how gnu make tells if file's been changed?

thinhare

Hi, all

I am writing a program to test if the file has been modified recently, and am thinking about how to implement it.

To know if a file has been changed, there has to be a reference to compared. My primitive idea was to check the modified stamp and the accessed stamp of the file, if they were not the same, probabaly the file has been changed. But after some experiments, it was not like what I supposed.

I also thought to keep a copy of the file, so I could compare the time stamps between them, but if the volume of the files were large, that certainly causes some overhead.

As I know, GNU make only compile those source files that have been modified. How does it do to tell if a file's changed or not?

thanks.

xi2008wang

应该是将原文件的一些属性(如果最近修改间)记录在一个文件中,之后与它比较吧

caijimin

假设 a.o 依赖a.c
make只是去看 a.c 的修改时间是否比 a.o 的修改时间新，是的话就需要重新编译，
不需要存上次的修改时间

flw2

make只是比较目标和依赖的时间，就一个文件而言没有什么是否修改，因为只有一个更改时间