man make: a Primer on the Make Utility
In a compiled language, the makefile is arguably the most important part of any programming project. To compile your project, you first have to compile each source file into an object file, which in turn needs to be linked with system libraries into the final executable file. Each command can have a considerable number of arguments added in. That's a lot of typing and a lot of potential for mistakes. The more source files you have, the more complex the compilation process becomes, unless you use makefiles. Most Linux users have at least a cursory knowledge of make and makefiles (because that's how we build software packages for our systems), but not much more than that. Most developers probably don't have too much in-depth experience with makefiles, because most Integrated Development Environments (IDEs) have the capability of managing makefiles for them. Although this is convenient most of the time, knowing more about how make works and what goes into makefiles can help you troubleshoot compilation errors down the road.
According to make's man page, "The purpose of the make utility is to determine automatically which pieces of a large program need to be recompiled, and issue the commands to recompile them." Essentially, make is used to determine efficiently (and without user error) which portions of the source code have been updated since the last compilation and recompile them. It can be used for more than just compiling programs. Because it isn't limited to any particular language, you can use it for anything you can come up with that relates to the modified date of a group of files.
Running make is a straightforward process. The more convoluted portion of using make is constructing the makefile. The makefile is a file that consists of a series of rules that define the dependencies of your project. These rules govern the behavior of make during execution.
Command lines must be indented with tab characters; spaces cause funky errors. This has been a design flaw in make for decades. Empty lines must still have a tab character or else make will throw a fit.
Comments start with a pound sign (#).
Continuation of a line is denoted by a back slash (\).
Lines containing equal signs (=) are variable definitions.
Each command line typically is executed in a separate Bourne shell—that is, sh1.
To execute more than one command line in the same shell, type them on the same line, separated by semicolons. Use a \ to continue the line if necessary.
Listing 1. Example Makefile
SOURCES=main.cpp hello.cpp factorial.cpp
all: $(SOURCES) $(EXECUTABLE)
$(CC) $(LDFLAGS) $(OBJECTS) -o $@
$(CC) $(CFLAGS) $< -o $@
Rules and Targets
<![CDATA[ CC=g++ CFLAGS=-c -Wall LDFLAGS= SOURCES=main.cpp hello.cpp factorial.cpp OBJECTS=$(SOURCES:.cpp=.o) EXECUTABLE=hello all: $(SOURCES) $(EXECUTABLE) $(EXECUTABLE): $(OBJECTS) $(CC) $(LDFLAGS) $(OBJECTS) -o $@ .cpp.o: $(CC) $(CFLAGS) $< -o $@ ]]>
Rules and Targets
Each rule in the makefile is an independent series of commands that are executed in order to build a target. Make does not necessarily run each rule in order. Make will run through the rules recursively, building each target in turn, based on modification. Rules are formatted like this:
target: dependency list ... commands ...
The target is typically the name of a file, but it can be a phony target (discussed later in this article). The dependency list is a space-separated list of files that designate whether the target needs to be rebuilt. The commands can be any shell command, so long as the target is up to date at the end of them. It is imperative that you indent the commands with a tab character and not spaces. This is a design flaw in make that has yet to be fixed, and it will cause some strange and obscure errors should you use spaces instead of tabs in your makefile.
When make encounters a rule, it first checks the files listed in the dependency list to ensure that they haven't changed. If one of them has, make looks through the makefile for the rule containing that file as the target. This recursion continues until a rule is found where all the dependencies are unchanged or rebuilt (or have no further dependencies), and then make executes the listed commands for that rule before returning to the previous rule, and so on, until the root rule has been satisfied and its commands run.
You may use pattern-matching characters to describe dependencies in the dependency list or in commands, but they may not be used in the target.
Practical Task Scheduling Deployment
July 20, 2016 12:00 pm CDT
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
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With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide