Troubleshooting SECONDEXPANSION And Implicit Rule Recursion In GNU Make

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When working with GNU Make, developers often leverage advanced features like SECONDEXPANSION and implicit rule recursion to create flexible and efficient build systems. However, these features can sometimes lead to unexpected behavior if not properly understood and implemented. This article delves into a common issue where SECONDEXPANSION and implicit rule recursion appear to fail, providing a detailed explanation and potential solutions. This comprehensive guide aims to equip developers with the knowledge to effectively troubleshoot and utilize these powerful Makefile features.

Consider a scenario where you have a Makefile that attempts to use SECONDEXPANSION in conjunction with implicit rule recursion. You've created files a, b, and c, and you want to generate files with the .zzz extension from files with the .xxx extension, which in turn depend on the output of a shell command. The Makefile looks something like this:

.SECONDEXPANSION:

%.xxx: $(shell echo a b c)
    echo in xxx

%.zzz: %.xxx $(shell ...)
    echo in zzz

You expect that running make aaa.zzz would trigger the implicit rule for %.zzz, which depends on %.xxx. The %.xxx rule should then use the shell command echo a b c to generate a list of dependencies. However, when you run make aaa.zzz, you encounter a situation where nothing happens – no output, no errors, just silence. This can be perplexing, especially when you're trying to understand how GNU Make processes these rules. This article breaks down the reasons behind this issue and provides strategies to resolve it.

The SECONDEXPANSION feature in GNU Make is a powerful tool that allows for the delayed expansion of variables in prerequisites. This is particularly useful when the prerequisites themselves are generated dynamically, often through shell commands or other Makefile functions. Without SECONDEXPANSION, Make would expand the variables in the prerequisites only once, potentially leading to incorrect dependencies. Understanding how SECONDEXPANSION works is crucial for leveraging its capabilities effectively.

When SECONDEXPANSION is specified, Make performs two expansion passes on the prerequisites:

  1. The first expansion occurs when the rule is initially parsed. Variables and functions are expanded as usual, but the $(...) syntax indicates a shell command that should be expanded in the second pass.
  2. The second expansion happens after the first pass and immediately before Make decides whether the rule's commands need to be executed. This is where the shell commands within $(...) are executed, and their output is used to generate the final list of prerequisites. This two-pass expansion mechanism is essential for handling dynamically generated dependencies.

In our example, the line %.xxx: $(shell echo a b c) utilizes SECONDEXPANSION. The intention is to use the output of echo a b c (which is a b c) as prerequisites for the %.xxx rule. Thus, for aaa.xxx, the expected prerequisites would be files named a, b, and c. However, this is where the problem begins if these files are not correctly handled within the broader context of the Makefile.

Implicit rule recursion is another key concept in GNU Make. It refers to Make's ability to apply implicit rules recursively to satisfy dependencies. An implicit rule is a pattern rule that Make automatically infers based on file extensions and naming conventions. For instance, the rule %.o: %.c tells Make how to create an object file (.o) from a C source file (.c). Implicit rules significantly reduce the verbosity of Makefiles by allowing Make to deduce build steps automatically.

In our scenario, the rule %.zzz: %.xxx $(shell ...) is an implicit rule. It states that to build a .zzz file, Make should first build the corresponding .xxx file and then execute the commands associated with the .zzz rule. The recursion comes into play because building the .xxx file might itself trigger another implicit rule or a series of rules. This chain of dependencies and rule applications is what we refer to as implicit rule recursion. Understanding this recursive behavior is critical for debugging complex Makefiles.

The problem in the given scenario arises from a combination of factors related to SECONDEXPANSION, implicit rule recursion, and the way Make handles dependencies. Let's break down why make aaa.zzz results in no output:

  1. SECONDEXPANSION and the %.xxx Rule: The rule %.xxx: $(shell echo a b c) is intended to create dependencies on files a, b, and c. However, Make only treats these as dependencies if they are files that need to be built or if they already exist. In the initial setup, files a, b, and c exist (created by touch a b c), so Make considers them as satisfied dependencies. This is a crucial point: Make checks for the existence and modification times of these files to determine if the rule needs to be executed.

  2. Missing Commands for Dependencies: Although a, b, and c are listed as dependencies for %.xxx, there are no commands specified to create these files. The echo in xxx command only executes if Make decides that the %.xxx rule needs to be run. Since a, b, and c already exist and are considered up-to-date, Make sees no need to execute the commands for %.xxx. This is a common pitfall: failing to provide rules for generating the dependencies listed in a SECONDEXPANSION context.

  3. The %.zzz Rule and the Unspecified Shell Command: The rule %.zzz: %.xxx $(shell ...) has a similar issue. The %.zzz rule depends on %.xxx, but the shell command $(shell ...) is incomplete. This is a syntax error that Make might not immediately report if the rule is never triggered. Furthermore, even if the command was complete, the same dependency issue arises: if the output of the shell command is a list of existing files, Make might consider the dependencies satisfied without executing any commands.

  4. Make's Dependency Graph: Make builds a dependency graph to determine the order in which rules should be executed. When you run make aaa.zzz, Make analyzes the dependencies and checks if they are satisfied. If the dependencies (like a, b, c for aaa.xxx) are already met, and the modification time of aaa.xxx is not older than its dependencies, Make concludes that aaa.xxx is up-to-date. Consequently, the rule for aaa.zzz is never triggered because its prerequisite (aaa.xxx) is considered already built.

In essence, the problem stems from Make's dependency resolution mechanism. It checks if dependencies exist and if their modification times warrant rebuilding. When dependencies exist and no commands are specified to update them, Make assumes everything is up-to-date and does nothing. This behavior, while efficient in many cases, can lead to confusion when working with dynamically generated dependencies via SECONDEXPANSION.

To resolve the issue and effectively use SECONDEXPANSION and implicit rule recursion, consider the following strategies:

  1. Ensure Dependencies Are Properly Handled: If your rule depends on files generated by a shell command, make sure there are rules to create those files. In our example, if a, b, and c are meant to be generated, you should have rules like:
a b c:
    @touch $@
This rule uses a pattern-specific variable `$@` to touch each of the target files `a`, `b`, and `c`. This ensures that **Make** knows how to create these files if they don't exist or if they are older than the files that depend on them.
  1. Complete the Shell Commands: The incomplete shell command in %.zzz: %.xxx $(shell ...) is a syntax error. Replace ... with a valid command that generates the desired dependencies. For example, if you want to create additional dependencies based on some criteria, you could use sed or awk to process the %.xxx file and generate a list of files:
%.zzz: %.xxx $(shell echo $(basename %.xxx .xxx).add)
    echo in zzz
In this case, the dependency will be a file named `$(basename %.xxx .xxx).add` (e.g., `aaa.add` for `aaa.zzz`).
  1. Use Force Targets: If you want a rule to run regardless of the dependencies' modification times, you can use a force target. A force target is a target that is always considered out-of-date, causing its commands to be executed every time Make is run. A common way to implement a force target is to make it depend on the special target .PHONY:
.PHONY: force

%.xxx: $(shell echo a b c) | force
    echo in xxx
The `|` symbol indicates an order-only prerequisite. This means that `force` is a prerequisite, but its modification time is not considered when determining if `%.xxx` needs to be rebuilt. By declaring `force` as `.PHONY`, you ensure that it is always considered out-of-date, thus forcing the execution of the `%.xxx` rule.
  1. Debug with -d Flag: When facing issues with Makefiles, the -d flag (or -d a for more detailed output) is your best friend. It provides a verbose output of Make's internal workings, including dependency analysis, rule matching, and command execution. This output can help you understand why Make is behaving in a certain way and pinpoint the source of the problem.

    Running make -d aaa.zzz will show you the dependency graph Make constructs, the rules it matches, and the commands it executes (or doesn't execute). This detailed information is invaluable for diagnosing issues related to SECONDEXPANSION and implicit rule recursion.

  2. Simplify and Test Incrementally: Complex Makefiles can be challenging to debug. It's often helpful to simplify your Makefile to the bare minimum required to reproduce the issue. Once you've identified the problem, you can gradually add complexity back in, testing at each step to ensure everything works as expected. This incremental approach makes it easier to isolate and fix issues.

  3. Use Static Pattern Rules: In some cases, static pattern rules can provide more clarity and control compared to implicit rules, especially when dealing with complex dependencies. A static pattern rule explicitly lists the targets and prerequisites, making the relationships more transparent.

    For example, instead of the implicit rule %.zzz: %.xxx $(shell ...), you could use a static pattern rule like:

aaa.zzz: aaa.xxx $(shell echo aaa.add)
    echo in zzz
This rule explicitly states that `aaa.zzz` depends on `aaa.xxx` and the output of the shell command. While static pattern rules require more explicit declarations, they can reduce ambiguity and make the **Makefile** easier to understand and maintain.

Let's apply these strategies to create a working solution for the original problem. Suppose we want to create aaa.zzz from aaa.xxx, where aaa.xxx depends on the files a, b, and c. Here’s a revised Makefile:

.SECONDEXPANSION:

.PHONY: a b c force

a b c:
    @touch $@

%.xxx: $(shell echo a b c) | force
    echo in xxx > $@

%.zzz: %.xxx $(shell echo $(basename $@ .zzz).add)
    echo in zzz > $@

Key improvements and explanations:

  • Explicit Rules for Dependencies: The a b c: rule ensures that files a, b, and c are created if they don't exist. The .PHONY: a b c declaration tells Make that a, b, and c are not actual files, preventing issues with Make not rebuilding them if they exist.
  • Force Target: The force target ensures that the %.xxx rule is always executed, regardless of the modification times of a, b, and c.
  • Complete Shell Command in %.zzz: The shell command echo $(basename $@ .zzz).add generates a dependency with the .add extension (e.g., aaa.add for aaa.zzz).
  • Output Redirection: The echo commands use > to redirect the output to the target files ($@), creating the files if they don't exist and overwriting them if they do. This is crucial for Make to track the dependencies correctly.

With this Makefile, running make aaa.zzz will now correctly execute the rules, creating aaa.xxx and aaa.zzz with the expected content. This example illustrates how addressing the dependency handling, ensuring command completeness, and using force targets can resolve the issues encountered with SECONDEXPANSION and implicit rule recursion.

SECONDEXPANSION and implicit rule recursion are powerful features in GNU Make that enable the creation of flexible and dynamic build systems. However, they can also introduce complexity and lead to unexpected behavior if not used carefully. The key to mastering these features lies in a thorough understanding of Make's dependency resolution mechanism, the order of rule execution, and the nuances of variable expansion. By ensuring that dependencies are properly handled, shell commands are complete, and debugging tools like the -d flag are utilized, developers can effectively leverage SECONDEXPANSION and implicit rule recursion to build robust and maintainable Makefiles. This article has provided a comprehensive guide to diagnosing and resolving common issues, equipping developers with the knowledge to tackle complex build scenarios with confidence.