### Resulting configure text from quoting example

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Autoconf-Language.html

Shows the output in configure after the previous quoting example is processed.

```autoconf
echo "Hard rock was here!  --oops"
echo "Hard rock was here!  --AC_DC"
echo "Hard rock was here!  --AC_DC"

```

--------------------------------

### Makefile.am for a simple 'Hello world' program

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Automake.html

Use this Makefile.am to build and install a simple 'Hello world' program with Automake. The resulting Makefile.in (~400 lines) automatically supports standard targets, Autoconf substitutions, automatic dependency tracking, and VPATH building. 'make' builds the 'hello' program, and 'make install' installs it in /usr/local/bin (or the prefix given to configure).

```makefile
bin_PROGRAMS = hello
hello_SOURCES = hello.c

```

--------------------------------

### Help string example for --with-readline

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/External-Software.html

Example of a help-string for the --with-readline option, showing proper formatting with leading whitespace. Avoid tabs in help strings.

```text
  --with-readline         support fancy command line editing

```

--------------------------------

### Basic Package Installation Commands

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Basic-Installation.html

The standard sequence of shell commands to configure, build, and install a package. The bootstrap step is only required for developer checkouts where the configure script is missing.

```shell
test -f configure || ./bootstrap
./configure
make
make install
```

--------------------------------

### m4_version_compare examples

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Number-processing-Macros.html

Examples of comparing version strings with m4_version_compare. Each example shows the expansion result after the ⇒ symbol.

```m4
m4_version_compare([1.1], [2.0])
⇒-1
m4_version_compare([2.0b], [2.0a])
⇒1
m4_version_compare([1.1.1], [1.1.1a])
⇒-1
m4_version_compare([1.2], [1.1.1a])
⇒1
m4_version_compare([1.0], [1])
⇒0
m4_version_compare([1.1pre], [1.1PRE])
⇒0
m4_version_compare([1.1a], [1,10])
⇒-1
m4_version_compare([2.61a], [2.61a-248-dc51])
⇒-1
m4_version_compare([2.61b], [2.61a-248-dc51])
⇒1
```

--------------------------------

### AC_PREPROC_IFELSE example

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Running-the-Preprocessor.html

Shows a complete configure.ac using AC_PREPROC_IFELSE to run the preprocessor on a C program. The macro uses CPPFLAGS, not CFLAGS, and on success the preprocessed output is available in conftest.i. The example defines HELLO_WORLD and checks that the program preprocesses successfully.

```autoconf
AC_INIT([Hello], [1.0], [bug-hello@example.org])
AC_DEFINE([HELLO_WORLD], ["Hello, World\n"],
  [Greetings string.])
AC_PREPROC_IFELSE(
   [AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]],
                    [[fputs (hw, stdout);]])],
   [AC_MSG_RESULT([OK])],
   [AC_MSG_FAILURE([unexpected preprocessor failure])])
```

--------------------------------

### Autoconf --program-transform-name example

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Transformation-Examples.html

Uses a sed script to prepend 'g' to most program names, except those already starting with 'g' (like gdb) and non-GNU programs like less and lesskey. The transformation is applied during 'make install'.

```shell
--program-transform-name='s/^/g/; s/^gg/g/; s/^gless/less/'

```

--------------------------------

### configure --help output for --with-foo

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Pretty-Help-Strings.html

Shows the expected output of configure --help after using the basic AS_HELP_STRING example. The option and description are aligned in columns.

```text
--enable and --with options recognized:
  --with-foo              use foo (default is no)

```

--------------------------------

### AC_ERLANG_CHECK_LIB Example

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Erlang-Libraries.html

Checks if the stdlib library is installed and prints its version and directory, or triggers an error if not found.

```m4
AC_ERLANG_CHECK_LIB([stdlib],
  [echo "stdlib version \"$ERLANG_LIB_VER_stdlib\""
   echo "is installed in \"$ERLANG_LIB_DIR_stdlib\""],
  [AC_MSG_ERROR([stdlib was not found!])])
```

--------------------------------

### configure --help output for MY_ARG_WITH

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Pretty-Help-Strings.html

Shows the expected output of configure --help after using the macro argument example. The option name is transformed from underscores to hyphens.

```text
--enable and --with options recognized:
  --with-a-b              use a_b (default is no)

```

--------------------------------

### Ash 0.2: Avoid using $? after expanding empty or unset variables or at the start of an eval

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Shellology.html

This snippet demonstrates a common pitfall in Ash 0.2 where using '$?' after expanding empty or unset variables, or at the start of an 'eval', can lead to incorrect results. The example shows two problematic usages: one after expanding an empty variable and one at the start of an 'eval'. Avoid these patterns for portability.

```shell
foo=
false
$foo
echo "Do not use it: $?"
false
eval 'echo "Do not use it: $?"'

```

--------------------------------

### AH_BOTTOM example with custom header include

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Autoheader-Macros.html

Use AH_BOTTOM to include text at the bottom of the header template file. The text is included verbatim, so it can get mangled when the template is processed.

```m4
AH_BOTTOM([#include <custom.h>])

```

--------------------------------

### Example conf.h.in Template

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Header-Templates.html

A template file for conf.h using #undef as a hook for HAVE_UNISTD_H.

```c
/* Define as 1 if you have unistd.h.  */
#undef HAVE_UNISTD_H
```

--------------------------------

### Preferred: Quoted AC_CACHE_CHECK with aligned arguments

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Coding-Style.html

Shows the recommended way to write AC_CACHE_CHECK with proper quoting and aligned arguments. This is the 'write' example that improves readability and follows the style guide.

```m4
AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2],
[AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])],
                   [ac_cv_emxos2=yes],
                   [ac_cv_emxos2=no])])

```

--------------------------------

### basename: Safe to use with -- option

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Limitations-of-Usual-Tools.html

Long ago some hosts lacked a working basename, but nowadays it is safe to use. The example shows using basename with -- to handle filenames starting with a dash.

```shell
base=`basename -- "$file"`

```

--------------------------------

### Example: Leading underscore in macro name fails on ancient make

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Special-Chars-in-Names.html

Shows a Makefile with macro names starting with an underscore, which fails on some ancient make implementations (e.g., NEWS-OS 4.2R) with the error 'Make: Must be a separator on rules line 2.  Stop.'

```shell
$ `cat Makefile`
_am_include = #
_am_quote =
all:; @echo this is test
$ `make`
Make: Must be a separator on rules line 2.  Stop.

```

```shell
$ `cat Makefile2`
am_include = #
am_quote =
all:; @echo this is test
$ `make -f Makefile2`
this is test

```

--------------------------------

### Sample config.site for changing defaults

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Site-Defaults.html

A sample site file read by configure when invoked with --prefix=/usr/share/local/gnu. It changes default values for prefix, exec_prefix, sharedstatedir, localstatedir, and runstatedir, and sets a shared cache file and CC compiler when the cache file is /dev/null.

```shell
# /usr/share/local/gnu/share/config.site for configure
#
# Change some defaults.
test "$prefix" = NONE && prefix=/usr/share/local/gnu
test "$exec_prefix" = NONE && exec_prefix=/usr/local/gnu
test "$sharedstatedir" = '${prefix}/com' && sharedstatedir=/var
test "$localstatedir" = '${prefix}/var' && localstatedir=/var
test "$runstatedir" = '${localstatedir}/run' && runstatedir=/run

# Give Autoconf 2.x generated configure scripts a shared default
# cache file for feature test results, architecture-specific.
if test "$cache_file" = /dev/null; then
  cache_file="$prefix/var/config.cache"
  # A cache file is only valid for one C compiler.
  CC=gcc
fi

```

--------------------------------

### m4_list_cmp examples

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Number-processing-Macros.html

Examples of comparing M4 lists of arithmetic expressions with m4_list_cmp. Each example shows the expansion result after the ⇒ symbol.

```m4
m4_list_cmp([1, 0],       [1])
⇒0
m4_list_cmp([1, [1 * 0]], [1, 0])
⇒0
m4_list_cmp([1, 2],       [1, 0])
⇒1
m4_list_cmp([1, [1+1], 3],[1, 2])
⇒1
m4_list_cmp([1, 2, -3],   [1, 2])
⇒-1
m4_list_cmp([1, 0],       [1, 2])
⇒-1
m4_list_cmp([1],          [1, 2])
⇒-1
```

--------------------------------

### Equivalent command for autom4te -l m4sugar

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/autom4te-Invocation.html

Shows that 'autom4te -l m4sugar foo.m4' is equivalent to using --prepend-include with the default installation path /usr/local.

```shell
autom4te --prepend-include /usr/local/share/autoconf \
  m4sugar/m4sugar.m4f foo.m4

```

--------------------------------

### Failed installation command

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Automatic-Rule-Rewriting.html

The specific command that fails during installation.

```shell
install -m 644 \
  `test -f ../pkg/src/f.h || \
    echo ../pkg/src/ \
  `../pkg/src/f.h \
  /usr/local/include/../pkg/src/f.h;
```

--------------------------------

### Preferred: test with $[@%:@]

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Coding-Style.html

Shows the recommended test command using $[@%:@] to avoid editor issues. This is the 'use' example.

```shell
test $[@%:@] != 0

```

--------------------------------

### Default configure --help output format

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Help-Formatting.html

Shows the default help output format, where options are grouped into sections such as 'Optional Features' and 'Optional Packages'. The ellipsis (...) indicates omitted options.

```text
Optional Features:
  …
  --enable-bar            include bar

Optional Packages:
  …
  --with-foo              use foo

```

--------------------------------

### AC_OBSOLETE usage example

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Obsolete-Macros.html

Example of calling AC_OBSOLETE with a suggestion for replacement.

```autoconf
AC_OBSOLETE([$0], [; use AC_CHECK_HEADERS(unistd.h) instead])dnl

```

--------------------------------

### Configure libdir for 64-bit platforms in config.site

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Site-Defaults.html

This snippet shows the content for /usr/local/share/config.site to ensure libraries are installed in lib64 instead of lib on platforms that prefer it.

```shell
# /usr/local/share/config.site for platforms that prefer
# the directory /usr/local/lib64 over /usr/local/lib.
test "$libdir" = '${exec_prefix}/lib' && libdir='${exec_prefix}/lib64'
```

--------------------------------

### Reduced installation command

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Automatic-Rule-Rewriting.html

The reduced command that installs the file in an incorrect directory.

```shell
install -m 644 \
  ../pkg/src/f.h \
  /usr/local/include/../pkg/src/f.h;
```

--------------------------------

### AC_CHECK_SIZEOF example

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Generic-Compiler-Characteristics.html

Example usage of the AC_CHECK_SIZEOF macro to determine the size of a pointer type.

```m4
AC_CHECK_SIZEOF([int *])
```

--------------------------------

### Configure with compiler and linker options

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Compilers-and-Options.html

Run the configure script with CC, CFLAGS, and LIBS variables set on the command line to specify the compiler, compiler flags, and linker libraries. This is useful when the system requires unusual options not known to configure. See 'Defining Variables' and 'Preset Output Variables' for more details.

```shell
./configure CC=gcc CFLAGS=-g LIBS=-lposix

```

--------------------------------

### Unquoted AC_CHECK_HEADERS example

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Changed-Quotation.html

An example of an improperly quoted AC_MSG_ERROR invocation within AC_CHECK_HEADERS.

```m4
AC_INIT
AC_CHECK_HEADERS(foo.h, ,
  AC_MSG_ERROR(cannot find foo.h, bailing out))
AC_OUTPUT

```

--------------------------------

### AC_CONFIG_COMMANDS with variable initialization

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Configuration-Commands.html

An example showing how to use AC_CONFIG_COMMANDS to execute commands with variable initialization.

```m4
fubar=42
AC_CONFIG_COMMANDS([fubar],
                   [echo this is extra $fubar, and so on.],
                   [fubar=$fubar])
```

--------------------------------

### AC_MSG_NOTICE usage example

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Printing-Messages.html

Example of using AC_MSG_NOTICE to print a general description of a feature check.

```m4
AC_MSG_NOTICE([checking if stack overflow is detectable])
```

--------------------------------

### Tru64 make dependency simplification example

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Automatic-Rule-Rewriting.html

A shell session showing a Makefile with VPATH and a dependency ../foo. Tru64 make simplifies ../foo to foo, demonstrating its automatic rule rewriting behavior. The sub/ directory does not exist, indicating simplification occurs before checking for the file.

```shell
$ `cat Makefile`
VPATH = sub
all: ../foo
        echo ../foo
$ `ls`
Makefile foo
$ `make`
echo foo
foo

```

--------------------------------

### Corrected quoted AC_CHECK_HEADERS example

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Changed-Quotation.html

The corrected version of the previous example with proper quotation for arguments.

```m4
AC_INIT([Example], [1.0], [bug-example@example.org])
AC_CHECK_HEADERS([foo.h], [],
  [AC_MSG_ERROR([cannot find foo.h, bailing out])])
AC_OUTPUT

```

--------------------------------

### config.status with a tag

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/config_002estatus-Invocation.html

Shows how to specify a tag (file) to configure. The file must be given without its dependencies.

```shell
./config.status foobar

```

--------------------------------

### C const keyword example

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/C-Compiler.html

An example of a const declaration that is valid in C but invalid in C++.

```c
const int foo;
```

--------------------------------

### Demonstrate Tru64 make automatic directory creation

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Tru64-Directory-Magic.html

This sequence shows how Tru64 make creates the directory structure 'foo/bar' in the current build directory when VPATH points to the parent directory.

```bash
$ `mkdir -p foo/bar build`
$ `cd build`
$ `cat >Makefile <<END
VPATH = ..
all: foo/bar
END`
$ `make`
mkdir foo
mkdir foo/bar
```

--------------------------------

### Check for POSIX system support

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Particular-Headers.html

Demonstrates how to verify if a system supports POSIX by checking for unistd.h and the _POSIX_VERSION macro.

```c
#ifdef HAVE_UNISTD_H
# include <sys/types.h>
# include <unistd.h>
#endif

#ifdef _POSIX_VERSION
/* Code for Posix systems.  */
#endif
```

--------------------------------

### Non-portable VPATH rule example

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Automatic-Rule-Rewriting.html

An example of a rule that is not portable to GNU make because of VPATH rewriting behavior.

```makefile
# This isn't portable to GNU make.
VPATH = ../pkg/src
f.c: if.c
        cp if.c f.c
```

--------------------------------

### Reusing configuration with --config

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/config_002estatus-Invocation.html

Shows how to use the --config option to retrieve configuration settings and apply them to a new configure invocation, useful for debugging builds in a different build directory.

```shell
args=`build-dir/config.status --config`
eval src-dir/configure "$args" CFLAGS=-g --srcdir=src-dir

```

--------------------------------

### Expected output of AC_PREPROC_IFELSE example

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Running-the-Preprocessor.html

The configure output that the AC_PREPROC_IFELSE example might produce, ending with 'OK' when the preprocessor succeeds.

```text
checking for gcc... gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables...
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether the compiler supports GNU C... yes
checking whether gcc accepts -g... yes
checking for gcc option to enable C11 features... -std=gnu11
checking how to run the C preprocessor... gcc -std=gnu11 -E
OK
```

--------------------------------

### AC_HEADER_RESOLV: Prerequisites for resolv.h

Source: https://www.gnu.org/software/autoconf/manual/autoconf-2.72/html_node/Particular-Headers.html

Use this code after running AC_HEADER_RESOLV to include the necessary headers before including resolv.h. It conditionally includes sys/types.h, netinet/in.h, arpa/nameser.h, and netdb.h based on their respective HAVE_ macros.

```c
#ifdef HAVE_SYS_TYPES_H
#  include <sys/types.h>
#endif
#ifdef HAVE_NETINET_IN_H
#  include <netinet/in.h>   /* inet_ functions / structs */
#endif
#ifdef HAVE_ARPA_NAMESER_H
#  include <arpa/nameser.h> /* DNS HEADER struct */
#endif
#ifdef HAVE_NETDB_H
#  include <netdb.h>
#endif
#include <resolv.h>
```