What is the difference between the GNU Makefile variable assignments =, ?=, := and +=?

Question

Can anybody give a clear explanation of how variable assignment really works in Makefiles. What is the difference between : ``` VARIABLE = value VARIABLE ?= value VARIABLE := value VARIABLE += value ``` I have read the section in GNU Make's manual, but it still doesn't make sense to me.

Accepted Answer

Lazy Set

```
VARIABLE = value
```

Normal setting of a variable, but any other variables mentioned with the `value` field are recursively expanded with their value at the point at which the variable is used, not the one it had when it was declared

Immediate Set

```
VARIABLE := value
```

Setting of a variable with simple expansion of the values inside - values within it are expanded at declaration time.

Lazy Set If Absent

```
VARIABLE ?= value
```

Setting of a variable only if it doesn't have a value. `value` is always evaluated when `VARIABLE` is accessed. It is equivalent to

```
ifeq ($(origin VARIABLE), undefined)
  VARIABLE = value
endif
```

See the documentation for more details.

Append

```
VARIABLE += value
```

Appending the supplied value to the existing value (or setting to that value if the variable didn't exist)

What is the difference between the GNU Makefile variable assignments =, ?=, := and +=?

Solution

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