Why automatic variable in my makefile doesn't find any target? - makefile

Question

I've written this hello world in hello.c:
#include <stdio.h>
int main() {
printf("Hello, World!\n");
exit( 0 );
}
my Makefile is:
%: %.c
When I run make I will get this error: make: *** No targets. Stop.

Answer 1

Your makefile provides a rule %: %.c specifying that it's extensionless executables and .c files that you're interested in (in fact, just the built-in rules do that much), but gives no hint that there's a source file named hello.c or a target named hello.
When you type make by itself, make takes the first target listed in the makefile as the target to be made, but your makefile contains no targets whatsoever, hence No targets. Stop. In short, make has no clue that there is anything nearby with a name like hello*.
With your makefile as is, typing make hello will do what you want, as it tells make what it is that you'd like to build.
If you tell make about hello, you'll also be able to type just make to do what you want:
hello: hello.c
%: %.c
or more idiomatically and flexibly, you would list all your "top-level" targets in an all target:
all: hello
%: %.c
.PHONY: all

Answer 2

Typical c hello world prgram:
hello.c:
#include <stdio.h>
int main() {
printf("Hello, World!\n");
return 0;
}
Typical Makefile, short but complete:
Makefile:
all: hello
hello: hello.o
gcc -o "$#" hello.o
hello.o: hello.c
g++ -c hello.c
.PHONY:clean
clean:
rm *.o hello
Example with pattern rules:
all: hello
hello: hello.o
gcc -o "$#" hello.o
%.o: %.c
gcc -c $<
.PHONY:clean
clean:
rm *.o hello
Example with delimiter(\n as enter, \t as tab):
all: hello\n
hello: hello.o\n
\tgcc -o "$#" hello.o
%.o: %.c\n
\tgcc -c $<
.PHONY:clean
clean:\n
\trm *.o hello