Function “patsubst” in Makefile

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In fact all is explained in the doc:

Finds whitespace-separated words in TEXT …

means that one or more spaces have to separate the words.

… that match PATTERN …

means that it select only words that match a pattern (which can include some spaces).

… and replaces them with REPLACEMENT.

means that the selected patterns will be replace by a replacement.

A picture is worth a thousand words.

For PATTERN = X:

           +----  SEPARATORS  ----+
           |                      |
   +-------+-------+     +--------+------+
   |               |     |               | 
X  space space space  X  space space space  x
|                     |                     |
+---------------------+---------------------+
                      |
                   PATTERNS

For PATTERN = X%:

                 +----  SEPARATORS  ---+
                 |                     |
               +-+-+                 +-+-+
               |   |                 |   | 
X  space space space  X  space space space  x
|            |        |            |        |
+------+-----+        +------+-----+        |
       |                     |              |
       +---  PATTERNS  ------+--------------+

Interesting thing:

When you use the % character in your pattern, you can re-use it in the replacement, like this:

$(patsubst x%,y%,xa xb xc)
# Will be "ya yb yc"

But when you have space character in the % variable, make will strip them in the replacement.

$(patsubst x%,y%,xa   xb   xc)
# Will also be "ya yb yc"

EDIT: After reading the source code, the interesting things are:

  • function.c +146: The function patsubst_expand_pat
  • misc.c +337: The function find_next_token
  • misc.c +325: The function next_token

So here is the behavior:

  1. If no % in the pattern, this is a simple substitution (which keep the spaces).
  2. Else it split the text by words and get rid of all spaces (using the isblank function).
  3. Finally, it does the replacement

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