Debugging Aids

The link-editor provides a debugging facility that allows you to trace the link-editing process in detail. This facility can help you understand and debug the link-edit of your applications and libraries. The type of information that is displayed by using this facility is expected to remain constant. However, the exact format of the information might change slightly from release to release.

Some of the debugging output might be unfamiliar if you do not have an intimate knowledge of the ELF format. However, many aspects might be of general interest to you.

Debugging is enabled by using the -D option. This option must be augmented with one or more tokens to indicate the type of debugging that is required.

The tokens that are available with -D can be displayed by typing -D help at the command line.

$ ld -Dhelp

By default, all debug output is sent to stderr, the standard error output file. Debug output can be directed to a file instead, using the output token. For example, the help text can be captured in a file named ld-debug.txt.

$ ld -Dhelp,output=ld-debug.txt

Most compiler drivers assign the -D option a different meaning, often to define preprocessing macros. The LD_OPTIONS environment variable can be used to bypass the compiler driver, and supply the -D option directly to the link-editor.

The following example shows how input files can be traced. This syntax can be useful to determine what libraries are used as part of a link-edit. Objects that are extracted from an archive are also displayed with this syntax.

$ LD_OPTIONS=-Dfiles cc -o prog main.o -L. -lfoo
....
debug: file=main.o  [ ET_REL ]
debug: file=./libfoo.a  [ archive ]
debug: file=./libfoo.a(foo.o)  [ ET_REL ]
debug: file=./libfoo.a  [ archive ] (again)
....

Here, the member foo.o is extracted from the archive library libfoo.a to satisfy the link-edit of prog. Notice that the archive is searched twice to verify that the extraction of foo.o did not warrant the extraction of additional relocatable objects. Multiple “(again)” diagnostics indicates that the archive is a candidate for ordering using lorder(1) and tsort(1).

By using the symbols token, you can determine which symbol caused an archive member to be extracted, and which object made the initial symbol reference.

$ LD_OPTIONS=-Dsymbols cc -o prog main.o -L. -lfoo
....
debug: symbol table processing; input file=main.o  [ ET_REL ]
....
debug: symbol[7]=foo  (global); adding
debug:
debug: symbol table processing; input file=./libfoo.a  [ archive ]
debug: archive[0]=bar
debug: archive[1]=foo  (foo.o) resolves undefined or tentative symbol
debug:
debug: symbol table processing; input file=./libfoo(foo.o)  [ ET_REL ]
....

The symbol foo is referenced by main.o. This symbol is added to the link-editor's internal symbol table. This symbol reference causes the extraction of the relocatable object foo.o from the archive libfoo.a.

By using the detail token together with the symbols token, the details of symbol resolution during input file processing can be observed.

$ LD_OPTIONS=-Dsymbols,detail cc -o prog main.o -L. -lfoo
....
debug: symbol table processing; input file=main.o  [ ET_REL ]
....
debug: symbol[7]=foo  (global); adding
debug:   entered  0x000000 0x000000 NOTY GLOB  UNDEF REF_REL_NEED
debug:
debug: symbol table processing; input file=./libfoo.a  [ archive ]
debug: archive[0]=bar
debug: archive[1]=foo  (foo.o) resolves undefined or tentative symbol
debug:
debug: symbol table processing; input file=./libfoo.a(foo.o)  [ ET_REL ]
debug: symbol[1]=foo.c
....
debug: symbol[7]=bar  (global); adding
debug:   entered  0x000000 0x000004 OBJT GLOB  3     REF_REL_NEED
debug: symbol[8]=foo  (global); resolving [7][0]
debug:       old  0x000000 0x000000 NOTY GLOB  UNDEF main.o
debug:       new  0x000000 0x000024 FUNC GLOB  2     ./libfoo.a(foo.o)
debug:  resolved  0x000000 0x000024 FUNC GLOB  2     REF_REL_NEED
....

The original undefined symbol foo from main.o has been overridden with the symbol definition from the extracted archive member foo.o. The detailed symbol information reflects the attributes of each symbol.

In the previous example, you can see that using some of the debugging tokens can produce a wealth of output. To monitor the activity around a subset of the input files, place the -D option directly in the link-edit command line. This option can be toggled on and toggled off. In the following example, the display of symbol processing is switched on only during the processing of the library libbar.

$ ld .... -o prog main.o -L. -Dsymbols -lbar -D!symbols ....