e.g. using supplied WSDL to generate C stubs and then using them in an app). However, I've found that the generated .c and object files is quite big (several megabytes), which is a problem in embedded environment where I work.
Do you know of any simpler SOAP libraries, or do I have to fall back to generic XML generators and parsers like ezXML?
Usually we fall back to creating the XML directly (mostly by string concatenation) where no good SOAP library can be used.
Another solution could be that you switch to JSON, which (usually) has smaller overhead and request/response sizes so it could be better in embedded programs. If you only have a SOAP WebService available you could use a proxy Script on the Server which translates JSON Requests to SOAP Requests and SOAP Responses to JSON Responses.
Is this a web service that you are creating? If so, consider using REST instead of SOAP. REST is far simpler, and you can use existing, tested, working now HTTP handlers instead of going through a huge HTTP - XML - SOAP translation layer.
If you are consuming someone else's web service, examine the SOAP schema and/or examples of responses. I cannot believe I am advocating this, but if the schema is not extensible or recursive, you may be better off using a simple LALR parser or even string matching in the raw HTTP responses instead of trying to parse SOAP or XML at all. This is far simpler to implement in embedded C.
Have you looked at Apache CXF. It has several code gen features
* Java to WSDL
* WSDL to Java
* XSD to WSDL
* WSDL to XML
* WSDL to SOAP
* WSDL to service
A more helpful guide to build a consumer is here.
I recently looked into this question too, and the best option I found was gSOAP, it is very mature and well tested. However, I decided to go a non-SOAP route, which was an option since I'm on both client and server sides. Before using gSOAP, make sure you can live with their license, you may be obliged to release your code or pay them, depending how you use it.
Another option is Apache Axis2/C, though I have no experience with it (I would guess that it has a similar sized footprint to gSOAP). Their client API is here. A tutorial on the client API is here.
If you decide to go the parsed XML route, you might be interested in this SO question (see answers).
You might also checkout boost::spirit for the parsed route. It has the ability to make small, fast, specialized (and general) parsers, if you're comfortable with C++ (they can be written to be reentrant, so a calling them through a static object with an extern "C" interface is kosher). I can vouch for it in the general sense (not specific to XML). Steep learning curve, but big payoff.