Linker and Libraries Guide

Debugging Aids

A debugging library is provided with the Solaris linkers. This library enables you to trace the link-editing process in more detail. This library helps you understand, or debug, the link-edit of your own applications or libraries. Although the type of information displayed using this library is expected to remain constant, 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, and all output produced is directed to the standard error. This option must be augmented with one or more tokens to indicate the type of debugging required. The tokens available can be displayed by typing -D help at the command line.


$ ld -Dhelp
debug:
debug:            For debugging the link-editing of an application:
debug:                  LD_OPTIONS=-Dtoken1,token2 cc -o prog ...
debug:            or,
debug:                  ld -Dtoken1,token2 -o prog ...
debug:            where placement of -D on the command line is significant
debug:            and options can be switched off by prepending with `!'.
debug:
debug:
debug: args       display input argument processing
debug: basic      provide basic trace information/warnings
debug: detail     provide more information in conjunction with other options
debug: entry      display entrance criteria descriptors
debug: files      display input file processing (files and libraries)
debug: got        display GOT symbol information
debug: help       display this help message
debug: libs       display library search paths; detail flag shows actual
debug:              library lookup (-l) processing
debug: map        display map file processing
debug: move       display move section processing
debug: reloc      display relocation processing
debug: sections   display input section processing
debug: segments   display available output segments and address/offset
debug:              processing; detail flag shows associated sections
debug: statistics display processing statistics
debug: strtab     display information about string table compression; detail
debug:              shows layout of string tables
debug: support    display support library processing
debug: symbols    display symbol table processing; detail flag shows
debug:              internal symbol table addition and resolution
debug: tls        display TLS processing info
debug: unused     display unused/unreferenced files; detail flag shows
debug:              unused sections
debug: versions   display version processing

Note –

This listing is an example, and shows the options meaningful to the link-editor. The exact options might differ from release to release.


Most compiler drivers interpret the -D option during their preprocessing phase. Therefore, the LD_OPTIONS environment variable is a suitable mechanism for passing this option to the link-editor.

The following example shows how input files can be traced. This syntax can be especially useful in determining what libraries have been located, or what relocatable objects have been extracted from an archive during a link-edit.


$ 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. More than one “(again)” display 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 and 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.


Note –

This output has been simplified for this document.


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. In cases where you are interested only in the activity around a subset of the input files, the -D option can be placed directly in the link-edit command-line, and toggled on and 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 .... 

Note –

To obtain the link-edit command line you might have to expand the compilation line from any driver being used. See Using a Compiler Driver.