How to understand AS_IF in autoconf?

Demystifying AS_IF in Autoconf: Conditional Logic for Portable Builds

Explore the AS_IF macro in Autoconf, understanding its syntax, purpose, and how to use it effectively for conditional compilation and configuration in portable software projects.

Autoconf is a powerful tool for generating configure scripts that automatically adapt software source code to various Unix-like systems. A core part of this adaptability comes from its ability to perform conditional checks and execute different actions based on the system's characteristics. Among the most fundamental macros for this purpose is AS_IF. This article will break down AS_IF, explaining its structure, common use cases, and how it contributes to robust, portable build systems.

What is AS_IF and Why is it Essential?

The AS_IF macro provides a way to implement if-then-else conditional logic within your configure.ac script. It evaluates a shell command or expression and executes different blocks of M4 code (which generate shell script code) based on the outcome. This is crucial for portability because different systems might have different compilers, libraries, header files, or even command-line utilities. AS_IF allows your configure script to detect these differences and adjust the build process accordingly.

flowchart TD
    A[Start Autoconf Script] --> B{AS_IF Test Condition?}
    B -- True --> C[Execute 'then' actions]
    B -- False --> D[Execute 'else' actions (optional)]
    C --> E[Continue Script]
    D --> E[Continue Script]

Basic flow of the AS_IF macro in an Autoconf script.

Syntax and Structure of AS_IF

The basic syntax of AS_IF is straightforward, mirroring the if-then-else construct found in many programming languages. It takes three arguments: a test condition, a 'then' block, and an optional 'else' block. Each block contains M4 code that will be expanded into shell commands in the generated configure script.

AS_IF([test-condition],
      [then-actions],
      [else-actions])

General syntax of the AS_IF macro.

Let's break down each part:

• test-condition: This is a shell command or expression that returns an exit status. An exit status of 0 (zero) is considered success (true), and any non-zero status is considered failure (false). Common commands used here include test, [ ], grep, command -v, or custom shell functions.
• then-actions: This block contains M4 code that will be executed if the test-condition evaluates to true (exit status 0). This typically involves setting Autoconf variables, defining preprocessor macros, or running other checks.
• else-actions: This optional block contains M4 code that will be executed if the test-condition evaluates to false (non-zero exit status). If omitted, nothing happens when the condition is false.

Practical Examples of AS_IF in Action

To illustrate the power of AS_IF, let's look at a few common scenarios where it's indispensable for creating robust configure scripts.

Example 1: Checking for a Specific Program

A common use case is to check if a particular program (like pkg-config or g++) is available on the system. If it is, we might set a variable or enable a feature.

AS_IF([test -x "`command -v pkg-config`"],
      [AC_MSG_NOTICE([pkg-config found, enabling feature X])
       AM_CONDITIONAL([HAVE_PKG_CONFIG], [true])],
      [AC_MSG_WARN([pkg-config not found, disabling feature X])
       AM_CONDITIONAL([HAVE_PKG_CONFIG], [false])])

Using AS_IF to check for 'pkg-config' and set a conditional.

In this example:

• test -x "command -v pkg-config" checks if pkg-config exists and is executable. command -v is used for portability.
• If true, a message is displayed, and the HAVE_PKG_CONFIG Automake conditional is set to true.
• If false, a warning is displayed, and the conditional is set to false.

Example 2: Conditional Compilation Based on OS

Sometimes, you need to include different source files or define different preprocessor macros based on the operating system. AS_IF can be combined with AC_CANONICAL_HOST to achieve this.

AC_CANONICAL_HOST

AS_IF([test "$host_os" = "linux-gnu"],
      [AC_DEFINE([_GNU_SOURCE], [1], [Define to 1 for GNU extensions.])
       AC_CONFIG_SRCDIR([src/linux_specific.c])],
      [test "$host_os" = "darwin"],
      [AC_DEFINE([_DARWIN_C_SOURCE], [1], [Define to 1 for Darwin extensions.])
       AC_CONFIG_SRCDIR([src/macos_specific.c])],
      [AC_MSG_WARN([Unknown OS, using generic source files])
       AC_CONFIG_SRCDIR([src/generic.c])])

Conditional logic for OS-specific source files and preprocessor definitions.

This example demonstrates nested AS_IF logic (though it's often cleaner to use case statements for multiple conditions, AS_IF can handle it). It checks the host_os variable (set by AC_CANONICAL_HOST) and defines appropriate macros or configures different source directories based on whether the OS is Linux or macOS. A fallback is provided for unknown systems.