notebook/notes/linkers/relocatable.md

title	TARGET DECK	FILE TAGS	tags
Relocatable Object Files	Obsidian::STEM	linker	linker

Overview

Relocatable object files are those, typically ending with a .o suffix, produced by the assembler. They contain binary code and data in a form that can be combined with other relocatable object files at compile time. The following diagram shows how one looks like when formatted using elf:

!

%%ANKI Basic Relocatable object files are outputs of which compiler driver component? Back: The assembler. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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%%ANKI Basic Relocatable object files are inputs into which compiler driver component? Back: The linker. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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%%ANKI Basic A relocatable object file is typically broken up into what three regions? Back: The header, sections, and the section header table. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: linker::elf

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%%ANKI Basic In a relocatable object file, what exists between the header and section header table? Back: The sections. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: linker::elf

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%%ANKI Cloze A relocatable object file consists of a {header}, {sections}, and a {section header table}, in that order. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: linker::elf

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%%ANKI Basic Where in a relocatable object file does the section header table exist? Back: At the end. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: linker::elf

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Symbols

Every relocatable object module m has a [[elf#.symtab|symbol table]] that contains information about the symbols defined and referenced by m. In the context of a linker, there are three different kinds of symbols:

1. Global symbols defined by m and that can be referenced by other modules.
2. Global symbols referenced by m but defined by another module.
3. Local symbols defined and referenced exclusively by m.

%%ANKI Basic With respect to linkers, how many kinds of symbols are there? Back: Three. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic With respect to linkers, what are the three kinds of symbols? Back: Global (defined), global (referenced), and local. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic What distinguishes the two types of global symbols a linker understands? Back: Whether or not the symbol is defined within the module in question. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic With respect to linkers, a global (defined) symbol corresponds to what kind of C construct? Back: A nonstatic function or global variable defined in the given module. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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%%ANKI Basic With respect to linkers, a global (referenced) symbol corresponds to what kind of C construct? Back: A nonstatic function or global variable defined in a different module. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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%%ANKI Basic With respect to linkers, a local symbol corresponds to what kind of C construct? Back: A static function or variable defined in the given module. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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Pseudosections

There are three special pseudosections specified in the section header table that do not have entries in the section header table. Note pseudosections only exist in relocatable object files.

%%ANKI Basic How many types of pseudosections can be found in relocatable object files? Back: Three. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic How many types of pseudosections can be found in executable object files? Back: Zero. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What are the three pseudosections possibly found in relocatable object files? Back: ABS, UNDEF, and COMMON. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic In what region of an ELF file can references to pseudosections be found? Back: The section header table. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Why are ELF pseudosections named the way they are? Back: They don't actually correspond to any ELF section. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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ABS

Marks symbols that should not be relocated.

%%ANKI Basic What does the ABS pseudosection indicate? Back: The corresponding symbol should not be relocated. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Why is the ABS pseudosection named the way it is? Back: It's short for absolute. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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UNDEF

Marks undefined symbols. These are referenced in the object module but (presumably) defined elsewhere.

%%ANKI Basic What does the UNDEF pseudosection indicate? Back: The corresponding symbol is (presumably) defined elsewhere. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Why is the UNDEF pseudosection named the way it is? Back: It's short for undefined. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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COMMON

Assuming -fcommon, marks unitialized data objects that are not yet allocated.

%%ANKI Basic What does the COMMON pseudosection indicate? Back: The corresponding symbol is uninitialized and not yet allocated. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic What C variables are marked COMMON instead of put in .bss? Back: Global uninitialized variables. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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%%ANKI Basic What C variables are put in .bss instead of marked COMMON? Back: Static variables and global variables initialized to zero. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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%%ANKI Basic Assuming -fcommon, what kind of C variables does the .bss section contain? Back: Static variables and global variables initialized to zero. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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%%ANKI Basic Assuming -fcommon, what kind of C variables does the COMMON section contain? Back: Global uninitialized variables. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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%%ANKI Basic Assuming -fcommon, which ELF section contains uninitialized global C variables? Back: N/A. These are "placed" into the COMMON pseudosection. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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%%ANKI Basic Assuming -fcommon, which ELF section contains global C variables initialized to a zero value? Back: .bss Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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%%ANKI Basic Consider the following translation unit. Assuming -fcommon, which ELF section will foo end up in?

int foo = 0;

Back: .bss Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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%%ANKI Basic Consider the following translation unit. Assuming -fcommon, which ELF section will foo end up in?

int foo;

Back: N/A. It is "placed" into the COMMON pseudosection. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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At compile time, the compiler exports each global symbol as either strong or weak, and the assembler encodes this information in the symbol table. Functions and initialized global variables get strong symbols whereas uninitialized global variables get weak symbols. The linker then resolves global symbols as follows:

1. Multiple strong symbols with the same name are not allowed.
2. Given a strong symbol and multiple weak symbols with the same name, choose the strong symbol.
3. Given multiple weak symbols with the same name, choose any of the weak symbols.

%%ANKI Basic Assuming -fcommon, global symbols are further categorized into what two buckets? Back: Strong and weak. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Which component of the compiler driver indicates whether a global variable is strong or weak? Back: The compiler. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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%%ANKI Basic Does a function correspond to a strong or weak symbol? Back: Strong. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Does a globally initialized variable correspond to a strong or weak symbol? Back: Strong. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Does a globally uninitialized variable correspond to a strong or weak symbol? Back: Weak. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Does a static variable correspond to a strong or weak symbol? Back: N/A. Strong and weak describe global variables. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Is foo considered strong or weak in the following translation unit?

int foo;

Back: Weak. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Is foo considered strong or weak in the following translation unit?

int foo = 0;

Back: Strong. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Is foo considered strong or weak in the following translation unit?

int foo = 1;

Back: Strong. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Is foo considered strong or weak in the following translation unit?

int foo();

Back: Strong. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic How does a linker resolve multiple strong symbols with the same name? Back: N/A. It throws an error. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic How does a linker resolve one strong symbol and multiple weak symbols with the same name? Back: It prefers the strong symbol. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic How does a linker resolve one weak symbol and multiple strong symbols with the same name? Back: N/A. It throws an error. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI How does a linker resolve multiple weak symbols with the same name? Back: By arbitrarily picking one. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. END%%

%%ANKI Cloze Assuming -fcommon, {1:strong} is to {2:.bss} whereas {2:weak} is to {1:COMMON}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Why is COMMON considered in conflict with the C standard? Back: C only allows a single definition for any object. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

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Static Libraries

A static library is a format for packaging multiple relocatable object files together. When the linker builds the output executable, it only copies the object modules in the library referenced by the application program.

On Linux systems, static libraries are typically stored on disk as an archive. An archive is a collection of concatenated relocatable object files with a header that describes the size and location of each member object file.

%%ANKI Cloze A {static library} packages multiple {relocatable object files} together. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic A static library is a collection of what kind of files? Back: Relocatable object files. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic What memory-saving strategy does static libraries allow linkers to employ? Back: Only copying relocatable object files actually used by the application program. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Linux typically uses what file format for its static libraries? Back: Archives. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Cloze On Linux machines, an {archive} typically has a {.a} suffix. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic On Linux machines, what kind of files usually have a .a suffix? Back: Archives. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic A Linux archive file is a specific example of what more general kind of file? Back: A static library. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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Symbol Resolution

During symbol resolution, the linker scans the relocatable object files and archives left to right in the same order they appear on the command line. During this scan, the linker maintains three sets:

• E, a set of relocatable object files to be merged into the final executable;
• U, a set of unresolved symbols,
• D, a set of symbols defined in previous input files.

For each input file, the linker determines if its an object file or archive. If the former, the three sets are updated. If the latter, the linker attempts to match symbols in U against those defined by members of the archive. For any match, the three sets are further updated.

If U is nonempty when the linker finishes scanning all input files, it prints an error and terminates. Otherwise it merges and relocates the object files in E to build the executable.

%%ANKI Basic In order, what input files does the linker scan?

$ cc -lvector main.c

Back: libvector.a, main.o. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Assuming libvector.a is necessary, why does the following fail?

$ cc -lvector main.c

Back: libvector.a is scanned before main.o so no relocatable object files are copied. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic During symbol resolution, what set(s) of files does the linker maintain? Back: The set of relocatable object files to merge into the final executable. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic During symbol resolution, what set(s) of symbols does the linker maintain? Back: The sets of unresolved and resolved symbols. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Assume success. What relocatable object files were merged into the output executable?

$ clang -lvector main.c

Back: Just main.o. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Assume success. What relocatable object files were merged into the output executable?

$ clang main.c -lvector

Back: main.o and those in libvector.a used by main.o. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Where on the command line are static libraries usually specified to the compiler driver? Back: At the end. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Why shouldn't a static library be the first argument to the linker? Back: No unresolved symbols exist at that point; the argument is effectively ignored. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Assuming at least one relocatable object file, when does symbol resolution fail? Back: When any unresolved symbols exist after all input files are scanned. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Linking fails at symbol resolution if what set(s) are nonempty? Back: The set of unresolved symbols. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Under what condition must a static library be repeated on the command line? Back: When there exist mutually recursive references between it and another module/library. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Let p.o depends on libx.a. What minimal command lets cc resolve all symbol references? Back:

$ cc p.o -lx

Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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%%ANKI Basic Let p.o depends on liby.a which depends on libx.a . What minimal command lets cc resolve all symbol references? Back:

$ cc p.o -ly -lx

Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

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Bibliography

• Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.