Check Headers

Can AC_CHECK_HEADERS be defined more than once in configure.ac ? Yes, AC_CHECK_HEADERS can be used multiple times in configure.ac. Each call to AC_CHECK_HEADERS checks for the presence of the specified headers and defines preprocessor macros (e.g., HAVE_HEADER_H) if the headers are found. For example:
```
AC_CHECK_HEADERS([dmapi.h ifaddrs.h libaio.h])
AC_CHECK_HEADERS([mm.h linux/module.h])
```
This is perfectly valid and allows you to organize the checks logically or add new checks without modifying existing ones.
Explanation of --with-kernel or similar options;

In the confFlags.log file you provided, there are two relevant options for specifying kernel-related paths:
- --with-linux-version=VERSION: This option allows you to specify the Linux kernel version for which the LTP (Linux Test Project) should be configured. This is useful if you are building kernel modules or tests that depend on a specific kernel version.
- --with-linux-dir=DIR : This option allows you to specify the path to the kernel development directory (e.g., /usr/src/k-ver). This directory typically contains the kernel headers and source files needed for building kernel modules or tests.
For example, if your kernel headers are located in /usr/src/k-ver, you can run:
```
./configure --with-linux-dir=/usr/src/k-ver
```
This tells the configure script to look for kernel headers in /usr/src/k-ver/include instead of the default locations like /usr/include.
Explanation of AC_MSG_CHECKING and AC_MSG_RESULT; These are Autoconf macros used to provide user-friendly output during the configuration process.
- AC_MSG_CHECKING([for header search paths]): This macro prints a message to the user indicating what the script is currently checking. In this case, it will print:
```
checking for header search paths...
```
- AC_MSG_RESULT([$CPPFLAGS]): This macro prints the result of the check. It takes a single argument, which is typically a variable or value. In this case, it will print the value of $CPPFLAGS, which contains the preprocessor flags (e.g., -I include paths). For example, if CPPFLAGS is set to -I/usr/src/k-ver/include, the output will look like:
```
checking for header search paths... -I/usr/src/k-ver/include
```
Together, these macros provide feedback to the user about the search paths being used for header files. This is useful for debugging or verifying that the correct paths are being used.

4. How to use CPPFLAGS to specify custom include paths, i59

If you want to ensure that the configure script searches for headers in /usr/src/k-ver/include, you can set the CPPFLAGS environment variable before running configure:
export CPPFLAGS="-I/usr/src/k-ver/include"
./configure
This adds /usr/src/k-ver/include to the list of directories where the preprocessor will look for header files.

Modifying configure.ac to handle custom paths, If you want to modify configure.ac to handle custom paths or provide better feedback, you can add the following:

# Check for custom header paths
AC_MSG_CHECKING([for header search paths])
AC_MSG_RESULT([$CPPFLAGS])

# Check for specific headers
AC_CHECK_HEADERS([dmapi.h mm.h linux/module.h sys/jfsdmapi.h], [], [
  AC_MSG_WARN([Required headers not found. Ensure kernel headers are installed.])
])

  This will:
          - Print the header search paths being used.
          - Check for the required headers.
          - Print a warning if any headers are missing.

Summary

AC_CHECK_HEADERS can be used multiple times in configure.ac.
Use --with-linux-dir to specify the kernel headers directory.
AC_MSG_CHECKING and AC_MSG_RESULT provide user-friendly output during configuration.
Set CPPFLAGS to add custom include paths for header searches.