Lecture 2 - Compiling, Object Files, Linking, and Execution

Question 1

Background on C/Why C?

Answer 1

• the linux kernel is written in C
• it is fast
• it is powerful
• it is simple (easy to do low level things)

Question 2

process C goes through to be executed

Answer 2

• start with code
• gets compiled
• equivalent program is made in machine language

Question 3

compiling for this class/our C standard

Question 4

gcc

Answer 4

-GNU compiler collection
- standard compiler for most UNIX-like systems

Question 5

gcc flag -c

Answer 5

• compile file and produce an object file
• however, the object file is not an executable

Question 6

gcc flag -g

Answer 6

include debug symbols

Question 7

gcc flag -Wall

Answer 7

• enable ALL warnings
• doesn’t flag them as errors on its own

Question 8

gcc flag -Werror

Answer 8

turn warnings into errors

Question 9

gcc flag -O

Answer 9

• used for optimization during compilation
• tells compiler to optimize the generated machine code for better performance

Question 10

gcc flag -o file

Answer 10

• outputs to ‘file’
• specifies the output file name for the compiled program
• now you can run the program with that file

Question 11

gcc flag -ansi

Answer 11

adhere to the ANSI standard

Question 12

gcc flag -std=X

Answer 12

adhere to some C standard X

Question 13

what is an object file?

Answer 13

• is an intermediate machine readable file made by a compiler when compiling a source code file
• to create an executable from multiple object files, we need to link them together

Question 14

example of linking two files

Answer 14

• compile two C files and link them together
  gcc -Wall -Werror -g -c file1.c
  gcc -Wall -Werror -g -c file1.c
  gcc -o my_progr file1.o file2.o
• do it all in the same line (but doesn’t make .o files)
  gcc -Wall -Werror -g file1.c file2.c -o file

Question 15

why object files?

Answer 15

• it takes a long time to compile “big” application if they have lots of C files
• its’s better to do incremental compilation of the application

Question 16

executing a program

Answer 16

• if there’s no errors compiling/linking the program invoke it with
  gcc -Wall -Werror -c hello.c
  gcc -o hello hello.o
  ./hello
• the .c compiles hello.c into a OBJECT file hello.o

Question 17

common errors with functions

Answer 17

when putting functions into separate files, they need to have forward declarations for functions
- ex. float calc(float first, float sec); in one file and the function in the other

Question 18

types of optimization -O to pass through

Answer 18

• no optimization: -O0
• basic optimization: -O1
• moderate optimization: -O2
• max optimizaiton: -O3