For example, the line: project. Obj states that project. Exe depends on main. At run time make compares the time that project. Exe was last changed to the times main. Obj were last changed. If either source is newer than project.
Makefile, variable, assignment, shell Command
C bcc ms. C This makefile shows three rules, one each for making project. The rules as shown above are called explicit rules since they are supplied explicitly in the makefile. Make also has inference rules that generalize the make process. Inference rules are discussed on Page and in the User's guide on Page. We will return to and modify this example during the course of this tutorial. Dependency lines: When to build a target Top The lines with the colon : in them are called dependency lines. They determine when the target is to be rebuilt. To the left essay of the colon is the target of the dependency. To the right of the colon are the sources 1 needed to make the target. A dependency line says the target depends on the sources.
For example: line_one line_two more_line_two line_three is the same as: line_one line_two line_three rules Top A rule tells make both when and how to make a file. As an example, suppose your project involves compiling source files main. C then linking them to produce the executable project. Withholding a detailed explanation for a bit, here is a makefile using Borland C which will manage the task of making project. Exe: The Example makefile project. Obj tlink c0s main. Exe cs /Lf:bclib main. C bcc ms c main.
With a few business exceptions, the reviews initialization file holds the same kind of information as does a makefile. Both the initialization file and the makefile are composed of the following components: comments, dependency lines, directives, macros, response files, rules and shell lines. Continued makefile lines, lines in the makefile can be very long. For easier reading a long line can be broken up by putting enter as the last characters of this line and the rest of this (logical) line on the next (physical) line of the makefile. For example: first_part_of_line second_part_of_line is the same as: first_part_of_line second_part_of_line. Comments, top The simplest makefile statement is a comment, which is indicated by the comment character. All text from the comment character to the end of the line is ignored. Here is a large comment as might appear in a makefile to describe its contents: makefile for Opus make.1 Compiler: Microsoft.0 Linker: Microsoft Link.10 The comment character could also be used at the end.
An initialization file is read first, followed by the makefile. The initialization file holds instructions for all makes and is used to customize the operation of make. Make automatically reads the initialization file whenever it starts. Typically the initialization file is named i and it resides in the directory of make. The name and location of the initialization file is discussed in detail on Page. The makefile has instructions for a specific project. The default name of the makefile is literally makefile, but the name can be specified with a command-line option.
Variable, expansion gnu make
In the game of Life program — the one linked at the top of this article — each cell is stored in a ireland macro named xxyy, after its position. The top-left most cell is named 0000, then going left to right, 0100, 0200, etc. Providing input is a matter of assigning each of these macros. I chose x for alive and - for dead, but, as youll see, any two characters permitted in macro names would work as well. The next part should be no surprise: The rules of the game of Life are encoded as a 512-entry lookup table. The key is formed by concatenating the cells value along with all its neighbors, with itself in the center. The beginning of the table looks like this: -x xx xxx- x -x- - x-x x-x- - xx-x- x.
Note: The two right-hand x values here are the cell coming to life (exactly three living neighbors). Computing the next value (n0101) for 0101 is done like so: n0101 given these results, constructing the input to the next loop is simple: args 0000(n0000) 0100(n0100) 0200(n0200). The display output, to be given to printf, is built similarly: output in the real version, this is decorated with an ansi escape code that clears the terminal. The printf interprets the escape byte (033) so that it doesnt need to appear literally in the source. And thats all there is to it: Conways Game of Life running in a makefile. Life, uh, finds a way. Tutorial - makefile, make reads its instructions from text files.
The program can clobber the make variable when its ready to halt. Loop (make) (args) output : @printf result)n" loop) to halt, the program just needs to clear loop. Suppose we want to count down. There will be an initial count: count 6 A decrement table: loop The last line will be used to halt by clearing the name on the right side. This is three star territory. (count) The result (current iteration) loop value is computed from the lookup table.
Result (count) The next loop value is passed via args. If loop was cleared above, this result will be discarded. Args count(result) With all that in place, invoking the makefile will print a countdown from 5 to 0 and quit. This is the general structure for the game of Life macro program. Game of Life a universal Turing machine has been implemented in Conways Game of Life. With all that heavy lifting done, one of the easiest methods today to prove a languages Turing-completeness is to implement Conways Game of Life. Ignoring the criminal inefficiency of it, the game of Life turing machine could be run on the game of Life simulation running on makes macro assignments.
Dictionary definition Successor In Interest
This is similar to the mov paper. Result : (op n) (op n) result(verbose) : (op n) And its usage: make n5 opsquare 25 make n5 opsquare verbose1 square(5) 25 looping is a tricky problem. However, one of the most common build ( anti?)patterns is the recursive makefile. Borrowing from the mov paper, which used an unconditional jump to restart the program from the beginning, for shredder a makefile turing-completeness I can invoke the makefile recursively, restarting the program with a new set of inputs. Remember the print target above? I can loop by invoking make again with new inputs in this target, output : @printf result)n" make) (args) Before going any further, now that loops have been added, the natural next question is halting. In reality, the operating system will take care of that after some millions of make processes have carelessly been invoked by this horribly inefficient scheme. However, we can do better.
make -f k -v result.828427 Without -v, a default target could be used instead: output : @printf result)n" There are no math operators, so performing arithmetic requires some creativity. For example, integers could be represented as a series of x characters. The number 4 is xxxx, the number 6 is xxxxxx, etc. Addition is concatenation (note: macros can have in their names a xxx b xxxx ab (A B) However, since theres no way to slice a value, subtraction isnt possible. A more realistic approach to arithmetic would require lookup tables. Branching Branching could be achieved through more lookup tables. For example, square_0 1 square_1 2 square_2 4. Result : (op n) And called as: biography make n5 opsqrt.236068 make n5 opsquare 25 Or using the debug trick above, use the condition to mask out the results of the unwanted branch.
make extra_flags -wbar make use_FOO0 extra_flags -wbar make use_FOO1 extra_flags -ffoo -lfoo before reading the posix specification, i didnt realize that the left side of an assignment can get the same treatment. For example, if I really want the if defined behavior back, i can use the macro to mangle the left-hand side. For example, extra_flags -o0 -g3 extra_flags(debug) -o3 -dndebug caveat: If debug is set to empty, it may still result in true for ifdef depending on which make flavor youre using, but will always appear to be unset in this hack. make extra_flags -o3 -dndebug make debugyes extra_flags -o0 -g3 This last case had me thinking: This is very similar to the (ab)use of the x86 mov instruction in mov is Turing-complete. These macro assignments alone should be enough to compute any algorithm. Macro Operations Macro names are just keys to a global associative array. This can be used to build lookup tables. Heres a makefile to compute the square root of integers between 0 and. Sqrt_0.000000 sqrt_1.000000 sqrt_2.414214 sqrt_3.732051 sqrt_4.000000 sqrt_5.236068 sqrt_6.449490 sqrt_7.645751 sqrt_8.828427 sqrt_9.000000 sqrt_10.162278 result : (sqrt n) The bsd flavors of make have a -v option for printing variables, which is an easy way to retrieve. I used an immediate assignment for result since some versions of make wont evaluate the expression before -v printing.
Make -f k, its 100 posix-compatible except for the sleep.1 (fractional sleep which is only needed for visual effect. A posix workaround, unlike virtually every real world implementation, posix make doesnt support conditional parts. For example, you might want your makefiles behavior to change depending on the value of certain variables. Make it looks like this: ifdef use_foo extra_flags -ffoo -lfoo else extra_flags -wbar endif, or bsd-style:.ifdef use_foo extra_flags -ffoo -lfoo. Endif, if the goal presentation is to write a strictly posix makefile, how could I work around the lack of conditional parts and maintain a similar interface? The selection of macro/variable to evaluate can be dynamically selected, allowing for some useful tricks. First define the options default: use_foo 0, then define both sets of flags: extra_flags_0 -wbar, extra_flags_1 -ffoo -lfoo. Now dynamically select one of these macros for assignment to extra_flags. Extra_flags (extra_flags use_FOO) The assignment on the command line overrides the assignment in the makefile, so the user gets to override use_FOO.
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April 30, 2016 m/blog/2016/04/30 for over a decade now, gnu make has almost exclusively presentation been my build system of choice, either directly or indirectly. Unfortunately this means i unnecessarily depend on some gnu extensions — an annoyance when porting to the bsds. In an effort to increase the portability. Makefiles, i recently read the posix make specification. I learned two important things: 1) posix make is so barren its not really worth striving for ( update : ive changed my mind and 2) makes macro assignment mechanism is Turing-complete. If you want to see it in action for yourself before reading further, heres a makefile that implements Conways Game of Life (40x40) using only macro assignments. Run it with any make program in an ansi terminal. It must literally be named. Beware: if you run it longer than a few minutes, your computer may begin thrashing.