Makefile变量扩展语法

Question

Please explain how does this TARGET get expanded?

PROJECT=helloworld
TARGET = $(PROJECT:=.so)

For example, I have generated helloworld.so.1.2.3, helloworld.so.1 and helloworld.so

Answer 1

This would get messy in the comments, reiterating the description from docs with this example. For:

PROJECT=helloworld
TARGET = $(PROJECT:=.so)

Value of $(TARGET) would be helloworld.so . As an empty string '' at the end of the word would gets replaced with .so . This "end of a word" is the difference from:

TARGET = $(PROJECT).so

With PROJECT being helloworld , the resulting value would look the same, but call make PROJECT="hello world" and in case of the former you end up with hello.so and world.so in TARGET , for the latter, it would be "hello world.so" . Or to quote the docs:

$(var:a=b)' ... replace every a at the end of a word with b ... “at the end of a word”, we mean that a must appear either followed by whitespace or at the end of the value in order to be replaced