C: linking

Question 1

True or False: c source files are compiled over each other and linked in order

Answer 1

False. c source files are compiled separately into relocatable object files, which then get linked.

Question 2

How do linkers offer modularity?

Answer 2

cus you can have multiple files duh

Question 3

why are linkers efficient?

Answer 3

because source files are compiled separately, when you edit something you just need to recompile the files changed, and relink.

Question 4

what is a symbol? what is the symbol table? where are symbols stored?

Answer 4

symbols are essentially variables, function names, things that get referenced and defined. its the name itself.

the symbol table holds symbols. its an array of structs. its stored in the object files (by assembler), but linkers need to resolve all symbol tables into a single one.

a symbol struct may look like this:

struct SymbolEntry {
char name[255]; // Symbol name, assuming a maximum of 255 characters
int dataType; // Data type (e.g., int, float)
void* memoryLocation; // Memory location where the symbol is stored
};

struct SymbolTable {
struct SymbolEntry entries[100]; // Array of symbol entries, assuming a max of 100 symbols
int count; // Number of entries in the table
};

Question 5

symbol relocation

Answer 5

separate code files need to be merged into a single data section.

Relocates symbols from their relative locations in the .o files to their final absolute memory locations in the executable.

Question 6

what is a relocatable object file?

Answer 6

(.o file)
Contains code and data in a form that can be combined with other relocatable object files to form executable object file.
each .c file becomes a .o file

Question 7

what is an executable object file?

Answer 7

(.out file)
Contains code and data in a form that can be copied directly into memory and then executed.

Question 8

what is a shared object file?

Answer 8

(.so on linux, .dll on windows)
Special type of relocatable object file that can be loaded into memory and linked dynamically, at either load time or run- time.

essentially, they are “global” libraries that just exist to you, at any time.

Question 9

executable and linkable format (ELF)

Answer 9

ELF object file format are a standard binary format for object files. .o, .out, .so (relocatable, executable, and shared)

also called ELF Binaries

(more info in the flash card set for elf binaries)

Question 10

global vs external vs local symbols

Answer 10

global: symbols defined by a module that can referenced by other modules. (non-static C functions, non-static globals, aka the default)

external: symbols referenced by a module, but defined in another

local: defined and referenced exclusively by a single module. (static functions/global variables)

note: local linker symbols are not local program variables. Linker symbols are used to manage functions and data across your program.

Question 11

Local non-static C variables vs. local static C variables

Answer 11

non static: stored on the stack

static: stored in .bss or .data

if i have a static int x = 0; in two different functions, they can both exist independently in the .data section. the symbol table may store these as x.1, x.2

regularly defined variables, however (int x = 0;), just go on the stack.

Question 12

strong vs weak symbol

Answer 12

strong: procedures & initialized globals

weak: initialized globals

strong: int foo = 5;
p1(), cry()

weak: int foo; (notice its not initialized)

RECOMMENDED: look at slide 17 of C: Linkers for some linker puzzles

Question 13

are multiple strong symbols allowed? if not, what error will occur

Answer 13

No. Linker error

Question 14

ilovec.c
int foo = 0;

ihatec.c
int foo;

allowed?

Answer 14

yes. references to the weak symbol will point to the strong symbol.

Question 15

are multiple weak symbol allowed

Answer 15

yes. it’ll pick a random one

overrideable via gcc -fno-common

Question 16

is it advisable to use extern when referencing an external global?

Answer 16

yes. do that.

Question 17

the two step process 😳

Answer 17

1. symbol resolution
2. relocation aka MERGE

Question 18

Executable Object File format (im just adding it here for fun ok probably dont memorize this) (memorize the notes tho lowkey)

Answer 18

ELF header

Program header table (required for executables)

.init section

.text section

.rodata section

.data section

.bss section

.symtab

.debug

.line

.strtab

 Section header table (required for relocatables)

notes:
stack grows down (%rsp)
heap grows up (malloc, brk)
between these areas is a memory mapping for shared libraries ( .so)

below heap is read/write (.data,.bss)
below that is read only (.init, .text, .rodata)

Question 19

static libraries (.a) archive files

Answer 19

Concatenate related relocatable object files into a single file
with an index (called an archive).

linker will look for unresolved references here and link it in.

each function of a library may exist as its own object file to be added to an archive

linker links object files and archive files (otherwise known as static libraries)

ex.
libc.a: Standard C Library, 4.6 MB of 1496 object files

io, memory allocation, signal handing, string handling, random, etc.

libm.a: the C Math Library, 2 MB of 444 object files

sin cos tan log exp sqrt etc

Question 20

dynamic linking

Answer 20

Dynamic linking can also occur after program has begun
(run-time linking).
In Linux, this is done by calls to the dlopen() interface.

uses shared libraries. ex. libc.so (the c standard library usually gets dynamically linked)

The linker creates a partially linked executable with object files and from .so files, relocation and symbol table info

the dynamic linker creates a fully linked executable in memory adding in code and data sections of .so files.

Shared libraries are typically loaded using memory- mapped files, which are a feature of the VM system.
.so libraries are compiled to position-independent code (PIC) to enable mapping at any address.

Question 21

when can linking occur?
a. compile time
b. load time
c. run time
d. all of the above

Answer 21

d