On some platforms, a dynamic loader (e.g. ld.so) runs in the same process as your application to manage libraries. In this case, gdb can use the loader's symbol table and normal memory operations to maintain a list of shared libraries. On other platforms, the operating system manages loaded libraries. gdb can not retrieve the list of currently loaded libraries through memory operations, so it uses the ‘qXfer:libraries:read’ packet (see qXfer library list read) instead. The remote stub queries the target's operating system and reports which libraries are loaded.
The ‘qXfer:libraries:read’ packet returns an XML document which lists loaded libraries and their offsets. Each library has an associated name and one or more segment or section base addresses, which report where the library was loaded in memory.
For the common case of libraries that are fully linked binaries, the library should have a list of segments. If the target supports dynamic linking of a relocatable object file, its library XML element should instead include a list of allocated sections. The segment or section bases are start addresses, not relocation offsets; they do not depend on the library's link-time base addresses.
gdb must be linked with the Expat library to support XML library lists. See Expat.
A simple memory map, with one loaded library relocated by a single offset, looks like this:
<library-list> <library name="/lib/libc.so.6"> <segment address="0x10000000"/> </library> </library-list>
Another simple memory map, with one loaded library with three allocated sections (.text, .data, .bss), looks like this:
<library-list> <library name="sharedlib.o"> <section address="0x10000000"/> <section address="0x20000000"/> <section address="0x30000000"/> </library> </library-list>
The format of a library list is described by this DTD:
<!-- library-list: Root element with versioning --> <!ELEMENT library-list (library)*> <!ATTLIST library-list version CDATA #FIXED "1.0"> <!ELEMENT library (segment*, section*)> <!ATTLIST library name CDATA #REQUIRED> <!ELEMENT segment EMPTY> <!ATTLIST segment address CDATA #REQUIRED> <!ELEMENT section EMPTY> <!ATTLIST section address CDATA #REQUIRED>
In addition, segments and section descriptors cannot be mixed within a single library element, and you must supply at least one segment or section for each library.