There are several schemes for dealing with include files: the
traditional N_SOL approach, Sun's N_BINCL approach, and the
XCOFF C_BINCL approach (which despite the similar name has little in
common with N_BINCL).
An N_SOL symbol specifies which include file subsequent symbols
refer to. The string field is the name of the file and the value is the
text address corresponding to the end of the previous include file and
the start of this one. To specify the main source file again, use an
N_SOL symbol with the name of the main source file.
The N_BINCL approach works as follows. An N_BINCL symbol
specifies the start of an include file. In an object file, only the
string is significant; the linker puts data into some of the other
fields. The end of the include file is marked by an N_EINCL
symbol (which has no string field). In an object file, there is no
significant data in the N_EINCL symbol. N_BINCL and
N_EINCL can be nested.
If the linker detects that two source files have identical stabs between
an N_BINCL and N_EINCL pair (as will generally be the case
for a header file), then it only puts out the stabs once. Each
additional occurrence is replaced by an N_EXCL symbol. I believe
the GNU linker and the Sun (both SunOS4 and Solaris) linker are the only
ones which supports this feature.
A linker which supports this feature will set the value of a
N_BINCL symbol to the total of all the characters in the stabs
strings included in the header file, omitting any file numbers. The
value of an N_EXCL symbol is the same as the value of the
N_BINCL symbol it replaces. This information can be used to
match up N_EXCL and N_BINCL symbols which have the same
filename. The N_EINCL value, and the values of the other and
description fields for all three, appear to always be zero.
For the start of an include file in XCOFF, use the .bi assembler
directive, which generates a C_BINCL symbol. A .ei
directive, which generates a C_EINCL symbol, denotes the end of
the include file. Both directives are followed by the name of the
source file in quotes, which becomes the string for the symbol.
The value of each symbol, produced automatically by the assembler
and linker, is the offset into the executable of the beginning
(inclusive, as you'd expect) or end (inclusive, as you would not expect)
of the portion of the COFF line table that corresponds to this include
file. C_BINCL and C_EINCL do not nest.