ld链接器的--start-group和--end-group参数说明

–start-group archives --end-group
The archives should be a list of archive files. They may be either explicit file names, or `-l’ options. The specified archives are searched repeatedly until no new undefined references are created. Normally, an archive is searched only once in the order that it is specified on the command line. If a symbol in that archive is needed to resolve an undefined symbol referred to by an object in an archive that appears later on the command line, the linker would not be able to resolve that reference. By grouping the archives, they all be searched repeatedly until all possible references are resolved. Using this option has a significant performance cost. It is best to use it only when there are unavoidable circular references between two or more archives.

正常情况，链接的时候库文件只会按它们出现在命令行的顺序搜索一遍，如果包里有未定义的引用标号，而且该包还被放在命令行的后面,这样链接器就无法解决该标号的引用问题。通过给包分组，这些包可以被循环搜索直到所有的引用都可以解决为止。使用该选项将降低性能。只有在无法避免多个包之间互相引用的情况下才使用。用法示例：

      cmd_vmlinux__ ?= $(LD) $(LDFLAGS) $(LDFLAGS_vmlinux) -o $@ \
      -T $(vmlinux-lds) $(vmlinux-init)                          \
      --start-group $(vmlinux-main) --end-group                  \ 
      $(filter-out $(vmlinux-lds) $(vmlinux-init) $(vmlinux-main) vmlinux.o FORCE ,$^)