在这一篇中有提到，井号与define之间空格，可能导致搜索上的一些问题。
https://mzhan017.blog.csdn.net/article/details/135289451
今天看到有专门做这个空格的修改：
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=fcf70d4114db9ff7923f5dfeb3fea6e2d623e5c2;hp=3f3822198993be18d4d9ccb1593eea274dbd2ba0
应该是为了阅读代码的方便性，因为如果不加空格会对代码阅读造成一定的困难；但是为什么不直接将整个#define都缩进呢？例如下面的一段
glibc/elf/dl-load.h
/* The right way to map in the shared library files is MAP_COPY, which
makes a virtual copy of the data at the time of the mmap call; this
guarantees the mapped pages will be consistent even if the file is
overwritten. Some losing VM systems like Linux's lack MAP_COPY. All we
get is MAP_PRIVATE, which copies each page when it is modified; this
means if the file is overwritten, we may at some point get some pages
from the new version after starting with pages from the old version.
To make up for the lack and avoid the overwriting problem,
what Linux does have is MAP_DENYWRITE. This prevents anyone
from modifying the file while we have it mapped. */
#ifndef MAP_COPY
# ifdef MAP_DENYWRITE
# define MAP_COPY (MAP_PRIVATE | MAP_DENYWRITE)
# else
# define MAP_COPY MAP_PRIVATE
# endif
#endif
应个人的阅读爱好，其实只要养成一种阅读习惯，就可以，不管是前加空格还是后加空格，只要可用表示出if缩进块的功能就好。