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On SVR4 platforms gdb can use the symbol table of a dynamic loader (e.g. ld.so) and normal memory operations to maintain a list of shared libraries. Still a special library list provided by this packet is more efficient for the gdb remote protocol.
The ‘qXfer:libraries-svr4:read’ packet returns an XML document which lists loaded libraries and their SVR4 linker parameters. For each library on SVR4 target, the following parameters are reported:
name, the absolute file name from the l_name field of
struct link_map.
lm with address of struct link_map used for TLS
(Thread Local Storage) access.
l_addr, the displacement as read from the field l_addr of
struct link_map. For prelinked libraries this is not an absolute
memory address. It is a displacement of absolute memory address against
address the file was prelinked to during the library load.
l_ld, which is memory address of the PT_DYNAMIC segment
Additionally the single main-lm attribute specifies address of
struct link_map used for the main executable. This parameter is used
for TLS access and its presence is optional.
gdb must be linked with the Expat library to support XML SVR4 library lists. See Expat.
A simple memory map, with two loaded libraries (which do not use prelink), looks like this:
<library-list-svr4 version="1.0" main-lm="0xe4f8f8">
<library name="/lib/ld-linux.so.2" lm="0xe4f51c" l_addr="0xe2d000"
l_ld="0xe4eefc"/>
<library name="/lib/libc.so.6" lm="0xe4fbe8" l_addr="0x154000"
l_ld="0x152350"/>
</library-list-svr>
The format of an SVR4 library list is described by this DTD:
<!-- library-list-svr4: Root element with versioning -->
<!ELEMENT library-list-svr4 (library)*>
<!ATTLIST library-list-svr4 version CDATA #FIXED "1.0">
<!ATTLIST library-list-svr4 main-lm CDATA #IMPLIED>
<!ELEMENT library EMPTY>
<!ATTLIST library name CDATA #REQUIRED>
<!ATTLIST library lm CDATA #REQUIRED>
<!ATTLIST library l_addr CDATA #REQUIRED>
<!ATTLIST library l_ld CDATA #REQUIRED>