file function allows the makefile to write to or read from
a file. Two modes of writing are supported: overwrite, where the text
is written to the beginning of the file and any existing content is
lost, and append, where the text is written to the end of the file,
preserving the existing content. In both cases the file is created if
it does not exist. It is a fatal error if the file cannot be opened
for writing, or if the write operation fails. The
function expands to the empty string when writing to a file.
When reading from a file, the
file function expands to the
verbatim contents of the file, except that the final newline (if there
is one) will be stripped. Attempting to read from a non-existent file
expands to the empty string.
The syntax of the
file function is:
$(file op filename[,text])
file function is evaluated all its arguments are
expanded first, then the file indicated by filename will be
opened in the mode described by op.
The operator op can be
> to indicate the file will be
overwritten with new content,
>> to indicate the current
contents of the file will be appended to, or
< to indicate the
contents of the file will be read in. The filename specifies
the file to be written to or read from. There may optionally be
whitespace between the operator and the file name.
When reading files, it is an error to provide a text value.
When writing files, text will be written to the file. If text does not already end in a newline a final newline will be written (even if text is the empty string). If the text argument is not given at all, nothing will be written.
For example, the
file function can be useful if your build
system has a limited command line size and your recipe runs a command
that can accept arguments from a file as well. Many commands use the
convention that an argument prefixed with an
@ specifies a
file containing more arguments. Then you might write your recipe in
program: $(OBJECTS) $(file >$@.in,$^) $(CMD) $(CMDFLAGS) @$@.in @rm $@.in
If the command required each argument to be on a separate line of the input file, you might write your recipe like this:
program: $(OBJECTS) $(file >$@.in) $(foreach O,$^,$(file >>$@.in,$O)) $(CMD) $(CMDFLAGS) @$@.in @rm $@.in