Some Makefile tips and questions

$(SHARED_LIBS:%.so=%.d): %.d: %.c
@-$(CC) -M -MG $(CFLAGS) $< | \
sed -e 's@^.*\.o:@$*.d $*_sh.o:@' > $@

Q. What doest this mean? $(SHARED_LIBS:%.so=%.d)
Q. What does here two colon mean?

-------------------------------------------------------
Makefile Rule Format:
In general, a rule looks like this:
targets : prerequisites
command
...

or like this:

targets : prerequisites ; command
command
-------------------------------------------------------

Another similar parts.
$(SHARED_LIBS): %.so : %_sh.o
$(CC) -shared $(EXT_LDFLAGS) -o $@ $<

$(SHARED_SE_LIBS:%.so=%.d): %.d: %.c
@-$(CC) -M -MG $(CFLAGS) $< | \
sed -e 's@^.*\.o:@$*.d $*_sh.o:@' > $@

$(SHARED_SE_LIBS): %.so : %_sh.o
$(LD) -shared $(EXT_LDFLAGS) -o $@ $< $(LDLIBS)

Understand the Linux Makefile system

Makefile 的組成 (compoments)
1. explicit rules
2. implicit rules
3. variable definitions
4. directives
5. comments


1. 利用 origin function 可以知道某一個變數是從何處來的

Example:
ifdef V
ifeq ("$(origin V)", "command line")



The result of this function is a string telling you how the variable variable was defined:

`undefined'
if variable was never defined.
`default'
if variable has a default definition, as is usual with CC and so on. See Variables Used by Implicit Rules. Note that if you have redefined a default variable, the origin function will return the origin of the later definition.
`environment'
if variable was defined as an environment variable and the `-e' option is not turned on (see Summary of Options).
`environment override'
if variable was defined as an environment variable and the `-e' option is turned on (see Summary of Options).
`file'
if variable was defined in a makefile.
`command line'
if variable was defined on the command line.
`override'
if variable was defined with an override directive in a makefile (see The override Directive).
`automatic'
if variable is an automatic variable defined for the execution of the commands for each rule (see Automatic Variables).