Lab Exam : Makefile Review

Question 1

How do you make gcc not link what it makes?

Answer 1

Use the -c option.
When -c is omitted, it looks for a main function for the entry point.

Question 2

What recipe compiles an object file?

Answer 2

module1.o: module1.c
$(CC) -c $(CFLAGS) module1.c -o module1.o

Question 3

What recipe links the files?

Answer 3

my_prog: module1.o module2.o
$(CC) $(CFLAGS) module1.o module2.o my_prog

Question 4

CFLAGS

Answer 4

-Wall -Wextra -pedantic -std=c99 -g

Question 5

CC

Answer 5

gcc

Question 6

What do header files contain?

Answer 6

.h files contain function prototypes (or “headers”), type definitions, and useful macros.

Question 7

What files need to include header files?

Answer 7

Files that implement the functions listed or use the functions listed.

Question 8

-I (captial i) option

Answer 8

Adds a place to search to the preprocessor’s list. This gives the preprocessor an additional place to search.
I.e. -I. for the current directory
Note that there can be many -I flags

Question 9

What do you store -I (capital I) in?

Answer 9

CPPFLAGS

Question 10

How do you write a header file?

Answer 10

ifndef FILENAME_H

#define FILENAME_H
//stuff goes here
#endif

Question 11

How do you link files together into a static library?

Answer 11

ar rcs libname.a module1.o module2.o ….

This creates a library called libname.a which contains the object files.
Note that they MUST begin with “lib” and end with “.a”.
(or “.so” for dynamic libraries).

Question 12

How do you link a library during the linking step?

Answer 12

use -L and -l (lowercase L) options when invoking gcc.
-L followed by the directory (i.e. -L/home/…) says where to look for library files.
-l followed by the name of a library tells gcc to link a specific library from one of the locations given by -L.
I.e.
-lname-name tells gcc to find libname-name.a
Use LDFLAGS for these.

Question 13

Where do you put the -L flags?

Answer 13

In LDFLAGS

Question 14

How to make a shared libary?

Answer 14

$(CC) -shared $(CFLAGS) module1.o module2.o …. -o libname.so

Question 15

Do you need to tell gcc the -L and -l options for dynamic libraries?

Answer 15

Yes. It still needs to be able to “search” for a particular dynamic library.

Question 16

How do you tell the system where to look for a dynamic library?

Answer 16

export LD_LIBRARY_PATH=/path/to/library-dir
This can correct the No such file or directory error when loading a shared library.

Question 17

What directories do we want our makefiles to use?

Answer 17

build/ to contain binary objects
lib/ to contain all of our libraries
bin/ to contain the executables

Call these
BUILD_DIR = build/$(shell uname -s)-$(uname -m)
LIB_DIR = lib/$(shell uname -s)-$(uname -m)
BIN_DIR = bin/$(shell uname -s)-$(uname -m)

Note that you need to use uname to identify the system.

Question 18

How do you use uname?

Answer 18

uname -s for the operating sytem
uname -m for the machine architecture

Question 19

How do you make a directory in a makefile?

Answer 19

I.e
$(BUILD_DIR):
mkdir -p $(BUILD_DIR)

Question 20

What does | do in pre-requestites (dependencies)?

Answer 20

On the left needs to be up to date (timestamps), but both sides need to exist.
Useful for directories.

Question 21

How do you symbolically link things to the root of the project (where the Makefile is)

Answer 21

my_prog: $(BIN_DIR)/my_prog
ln -sf $(BIN_DIR)/my_prog my_prog

-sf is for symbolic force (force updates/re-creates symlink if needed).

Question 22

Where does $(LDFLAGS) go?

Answer 22

After $(CFLAGS)

Question 23

How to use -l (lowercase L)

Answer 23

-l nameoflib

Says to use libnameoflib.so