What does the `%` character do in GNU Make?

I am using GNU make. In a Makefile that I got, i see the symbol '%'. e.g. in context of %.c, %.开发者_如何学Casm , %.o etc.. (Is it a wildcard that will return all the files of that extension)

What does % mean?

What will the below rule do:

%.o:  %.c
  gcc $< -o:$@

thank you,

-AD

The rule:

%.o:  %.c
    gcc $< -o:$@

Will for each c file build an object file using the command below it. So yes, % is like a wildcard, but I tend to think of it as "for each" because that's what happens as a result of the rule. $< means dependent files and $@ means the object to be built. If you did this:

fred.o: fred.c
    gcc $< -o:$@

Here $@=fred.o and $<=fred.c Likewise

fred.o: fred.c george.c
    gcc $< -o:$@

gives $@=fred.o and $<=fred.c george.c.

Edit: from the commands I shall expand on this.

% will match everything matching that around it. So %.c means all the .c files in the current directory. asm/%.asm means all the files in the subdir asm with the .asm extension. It acts like a token, so whatever is found will be used whenever you next use % i.e. in the label.

So, the rule:

objs/asm/%.o: arch/%.S
     nasm -felf64 $< -o $@

Given arch/hello.S, arch/bye.S, arch/somethingelse.S will create obj/asm/hello.o, obj/asm/bye.S, obj/asm/somethingelse.o.