CODEWITHSUNDEEP
cgccmakefile
Happy Mittal
Happy Mittal

Reputation: 3747

How does make automatically create .o file

I have a file hello.c (no hello.o yet).
When I give command

gcc -o hello hello.o

It gives error gcc: error: hello.o: No such file or directory, but when I create a makefile containing following rule,

hello: hello.o
    gcc -o hello hello.o

and run make, it automatically creates hello.o from hello.c and then successfully creates hello executable.
Since I didn't write command to create hello.o from hello.c in makefile, how does make know which command it should run?

Upvotes: 0

Views: 270

Answers (2)

Wojtek Surowka
Wojtek Surowka

Reputation: 21003

make has not only uses the rules you added explicitly in makefiles, it has implicit rules as well. The full list you can find at https://www.gnu.org/software/make/manual/html_node/Catalogue-of-Rules.html#Catalogue-of-Rules and the first implicit rule in the list is

Compiling C programs

n.o is made automatically from n.c with a recipe of the form ‘$(CC) $(CPPFLAGS) $(CFLAGS) -c’.

Upvotes: 4

unwind
unwind

Reputation: 399949

Because make has been around the block enough to have implicit rules about how to transform a .c file to a .o file:

Compiling C programs

n.o is made automatically from n.c with a recipe of the form
‘$(CC) $(CPPFLAGS) $(CFLAGS) -c’.

Upvotes: 0

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