Using the DIRECT mapfile Keyword

The DIRECT mapfile keyword provides a means of establishing a direct binding for individual symbols. This mechanism is intended for specialized link-editing scenarios.

From the components used in the previous example, the function main() references the external functions W() and X(). The binding of these functions follow the default search model.

$ LD_DEBUG=symbols,bindings prog3
.....
18754: symbol=W;  lookup in file=prog3  [ ELF ]
18754: symbol=W;  lookup in file=./W.so.2  [ ELF ]
18754: binding file=prog3 to file=./W.so.2: symbol `W'
.....
18754: symbol=X;  lookup in file=prog3  [ ELF ]
18754: symbol=X;  lookup in file=./W.so.2  [ ELF ]
18754: symbol=X;  lookup in file=./X.so.2  [ ELF ]
18754: binding file=prog3 to file=./X.so.2: symbol `X'

prog3 can be rebuilt with DIRECT mapfile keywords so that direct bindings are established to the functions W() and X().

$ cat mapfile
$mapfile_version 2
SYMBOL_SCOPE {
        global:
                W       { FLAGS = EXTERN DIRECT };
                X       { FLAGS = EXTERN DIRECT };
};
$ cc -o prog4 -R. main.c W.so.2 X.so.2 -Mmapfile

The LD_DEBUG environment variable can be used to observe the runtime bindings.

$ LD_DEBUG=symbols,bindings,detail prog4
.....
23432: symbol=W;  lookup in file=./W.so.2  [ ELF ]
23432: binding file=prog4 to file=./W.so.2: symbol `W'  (direct)
23432: symbol=X;  lookup in file=./X.so.2  [ ELF ]
23432: binding file=prog4 to file=./x.so.2: symbol `X'  (direct)

The lari(1) utility can also reveal the direct binding information. However in this case, the functions W() and X() are not multiply defined. Therefore, by default lari does not find these functions interesting. The -a option must be used to display all symbol information.

$ lari -a prog4
....
[1:1ED]: W(): ./W.so.2
.....
[2:1ED]: X(): ./X.so.2
.....

The same direct binding to W.so.2 and X.so.1, can be produced by building prog4 with the -B direct option or the -z direct option. The intent of this example is solely to convey how the mapfile keyword can be used.