请教Makefile中wildcard执行疑问

zmdesperado

1. $(TARGET):
   
2. >---$(MKDIR) $(OUTDIR)
   
3. >---for dir in $(SUBDIRS); do \
   
4. >--->---$(MAKE) -C $dir; done
   
5. >---$(warning $(OUTDIR))
   
6. >---$(warning $(OUTDIR)/socket.o)
   
7. >---$(warning before: $(wildcard $(OUTDIR)/*.o))
   
8. >---$(CC) $(CFLAGS) $(filter-out $(OUTDIR)/socket.o,$(wildcard $(OUTDIR)/*.o)) -o $(TARGET) $(LDFLAGS)
   
9. >---$(warning after: $(wildcard $(OUTDIR)/*.o))

复制代码

执行makefile后有两个疑问：
1. 为什么四个warning先打印出来，先于其他语句执行？并且before和after中wildcard都返回null。
2. 在执行完$(MAKE) -C $$dir后OUTDIR中已经有.o文件了，但$(CC) $(CFLAGS) $(filter-out $(OUTDIR)/socket.o,$(wildcard $(OUTDIR)/*.o))仍然失败，即wildcard没有返回.o文件。难道wildcard在整个make file执行前就解析了？

请大家帮忙看一下，谢谢了！

zmdesperado

回复 1# zmdesperado


    原来makefile的wildcard是从cache中获得的，而OUTDIR下的.o file不是通过target生成的：
> The original poster's problem is almost certainly due to the
> combination of caching and a Makefile which doesn't tell the full
> truth.  In particular: make caches the list of files in each directory
> as it happens to read them, and will answer $(wildcard) expansions
> from that cache.  As part of that, make assumes that your makefile
> tells the truth and that a file won't be created unless it was the
> target of a rule that was invoked at some point.
>
> My guess is that the *.out files are created as the side-effect of
> some other rule.  Make doesn't know that, so it doesn't know to add
> them to the cache, so $(wildcard *.out) gives the wrong result.

However, the way this is being invoked via a submake $(MAKE) leads me to
believe from the observed behaviour that the changes to the cache are
not getting propagated back up the stack.  Thus the top-level make
(where the wildcard gets expanded) doesn't have them in it's cache.  I
guess I'll have to do a $(shell ...) for things to work as they are.

captivated

Makefile 是两阶段执行的，
warning, info, error, 这种东西是第一阶段执行的，
规则是第二阶段执行的。

wildcard 没问题，检查你的逻辑。

zmdesperado

回复 3# captivated


    Makefile 是两阶段执行的，
warning, info, error, 这种东西是第一阶段执行的，
规则是第二阶段执行的。
-->是的，但GNU make manual中好像没有说明。


wildcard 没问题，检查你的逻辑。
-->wildcard感觉不适合用在规则中，用在解析时比较安全。我是这么理解的：在解析时没有OUTDIR，所以主Makefile的cache中没有OUTDIR；在规则中创建OUTDIR，并在sub-Makefile中生成.o files时并没有update。在主Makefile中将$(OUTDIR)/*.o作为target，会update cache。GNU make manual中也没有关于cache的说明。