Writing a userspace component for comp that uses extra args

Table of Contents

Introduction
Writing an userspace component that uses extra args

Introduction

comp has had the facility to specify extra arguments to gcc for some while, but they have hitherto been undocumented

When writing a userspace component to be built with comp, one can specify 2 extra options.

option extra_compile_args allows an argument string to be specified which will be added to existing CFLAGS

The most usual type of argument is additional include search paths for headers, but it could be any legitimate gcc flag
eg.
option extra_compile_args "-I/path/to/extra/headers"

option extra_link_args allows an argument string to be specified which will be added to existing LDFLAGS

The use for this is almost exclusively add additional library search paths and to add specific libraries to be linked against.
eg.
option extra_link_args "-L/path/to/extra/libs -lfoo

Writing an userspace component that uses extra args

The easiest way to demonstrate is by way of example.

The below .h and .c files create a function foo() which just outputs a message

// contents of foo.h
#ifndef FOO_H_
#define FOO_H_

extern void foo(void);

#endif  // FOO_H_

// contents of foo.c
#include <stdio.h>

void foo(void)
{
    puts("This message is from library libfoo");
}

Then we compile it and link into a shared library called libfoo

$ gcc -c -Wall -Werror -fpic foo.c
$ gcc -shared -o libfoo.so foo.o

Next we write a very simple userspace component that waits for a signal.
When received it uses the library function to write a message and exits.

It specifies the locations of foo.h and libfoo.so in its extra args
and specifies linkage against libfoo

// component tester.comp
component tester;

option userspace;

option extra_compile_args "-I/prog/MK/comp-example";
option extra_link_args "-L/prog/MK/comp-example -lfoo";

pin in bit signal = 0;
license "GPL"; // indicates GPL v2 or later
;;

#include <stdio.h>
#include <unistd.h>
#include "foo.h"

void user_mainloop(void).
{
int once = 1;

    while(once).
	{
        usleep(1000);
        FOR_ALL_INSTS()
	    {
	    if(signal)
		{
		puts("This is a shared library test...");
		foo();
		once = 0;
		}
	    }
	}
}

Now we build the userspace component tester (note that the extra args are injected into the gcc arguments)

$ comp --compile tester.comp
    gcc -I. -pthread -DTHREAD_FLAVOR_ID=0 -DRTAPI -D_GNU_SOURCE -D_FORTIFY_SOURCE=0 -D__MODULE__ -mieee-fp  -I/usr/src/machinekit/include -fPIC -g -funwind-tables.
    -I/media/sdd/sdd3/prog/MK/comp-example -I/media/sdd/sdd3/prog/MK/comp-example -URTAPI -U__MODULE__ -DULAPI -Os -I/prog/MK/comp-example -o tester.
    /tmp/tmpibcrEK/tester.c -Wl,-rpath,/usr/src/machinekit/lib -L/usr/src/machinekit/lib -llinuxcnchal -L/prog/MK/comp-example -lfoo

Now to test the userspace component - tester

$ DEBUG=5 realtime restart
$ halcmd loadusr ./tester
    <commandline>:0: Program './tester' started
$ halcmd show pin
    Component Pins:
      Comp   Inst Type  Dir         Value  Name                                            Epsilon Flags  linked to:
        80        bit   IN          FALSE  tester.0.signal
$ halcmd setp tester.0.signal 1
    <commandline>:0: setting parameter 'tester.0.signal' to '1'
    <commandline>:0: Pin 'tester.0.signal' set to 1
    This is a shared library test...
    This message is from library libfoo
$ halrun -U
    <commandline>:0: Realtime threads stopped
$

Success, tester prints to stdout using the library function foo() to print the second line.

Machinekit

Machinekit

Writing a userspace component for comp that uses extra args

Introduction

Writing an userspace component that uses extra args