C++ Compiling and Linking

Question 1

What are the 5 stages of program lifetime?

Answer 1

Code writing, compilation, linking, loading, execution

Question 2

What is a translation unit?

Answer 2

Source code giving rise to an object file.

Question 3

What is the output of compilation?

Answer 3

Object file.

Question 4

What is the compilation?

Answer 4

Process of translating the code from a higher level language to a lower level language like assembly.

Question 5

What is the decompilation?

Answer 5

Process of translating the code from a lower level language like assembly to a higher level langauge.

Question 6

What are the compilation stages?

Answer 6

Pre-processing, linguistic analysis, assembly, optimization, code emission.

Question 7

What does pre-processing do ?

Answer 7

Replaces all macros with appropriate constants, conditionally includes/excludes parts of code, includes files based on include directives.

Question 8

What gcc flag can be used to get only the preprocesing output?

Answer 8

The flag is -E:

gcc -E input.c -o output.i

Question 9

What gcc flags can be used to get a more readable preprocessed output?

Answer 9

The flags are: -E and -P:

gcc -E -P input.c -o output.i

Question 10

What do lines starting with # in preprocessed output denote?

Answer 10

They usually mark the lines and file to which the code that follows belong to. Example:

25 hello.c

This denotes that the code which follows belongs to file hello.c starting from line 25.

Question 11

What does -E flag do to preprocessor?

Answer 11

It removes the suplementary information in the preprocessed output like line numbers and similar.

Question 12

What is lexical analysis?

Answer 12

Lexical analysis is a stage in linguistic analysis which splits the source into tokens.

Question 13

What is parsing/syntax analysis?

Answer 13

It is a stage in linguistic analysis in which the order of tokens is checked against language rules and specifications.

Question 14

What is linguistic analysis?

Answer 14

Linguistic analysis is a stage in which source is checked against language defined rules like syntax, semantics etc.

Question 15

What is semantic analysis?

Answer 15

It is a stage in linguistic analysis in which the source statements and are checked for validity, like two objects being concatenated via operator + or similar.

Question 16

What is assembling?

Answer 16

Compilation stage in which the source is translated to assembly language.

Question 17

What are the two known assembly instruction printing formats?

Answer 17

AT&T and Intel.

Question 18

What is different between AT&T and Intel assembly format?

Answer 18

One significant difference is that AT&T format uses the (operation, source, destination) order and Intel format uses the (operation, destination, source) order.

Question 19

What is the purpose of the optimization stage?

Answer 19

To remove unused code, inline operations where possible, remove unnecessary computations and similar.

Question 20

What is the purpose of the code emission stage?

Answer 20

To create object files from assembly.

Question 21

What flag can be used to generate the assembly output?

Answer 21

The -S flag,

gcc -S -masm=att input.c -o input.s

Question 22

What flag can be used to generate the object files without linking?

Answer 22

The -c flag

Question 23

What flag can be used to get the dissasembled output of objdump ?

Answer 23

-D

Question 24

What is objdump?

Answer 24

Utility program to check object files.

Question 25

What do object files consist of?

Answer 25

Symbols and sections

Question 26

What do symbols represent in object files?

Answer 26

References to memory addresses or data memory.

Question 27

What is contained in .txt section in obj files?

Answer 27

Code

Question 28

What is contained in .data section of obj files?

Answer 28

Initialized data.

Question 29

What is contained in .bss section of obj files?

Answer 29

Uninitialized data.

Question 30

Why is C/C++ building implemented as a two step procedure?

Answer 30

Because of code reuse and flexibility with libraries inclusion.

Question 31

What is the main task of the linker?

Answer 31

To resolve all the references and create a final memory map section out of all the object files included.

Question 32

What are the two stages of linking?

Answer 32

Relocation and reference resolving.

Question 33

What is relocation in linking step?

Answer 33

Process in which sections from different individual obj files are combined together.

Question 34

What is reference resolving in linking step?

Answer 34

Process in which memory addresses are found for the externally dependent variables and functions.

Question 35

What is the most significant difference between the executable and the dynamic library?

Answer 35

Dynamic library does not include the startup routine section (crt0 or crt1) in its memory map.

Question 36

What is the role of the shell in program execution stages?

Answer 36

Shell is usually used to launch the program.

Question 37

What is the first action a shell does upon program launch?

Answer 37

It clones itself by calling fork().

Question 38

Why does the shell create a copy of itself when launching a program?

Answer 38

To pass all the environment variables to the child process.

Question 39

What is the role of the kernel in program launch?

Answer 39

It invokes one of the exec() functions which is supposed to determine the executable format and start the preparation of the process memory map.

Question 40

What does loader do?

Answer 40

Loader copies the sections created by the linker into the memory map.

Question 41

How does loader group the sections provided by the linker?

Answer 41

Into segments by their loading requirements. Whether the attributes are read-only, read-write, write etc.

Question 42

What utility can be used to check the segments defined by the loader?

Answer 42

readelf --segments

Question 43

What is the role of the _start() function?

Answer 43

To prepare arguments for the __libc_start_main function which loader calls in the final stage of loading the program.

Question 44

What is the role of the __libc_start_main() function?

Answer 44

It initializes things needed before the main function starts, it registers cleanup handlers, it starts the main thread and executes the main function.

Question 45

Where is the information about the memory address from which the program execution will start (_start() function) usually stored?

Answer 45

In ELF header's e_entry field.