GNU M4 Macro Format

Overview

GNU M4 is a general-purpose macro processor that is part of the POSIX standard. M4 files contain macro definitions and processing instructions that generate output text by expanding macros. Originally designed for text processing, M4 is widely used in software build systems, configuration management, and code generation.

File Format Details

File Extensions

• .m4 (primary extension)
• .ac (Autoconf files)
• .in (template files)

MIME Type

• text/plain
• text/x-m4

Format Specifications

• Type: Plain text macro processor input
• Encoding: UTF-8 or ASCII
• Syntax: Functional macro language
• Comments: Lines starting with dnl or #
• Case Sensitivity: Function names are case-sensitive

Technical Specifications

M4 Syntax Elements

dnl This is a comment
define(`HELLO', `Hello, World!')
HELLO
dnl Output: Hello, World!

define(`SQUARE', `eval($1 * $1)')
SQUARE(5)
dnl Output: 25

define(`FOREACH', `pushdef(`$1')_foreach($@)popdef(`$1')')
define(`_foreach', `ifelse(`$2', `', `', 
  `define(`$1', `$2')$3`'
  `$0(`$1', shift(shift($@)), `$3')')')

Built-in Macros

• define: Create new macros
• undefine: Remove macro definitions
• ifdef: Conditional macro existence
• include: Include other files
• eval: Arithmetic evaluation
• substr: Substring extraction
• len: String length
• index: String search
• regexp: Regular expression matching

Advanced Features

dnl Conditional processing
ifdef(`DEBUG', `define(`LOG', `echo $1')', `define(`LOG', `')')

dnl File inclusion
include(`common.m4')

dnl System command execution
syscmd(`date')

dnl Arithmetic operations
define(`FACTORIAL', 
  `ifelse(`$1', `0', `1', 
    `eval($1 * FACTORIAL(eval($1 - 1)))')')

History and Development

Timeline

• 1977: Original M4 developed at Bell Labs by Brian Kernighan and Dennis Ritchie
• 1980s: Integration into Unix systems
• 1990s: GNU M4 implementation with extensions
• 2000s: POSIX standardization
• Present: Continued use in build systems and configuration tools

Key Milestones

• Integration with Autotools (Autoconf, Automake)
• Adoption by major software projects
• POSIX standardization ensuring portability
• Modern extensions for complex preprocessing

Common Use Cases

Build Systems

• Autoconf: Configure script generation
• Automake: Makefile template processing
• CMake: Configuration file preprocessing
• Custom build scripts: Dynamic build configuration

Code Generation

• Template processing: Generate source code from templates
• Configuration files: Dynamic configuration generation
• Documentation: Generate docs from templates
• Test case generation: Automated test creation

Text Processing

• Report generation: Dynamic report creation
• Mail templates: Personalized email generation
• Web content: Dynamic HTML generation
• Configuration management: System configuration files

Technical Implementation

Basic M4 Usage

# Process M4 file
m4 input.m4 > output.txt

# Define macros from command line
m4 -DDEBUG=1 -DVERSION=2.0 template.m4

# Include additional files
m4 -I./includes main.m4

Simple M4 Template

dnl config.m4 - Configuration template
define(`PROJECT_NAME', `MyProject')
define(`PROJECT_VERSION', `1.0.0')
define(`AUTHOR', `John Doe')

# PROJECT_NAME Configuration
Version: PROJECT_VERSION
Author: AUTHOR
Build Date: syscmd(`date +%Y-%m-%d')

ifdef(`DEBUG', `
Debug Mode: Enabled
Log Level: DEBUG
', `
Debug Mode: Disabled
Log Level: INFO
')

Complex Macro Example

dnl HTML table generator
define(`TABLE_START', `<table class="$1">')
define(`TABLE_END', `</table>')
define(`ROW', `<tr>$1</tr>')
define(`CELL', `<td>$1</td>')

define(`GENERATE_TABLE', `
TABLE_START(`data-table')
ROW(`CELL(`Name')CELL(`Age')CELL(`City')')
forloop(`i', 1, $1, `
  ROW(`CELL(`Person i')CELL(eval(20 + i))CELL(`City i')')
')
TABLE_END
')

dnl Loop macro
define(`forloop', `pushdef(`$1', `$2')_forloop($@)popdef(`$1')')
define(`_forloop',
  `$4`'ifelse($1, `$3', `', 
    `define(`$1', incr($1))$0($@)')')

Integration with Autoconf

dnl configure.ac example
AC_INIT([myproject], [1.0], [[email protected]])
AC_PREREQ([2.69])
AC_CONFIG_SRCDIR([src/main.c])
AC_CONFIG_HEADERS([config.h])

dnl Check for programs
AC_PROG_CC
AC_PROG_INSTALL

dnl Check for libraries
AC_CHECK_LIB([pthread], [pthread_create])
AC_CHECK_HEADERS([stdio.h stdlib.h])

dnl Custom macros
AC_DEFUN([CHECK_CUSTOM_FEATURE], [
  AC_MSG_CHECKING([for custom feature])
  if test -f /usr/include/custom.h; then
    AC_MSG_RESULT([yes])
    AC_DEFINE([HAVE_CUSTOM], [1], [Define if custom feature is available])
  else
    AC_MSG_RESULT([no])
  fi
])

CHECK_CUSTOM_FEATURE

AC_CONFIG_FILES([
  Makefile
  src/Makefile
  doc/Makefile
])

AC_OUTPUT

Tools and Software

GNU M4 Tools

• m4: Primary GNU M4 processor
• autom4te: Autotools M4 wrapper
• autoreconf: Autotools regeneration utility
• aclocal: Autoconf macro collection
• autoheader: Header template generator

Development Environment

• Emacs: M4 mode with syntax highlighting
• Vim: M4 syntax support
• VS Code: M4 extensions available
• Text editors: Basic M4 syntax support
• IDEs: Integration with build systems

Build Integration

• Autotools: Primary M4 consumer
• CMake: Limited M4 support
• Make: M4 preprocessing in Makefiles
• Custom scripts: Shell script integration
• CI/CD: Build pipeline integration

Best Practices

Macro Design

• Use descriptive macro names
• Document macro parameters and behavior
• Implement error checking in complex macros
• Avoid deeply nested macro calls

Code Organization

dnl common.m4 - Common macro definitions
dnl Include guard pattern
ifdef(`COMMON_M4_INCLUDED', `', `
define(`COMMON_M4_INCLUDED', `1')

dnl Version checking
define(`REQUIRE_VERSION', `
  ifdef(`M4_VERSION', `
    ifelse(eval(M4_VERSION >= $1), 1, `', `
      errprint(`Requires M4 version $1 or higher')
      m4exit(1)
    ')
  ', `
    errprint(`Cannot determine M4 version')
    m4exit(1)
  ')
')

dnl Debug utilities
define(`DEBUG_PRINT', `
  ifdef(`DEBUG', `errprint(`DEBUG: $1
')')
')

') dnl End include guard

Error Handling

define(`SAFE_INCLUDE', `
  ifdef(`$1_INCLUDED', `
    DEBUG_PRINT(`File $1 already included')
  ', `
    define(`$1_INCLUDED', `1')
    include(`$1.m4')
  ')
')

define(`VALIDATE_PARAM', `
  ifelse(`$1', `', `
    errprint(`Error: Parameter $2 is required')
    m4exit(1)
  ')
')

Performance Optimization

• Minimize recursive macro calls
• Use built-in functions when possible
• Cache expensive computations
• Avoid unnecessary string operations

Security Considerations

Input Validation

• Validate macro parameters
• Sanitize external input
• Prevent code injection through macro expansion
• Use safe file inclusion practices

Safe Execution

dnl Safe file inclusion with validation
define(`SAFE_FILE_INCLUDE', `
  syscmd(`test -f "$1" -a -r "$1"')
  ifelse(sysval, 0, `
    include(`$1')
  ', `
    errprint(`Warning: Cannot safely include file $1')
  ')
')

dnl Validate numeric parameters
define(`VALIDATE_NUMBER', `
  ifelse(regexp(`$1', `^[0-9]+$'), -1, `
    errprint(`Error: Invalid number: $1')
    m4exit(1)
  ')
')

Access Control

• Restrict file system access
• Validate command execution
• Use safe directory for temporary files
• Implement proper error handling

Integration Examples

Configuration File Generation

dnl nginx.conf.m4 template
define(`SERVER_NAME', `example.com')
define(`DOCUMENT_ROOT', `/var/www/html')
define(`ACCESS_LOG', `/var/log/nginx/access.log')

server {
    listen 80;
    server_name SERVER_NAME;
    root DOCUMENT_ROOT;
    
    access_log ACCESS_LOG;
    
    location / {
        try_files $uri $uri/ =404;
    }
    
    ifdef(`SSL_ENABLED', `
    listen 443 ssl;
    ssl_certificate /etc/ssl/certs/SERVER_NAME.crt;
    ssl_certificate_key /etc/ssl/private/SERVER_NAME.key;
    ')
}

Build Script Generation

dnl build.sh.m4 template
#!/bin/bash
define(`PROJECT_NAME', `myproject')
define(`BUILD_TYPE', `release')

set -e

echo "Building PROJECT_NAME (BUILD_TYPE mode)"

ifdef(`BUILD_TYPE', `
ifelse(BUILD_TYPE, `debug', `
    CFLAGS="-g -O0 -DDEBUG"
    echo "Debug build enabled"
', BUILD_TYPE, `release', `
    CFLAGS="-O2 -DNDEBUG"
    echo "Release build enabled"
', `
    echo "Unknown build type: BUILD_TYPE"
    exit 1
')
')

make CFLAGS="$CFLAGS" PROJECT_NAME
echo "Build completed successfully"

Documentation Generation

dnl README.md.m4 template
# PROJECT_NAME

define(`PROJECT_DESC', `A sample project demonstrating M4 usage')
define(`PROJECT_LICENSE', `MIT')

PROJECT_DESC

## Installation

```bash
./configure
make
make install

Usage

Basic usage example:

PROJECT_NAME --help

License

This project is licensed under the PROJECT_LICENSE License.

Version Information

• Version: PROJECT_VERSION
• Build Date: syscmd(`date +%Y-%m-%d')
• Git Commit: syscmd(`git rev-parse --short HEAD 2>/dev/null || echo "unknown"')

GNU M4 provides powerful text processing capabilities that are essential for build systems, configuration management, and automated code generation, making it a fundamental tool in Unix-like development environments.