ExtUtils::CChecker(3)

NAME
"ExtUtils::CChecker" - configure-time utilities for using C headers, libraries, or OS features

SYNOPSIS
 use Module::Build;
 use ExtUtils::CChecker;

 my $cc = ExtUtils::CChecker->new;

 $cc->assert_compile_run(
    diag => "no PF_MOONLASER",
    source => <<'EOF' );
 #include <stdio.h>
 #include <sys/socket.h>
 int main(int argc, char *argv[]) {
   printf("PF_MOONLASER is %d\n", PF_MOONLASER);
   return 0;
 }
 EOF

 Module::Build->new(
   ...
 )->create_build_script;

DESCRIPTION
Often Perl modules are written to wrap functionallity found in existing C headers, libraries, or to use OS-specific features. It is useful in the Build.PL or Makefile.PL file to check for the existance of these requirements before attempting to actually build the module.

Objects in this class provide an extension around ExtUtils::CBuilder to simplify the creation of a .c file, compiling, linking and running it, to test if a certain feature is present.

It may also be necessary to search for the correct library to link against, or for the right include directories to find header files in. This class also provides assistance here.

CONSTRUCTOR

  $cc = ExtUtils::CChecker->new( %args )
Returns a new instance of a "ExtUtils::CChecker" object. Takes teh following named parameters:

defines_to => PATH
    If given, defined symbols will be written to a C preprocessor .h file of the given name, instead of by adding extra "-DSYMBOL" arguments to the compiler flags.

METHODS

  $dirs = $cc->include_dirs
Returns the currently-configured include directories in an ARRAY reference.

  $flags = $cc->extra_compiler_flags
Returns the currently-configured extra compiler flags in an ARRAY reference.

  $flags = $cc->extra_linker_flags
Returns the currently-configured extra linker flags in an ARRAY reference.

  $cc->push_include_dirs( @dirs )
Adds more include directories

  $cc->push_extra_compiler_flags( @flags )
Adds more compiler flags

  $cc->push_extra_linker_flags
Adds more linker flags

  $success = $cc->try_compile_run( %args )

  $success = $cc->try_compile_run( $source )
Try to compile, link, and execute a C program whose source is given. Returns true if the program compiled and linked, and exited successfully. Returns false if any of these steps fail.

Takes the following named arguments. If a single argument is given, that is taken as the source string.

•   source => STRING

    The source code of the C program to try compiling, building, and running.

•   extra_compiler_flags => ARRAY

    Optional. If specified, pass extra flags to the compiler.

•   extra_linker_flags => ARRAY

    Optional. If specified, pass extra flags to the linker.

•   define => STRING

    Optional. If specified, then the named symbol will be defined if the program ran successfully. This will either on the C compiler commandline (by passing an option "-DSYMBOL"), or in the "defines_to" file.

  $cc->assert_compile_run( %args )
Calls "try_compile_run". If it fails, die with an "OS unsupported" message. Useful to call from Build.PL or Makefile.PL.

Takes one extra optional argument:

•   diag => STRING

    If present, this string will be appended to the failure message if one is generated. It may provide more useful information to the user on why the OS is unsupported.

  $success = $cc->try_find_include_dirs_for( %args )
Try to compile, link and execute the given source, using extra include directories.

When a usable combination is found, the directories required are stored in the object for use in further compile operations, or returned by "include_dirs". The method then returns true.

If no a usable combination is found, it returns false.

Takes the following arguments:

•   source => STRING

    Source code to compile

•   dirs => ARRAY of ARRAYs

    Gives a list of sets of dirs. Each set of dirs should be strings in its own array reference.

•   define => STRING

    Optional. If specified, then the named symbol will be defined if the program ran successfully. This will either on the C compiler commandline (by passing an option "-DSYMBOL"), or in the "defines_to" file.

  $success = $cc->try_find_libs_for( %args )
Try to compile, link and execute the given source, when linked against a given set of extra libraries.

When a usable combination is found, the libraries required are stored in the object for use in further link operations, or returned by "extra_linker_flags". The method then returns true.

If no usable combination is found, it returns false.

Takes the following arguments:

•   source => STRING

    Source code to compile

•   libs => ARRAY of STRINGs

    Gives a list of sets of libraries. Each set of libraries should be space-separated.

•   define => STRING

    Optional. If specified, then the named symbol will be defined if the program ran successfully. This will either on the C compiler commandline (by passing an option "-DSYMBOL"), or in the "defines_to" file.

  $cc->find_include_dirs_for( %args )

  $cc->find_libs_for( %args )
Calls "try_find_include_dirs_for" or "try_find_libs_for" respectively. If it fails, die with an "OS unsupported" message.

Each method takes one extra optional argument:

•   diag => STRING

    If present, this string will be appended to the failure message if one is generated. It may provide more useful information to the user on why the OS is unsupported.

  $mb = $cc->new_module_build( %args )
Construct and return a new Module::Build object, preconfigured with the "include_dirs", "extra_compiler_flags" and "extra_linker_flags" options that have been configured on this object, by the above methods.

This is provided as a simple shortcut for the common use case, that a Build.PL file is using the "ExtUtils::CChecker" object to detect the required arguments to pass.

EXAMPLES

  Socket Libraries
Some operating systems provide the BSD sockets API in their primary libc. Others keep it in a separate library which should be linked against. The following example demonstrates how this would be handled.

 use ExtUtils::CChecker;

 my $cc = ExtUtils::CChecker->new;

 $cc->find_libs_for(
    diag => "no socket()",
    libs => [ "", "socket nsl" ],
    source => q[
 #include <sys/socket.h>
 int main(int argc, char *argv) {
   int fd = socket(PF_INET, SOCK_STREAM, 0);
   if(fd < 0)
     return 1;
   return 0;
 }
 ] );

 $cc->new_module_build(
    module_name => "Your::Name::Here",
    requires => {
       'IO::Socket' => 0,
    },
    ...
 )->create_build_script;

By using the "new_module_build" method, the detected "extra_linker_flags" value has been automatically passed into the new "Module::Build" object.

  Testing For Optional Features
Sometimes a function or ability may be optionally provided by the OS, or you may wish your module to be useable when only partial support is provided, without requiring it all to be present. In these cases it is traditional to detect the presence of this optional feature in the Build.PL script, and define a symbol to declare this fact if it is found. The XS code can then use this symbol to select between differing implementations. For example, the Build.PL:

 use ExtUtils::CChecker;

 my $cc = ExtUtils::CChecker->new;

 $cc->try_compile_run(
    define => "HAVE_MANGO",
    source => <<'EOF' );
 #include <mango.h>
 #include <unistd.h>
 int main(void) {
   if(mango() != 0)
     exit(1);
   exit(0);
 }
 EOF

 $cc->new_module_build(
    ...
 )->create_build_script;

If the C code compiles and runs successfully, and exits with a true status, the symbol "HAVE_MANGO" will be defined on the compiler commandline. This allows the XS code to detect it, for example

 int
 mango()
   CODE:
 #ifdef HAVE_MANGO
     RETVAL = mango();
 #else
     croak("mango() not implemented");
 #endif
   OUTPUT:
     RETVAL

This module will then still compile even if the operating system lacks this particular function. Trying to invoke the function at runtime will simply throw an exception.

  Linux Kernel Headers
Operating systems built on top of the Linux kernel often share a looser association with their kernel version than most other operating systems. It may be the case that the running kernel is newer, containing more features, than the distribution's libc headers would believe. In such circumstances it can be difficult to make use of new socket options, "ioctl()"s, etc.. without having the constants that define them and their parameter structures, because the relevant header files are not visible to the compiler. In this case, there may be little choice but to pull in some of the kernel header files, which will provide the required constants and structures.

The Linux kernel headers can be found using the /lib/modules directory. A fragment in Build.PL like the following, may be appropriate.

 chomp( my $uname_r = `uname -r` );

 my @dirs = (
    [],
    [ "/lib/modules/$uname_r/source/include" ],
 );

 $cc->find_include_dirs_for(
    diag => "no PF_MOONLASER",
    dirs => \@dirs,
    source => <<'EOF' );
 #include <sys/socket.h>
 #include <moon/laser.h>
 int family = PF_MOONLASER;
 struct laserwl lwl;
 int main(int argc, char *argv[]) {
   return 0;
 }
 EOF

This fragment will first try to compile the program as it stands, hoping that the libc headers will be sufficient. If it fails, it will then try including the kernel headers, which should make the constant and structure visible, allowing the program to compile.

  Creating an "#include" file
Sometimes, rather than setting defined symbols on the compiler commandline, it is preferrable to have them written to a C preprocessor include (.h) file. This may be beneficial for cross-platform portability concerns, as not all C compilers may take extra "-D" arguments on the command line, or platforms may have small length restrictions on the length of a command line.

 use ExtUtils::CChecker;

 my $cc = ExtUtils::CChecker->new(
    defines_to => "mymodule-config.h",
 );

 $cc->try_compile_run(
    define => "HAVE_MANGO",
    source => <<'EOF' );
 #include <mango.h>
 #include <unistd.h>
 #include "mymodule-config.h"
 int main(void) {
   if(mango() != 0)
     exit(1);
   exit(0);
 }
 EOF

Because the mymodule-config.h file is written and flushed after every define operation, it will still be useable in later C fragments to test for features detected in earlier ones.

It is suggested not to name the file simply config.h, as the core of Perl itself has a file of that name containing its own compile-time detected configuration. A confusion between the two could lead to surprising results.

AUTHOR
Paul Evans <leonerd@leonerd.org.uk>