Here are some functions that operate on strings:
$(subst ee,EE,feet on the street)substitutes the string `fEEt on the strEEt'.
patsubstfunction invocations can be quoted with preceding backslashes (`\'). Backslashes that would otherwise quote `%' characters can be quoted with more backslashes. Backslashes that quote `%' characters or other backslashes are removed from the pattern before it is compared file names or has a stem substituted into it. Backslashes that are not in danger of quoting `%' characters go unmolested. For example, the pattern `the\%weird\\%pattern\\' has `the%weird\' preceding the operative `%' character, and `pattern\\' following it. The final two backslashes are left alone because they cannot affect any `%' character. Whitespace between words is folded into single space characters; leading and trailing whitespace is discarded. For example,
$(patsubst %.c,%.o,x.c.c bar.c)produces the value `x.c.o bar.o'. Substitution references (see section Substitution References) are a simpler way to get the effect of the
$(var:pattern=replacement)is equivalent to
$(patsubst pattern,replacement,$(var))The second shorthand simplifies one of the most common uses of
patsubst: replacing the suffix at the end of file names.
$(var:suffix=replacement)is equivalent to
$(patsubst %suffix,%replacement,$(var))For example, you might have a list of object files:
objects = foo.o bar.o baz.oTo get the list of corresponding source files, you could simply write:
$(objects:.o=.c)instead of using the general form:
stripcan be very useful when used in conjunction with conditionals. When comparing something with the empty string `' using
ifneq, you usually want a string of just whitespace to match the empty string (see section Conditional Parts of Makefiles). Thus, the following may fail to have the desired results:
.PHONY: all ifneq "$(needs_made)" "" all: $(needs_made) else all:;@echo 'Nothing to make!' endifReplacing the variable reference `$(needs_made)' with the function call `$(strip $(needs_made))' in the
ifneqdirective would make it more robust.
$(findstring a,a b c) $(findstring a,b c)produce the values `a' and `' (the empty string), respectively. See section Conditionals that Test Flags, for a practical application of
patsubstfunction above. The
filterfunction can be used to separate out different types of strings (such as file names) in a variable. For example:
sources := foo.c bar.c baz.s ugh.h foo: $(sources) cc $(filter %.c %.s,$(sources)) -o foosays that `foo' depends of `foo.c', `bar.c', `baz.s' and `ugh.h' but only `foo.c', `bar.c' and `baz.s' should be specified in the command to the compiler.
filterfunction. For example, given:
objects=main1.o foo.o main2.o bar.o mains=main1.o main2.othe following generates a list which contains all the object files not in `mains':
$(sort foo bar lose)returns the value `bar foo lose'. Incidentally, since
sortremoves duplicate words, you can use it for this purpose even if you don't care about the sort order.
Here is a realistic example of the use of
patsubst. Suppose that a makefile uses the
to specify a list of directories that
make should search for
VPATH: Search Path for All Dependencies).
This example shows how to
tell the C compiler to search for header files in the same list of
The value of
VPATH is a list of directories separated by colons,
such as `src:../headers'. First, the
subst function is used to
change the colons to spaces:
$(subst :, ,$(VPATH))
This produces `src ../headers'. Then
patsubst is used to turn
each directory name into a `-I' flag. These can be added to the
value of the variable
CFLAGS, which is passed automatically to the C
compiler, like this:
override CFLAGS += $(patsubst %,-I%,$(subst :, ,$(VPATH)))
The effect is to append the text `-Isrc -I../headers' to the
previously given value of
override directive is
used so that the new value is assigned even if the previous value of
CFLAGS was specified with a command argument (see section The
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