makefile +define

Makefile is a build automation tool used to compile and build software projects. The "+define" you mentioned is not a standard directive in Makefile, but it can be used as a convention to pass custom-defined variables or flags to the make command.

In a Makefile, you can define variables using the syntax "VAR_NAME = value". These variables can be referenced and used in the rules or recipes defined in the Makefile.

To use "+define" convention, you can pass the values to the make command using the "-D" flag. For example:

# Makefile

# Define a variable with a default value
MY_VARIABLE ?= default_value

# Use the variable in a rule
my_rule:
    echo $(MY_VARIABLE)

To override the default value of the variable, you can pass a new value using the "+define" convention when invoking the make command:

$ make +define=custom_value my_rule

This will set the value of MY_VARIABLE to "custom_value" during the execution of the "my_rule" rule.