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IRIX Base Documentation 2001 May
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SGI IRIX Base Documentation 2001 May.iso
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7.2.1 (BETA) C++ Release Notes
DDDDooooccccuuuummmmeeeennnntttt NNNNuuuummmmbbbbeeeerrrr 000000007777....2222666600006666----000011110000
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1. _D_y_n_a_m_i_c__S_h_a_r_e_d__O_b_j_e_c_t_s
A Dynamic Shared Object, or DSO, is an ELF format object
file, very similar in structure to an executable program but
with no "main". It has a shared component, consisting of
shared text and read-only data; a private component,
consisting of data and the GOT (Global Offset Table);
several sections that hold information necessary to load and
link the object; and a liblist, the list of other shared
objects referenced by this object. Most of the libraries
supplied by SGI are available as dynamic shared objects.
_N_O_T_E: When building a DSO containing C++ objects, the link
step mmmmuuuusssstttt be performed using the _C_C command, instead
of using _l_d directly. Follow the instructions in the
_d_s_o(_5) man page exactly, except that you must
substitute _C_C wherever that man page says _l_d.
A DSO is relocatable at runtime; it can be loaded at any
virtual address. A consequence of this is that all
references to external symbols must be resolved at runtime.
References from the private region (.e.g. from private data)
are resolved once at load-time; references from the shared
region (e.g. from shared text) must go through an
indirection table (GOT) and hence have a small performance
penalty associated with them.
Code compiled for use in a shared object is referred to as
Position Independent Code (PIC), whereas non-PIC is usually
referred to as non-shared. Non-shared code and PIC cannot
be mixed in the same object.
At Runtime, eeeexxxxeeeecccc loads the main program and then loads rrrrlllldddd,,,,
the runtime linking loader, which finishes the exec
operation. Starting with main's liblist, rrrrlllldddd loads each
shared object on the list, reads that object's liblist, and
repeats the operation until all shared objects have been
loaded. Next, rrrrlllldddd allocates common and fixes up symbolic
references in each loaded object. (This is necessary
because we don't know until runtime where the object will be
loaded.) Next, each object's _i_n_i_t code is executed.
Finally, control is transferred to "__start".
For a more complete discussion of DSOs, including answers to
questions frequently asked about them, see the _d_s_o(_5) man
page.