I am writing a kernel driver to send/receive data with a PCI Express device. For this first version of the driver I am creating a character device interface where the user can read data using a file.
Background
I want to implement a blocking read where the user requests data and the driver populates a user buffer. In order to block the user's read call, I am using a completion structure.
When the driver is loaded and the user requests a read the driver blocks as expected. If I were to finish the read then everything runs fine.
The problem
In order to be safe, whenever the module is removed I call the complete_all function, just in case someone removes the module or device in the middle of a read transaction.
Neither the remove or exit function is called and both the module and user application is blocked. I've tried the following three functions (shown with their associated result).
wait_completion(&dev->read_complete); //Blocks indefinitely, I need to reset the computer
retval = wait_for_completion_interruptible(&dev->read_complete); //I can kill the user application manually and then remove the driver
retval = wait_for_completion_killable(&dev->read_complete); //Same as interruptible
My expectation is that when the remove function is called I can call complete_all(&dev->read_complete) and the read function will return an error.
In order to remove external factors I've made a repo on github, so if anyone wants to see the behavior for themselves they just need to clone and follow the instructions:
Kernel Module Completion Test
The relevant parts of the module are here (/src/mymodule.c)
typedef struct {
struct cdev cdv;
struct class *cls;
struct device *dev;
struct completion complete;
} mymodule_t;
mymodule_t mymod;
//Sysfs 'mymodule_test' attribute (all but the actual function is left out for brevity)
static ssize_t mymodule_test_store(struct device *dev, struct device_attribute *attr, const char *buf, size_t count)
{
int retval = 0;
int value = 0;
if (sscanf(buf, "%d", &value) == 1)
{
retval = strlen(buf);
}
if (value)
{
printk("Value is: %d\n", value);
if (!completion_done(&mymod.complete))
{
complete(&mymod.complete);
}
printk("Sent Completion\n");
}
return retval;
}
//FOPS (all but 'read' function is left out for brevity)
ssize_t mymodule_read(struct file *filp, char * buf, size_t count, loff_t *f_pos)
{
printk("Read!\n");
if (completion_done(&mymod.complete))
{
reinit_completion(&mymod.complete);
}
printk("Wait for Completion\n");
wait_for_completion_interruptible(&mymod.complete);
printk("After Completion\n");
return 0;
}
static int __init mymodule_init(void)
{
...
//Register class and device
//Configure character driver with fops
init_completion(&mymod.complete);
...
}
static void __exit mymodule_exit(void)
{
...
if (!completion_done(&mymod.complete))
{
printk("Send a completion!\n");
complete(&mymod.complete);
}
//Clean up the rest of the module
...
}
module_init(mymodule_init);
module_exit(mymodule_exit);
Here is the userland application I use to exercise this:
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <termios.h>
#include "mymodule.h"
#define FILEPATH "/dev/mymodule0"
#define TEST_SIZE 10
int main(void)
{
int fn = -1;
char buf[TEST_SIZE];
printf("Attempting to open file module file...\n");
fn = open(FILEPATH, O_RDWR);
if (fn < 0)
{
printf("Failed to open file!\n");
return -1;
}
printf("Attempting to read from the file...\n");
read(fn, &buf, TEST_SIZE);
printf("Finished reading from file\n");
return 0;
}
Here is the dmesg output when I
load the module
run the user application (it opens the file, attempts to read 10 characters, then exits)
write '1' to the sysfs attribute
unload the module
[3217633.993937] Registering Driver
[3217633.993995] Driver Initialized!
[3217643.747791] Opened!
[3217643.747800] Read!
[3217643.747801] Wait for Completion
[3217646.436780] Value is: 1
[3217646.436792] Sent Completion
[3217646.436806] After Completion
[3217646.437010] Closed!
[3217727.378388] Cleanup Module
[3217727.378393] Check if we need to complete anything
[3217727.378395] Send a completion!
[3217727.378397] Unregistering Character Driver
[3217727.378400] Give back all the numbers we requested
[3217727.378402] Remove the class driver
[3217727.378571] Release the class
[3217727.378593] Finished Cleanup Module, Exiting
If I run the following commands:
load the module
run the user application
unload the module
[3218223.442777] Registering Driver
[3218223.442934] Driver Initialized!
[3218229.378396] Opened!
[3218229.378419] Read!
[3218229.378422] Wait for Completion
then the module doesn't unload. If this were a real device, like a USB hard drive, it is possible that the user could remove the device in the middle of a read transaction. It seems like something is wrong, or perhaps I'm missing something. Am I missing something?
While USB device can be removed at any time, its driver (e.g. kernel module) cannot be unloaded during certain operations with that device (e.g. reading). It is driver who reports to the upper level(e.g. filesystem) about absence of device.
I am creating a kernel module to find the resident pages for all the process. I am using get_mm_rss() and for_each_process but it works
only for init process / first time after first iteration it doesn't work.
int __init schedp(void){
struct task_struct *p;
for_each_process(p) {
int pid = task_pid_nr(p);
printk("Process: %s (pid = %d) , (rpages : %lu)\n",
p->comm, pid, get_mm_rss(p->mm));
}
return 0;
}
Results:
BUG: unable to handle kernel NULL pointer dereference at 00000160,
You're probably getting NULL in p->mm, because some tasks may have invalid mm pointer because they are exiting or don't have mm (because they are kernel-threads, not sure).
When you confused on how to use kernel API, always look for examples inside kernel itself. Quick search with cross-reference tool gave me kernel/cpu.c:
for_each_process(p) {
struct task_struct *t;
/*
* Main thread might exit, but other threads may still have
* a valid mm. Find one.
*/
t = find_lock_task_mm(p);
if (!t)
continue;
cpumask_clear_cpu(cpu, mm_cpumask(t->mm));
task_unlock(t);
}
Note that you need to call find_lock_task_mm() and task_unlock() and explicitly check for NULL.
finally it works after creating a function which checks a mm_struct is valid or not for_each_process
struct task_struct *task_mm(struct task_struct *p){
struct task_struct *t;
rcu_read_lock();
for_each_thread(p, t) {
task_lock(t);
if (likely(t->mm))
goto found;
task_unlock(t);
}
t = NULL;
found:
rcu_read_unlock();
return t;
}
I know I can set thread name (the one visible in gdb and htop) in Linux using prctl(). But with another OSes this most likely won't work. Also, I could try using pthread_setname_np(), which is a bit more available across POSIX systems, but still lacks full compatibility.
So I'd like to have some more portable way, maybe something QThread provides which I've not found. Is there any such way?
There's nothing in the QThread API to manually manage the system name of the thread, however, since version 4.8.3, Qt will automatically set the name of your thread to the name of the thread object (QObject::objectName()).
This is handled in the implementations of QThread as described below.
You have something like this in qthread_unix.cpp:
#if (defined(Q_OS_LINUX) || defined(Q_OS_MAC) || defined(Q_OS_QNX))
static void setCurrentThreadName(pthread_t threadId, const char *name)
{
# if defined(Q_OS_LINUX) && !defined(QT_LINUXBASE)
Q_UNUSED(threadId);
prctl(PR_SET_NAME, (unsigned long)name, 0, 0, 0);
# elif defined(Q_OS_MAC)
Q_UNUSED(threadId);
pthread_setname_np(name);
# elif defined(Q_OS_QNX)
pthread_setname_np(threadId, name);
# endif
}
#endif
/*
* [...]
*/
QString objectName = thr->objectName();
if (Q_LIKELY(objectName.isEmpty()))
setCurrentThreadName(thr->d_func()->thread_id, thr->metaObject()->className());
else
setCurrentThreadName(thr->d_func()->thread_id, objectName.toLocal8Bit());
And the equivalent in qthread_win.cpp:
typedef struct tagTHREADNAME_INFO
{
DWORD dwType; // must be 0x1000
LPCSTR szName; // pointer to name (in user addr space)
HANDLE dwThreadID; // thread ID (-1=caller thread)
DWORD dwFlags; // reserved for future use, must be zero
} THREADNAME_INFO;
void qt_set_thread_name(HANDLE threadId, LPCSTR threadName)
{
THREADNAME_INFO info;
info.dwType = 0x1000;
info.szName = threadName;
info.dwThreadID = threadId;
info.dwFlags = 0;
__try
{
RaiseException(0x406D1388, 0, sizeof(info)/sizeof(DWORD), (const ULONG_PTR*)&info);
}
__except (EXCEPTION_CONTINUE_EXECUTION)
{
}
}
/*
* [...]
*/
QByteArray objectName = thr->objectName().toLocal8Bit();
qt_set_thread_name((HANDLE)-1, objectName.isEmpty() ? thr->metaObject()->className() : objectName.constData());
Note that on Windows, the above code won't be executed if QT_NO_DEBUG is set, thus it won't work in Release mode.
In Qt documentation you can find:
To choose the name that your thread will be given (as identified by
the command ps -L on Linux, for example), you can call setObjectName()
before starting the thread. If you don't call setObjectName(), the
name given to your thread will be the class name of the runtime type
of your thread object (for example, "RenderThread" in the case of the
Mandelbrot Example, as that is the name of the QThread subclass). Note
that this is currently not available with release builds on Windows.
Hi i would like to know how is it possible to call/run the following function from user space.
static ssize_t lm70_sense_temp(struct device *dev,
struct device_attribute *attr, char *buf)
{
//some code
.
.
status = sprintf(buf, "%d\n", val); /* millidegrees Celsius */
.
.
//some code
}
This function is defined in lm70.c driver located in the kernel/drivers/hwmon folder of the linux source? Is it possible to pass the values of this functions internal variables to the user application? I would like to retrieve the value of val variable in the above function...
I don't know well the kernel internals. However, I grepped for lm70_sense_temp in the entire kernel source tree, and it appears only in the file linux-3.7.1/drivers/hwmon/lm70.c, first as a static function, then as the argument to DEVICE_ATTR.
Then I googled for linux kernel DEVICE_ATTR and found immediately device.txt which shows that you probably should read that thru the sysfs, i.e. under /sys; read sysfs-rules.txt; so a user application could very probably read something relevant under /sys/
I'm downvoting your question because I feel that you could have searched a few minutes like I did (and I am not a kernel expert).
You don't need to call this function from user space to get that value - it is already exported to you via sysfs.
You could use grep to find which hwmon device it is:
grep -rl "lm70" /sys/class/hwmon/*/name /sys/class/hwmon/*/*/name
Then you can read the temperature input from your user space program, e.g:
#include <stdio.h>
#include <fcntl.h>
#define SENSOR_FILE "/sys/class/hwmon/hwmon0/temp1_input"
int readSensor(void)
{
int fd, val = -1;
char buf[32];
fd = open(SENSOR_FILE, O_RDONLY);
if (fd < 0) {
printf("Failed to open %s\n", SENSOR_FILE);
return val;
}
if (read(fd, &buf, sizeof(buf)) > 0) {
val = atoi(buf);
printf("Sensor value = %d\n", val);
} else {
printf("Failed to read %s\n", SENSOR_FILE);
}
close(fd);
return val;
}
As others have already stated - you can't call kernel code from user space, thems the breaks.
You cannot call a driver function directly from user space.
If that function is exported with EXPORT_SYMBOL or EXPORT_SYMBOL_GPL then we can write a simple kernel module and call that function directly. The result can be sent to user space through FIFO or shared memory.
But in your case, this function is not exported. so you should not do in this way.
I'm trying to write some code to communicate with wpa_supplicant using DBUS. As I'm working in an embedded system (ARM), I'd like to avoid the use of Python or the GLib. I'm wondering if I'm stupid because I really have the feeling that there is no nice and clear documentation about D-Bus. Even with the official one, I either find the documentation too high level, or the examples shown are using Glib! Documentation I've looked at: http://www.freedesktop.org/wiki/Software/dbus
I found a nice article about using D-Bus in C: http://www.matthew.ath.cx/articles/dbus
However, this article is pretty old and not complete enough! I also found the c++-dbus API but also here, I don't find ANY documentation! I've been digging into wpa_supplicant and NetworkManager source code but it's quite a nightmare! I've been looking into the "low-level D-Bus API" as well but this doesn't tell me how to extract a string parameter from a D-Bus message! http://dbus.freedesktop.org/doc/api/html/index.html
Here is some code I wrote to test a little but I really have trouble to extract string values. Sorry for the long source code but if someone want to try it ... My D-Bus configuration seems fine because it "already" catches "StateChanged" signals from wpa_supplicant but cannot print the state:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <string.h>
#include <dbus/dbus.h>
//#include "wpa_supp_dbus.h"
/* Content of wpa_supp_dbus.h */
#define WPAS_DBUS_SERVICE "fi.epitest.hostap.WPASupplicant"
#define WPAS_DBUS_PATH "/fi/epitest/hostap/WPASupplicant"
#define WPAS_DBUS_INTERFACE "fi.epitest.hostap.WPASupplicant"
#define WPAS_DBUS_PATH_INTERFACES WPAS_DBUS_PATH "/Interfaces"
#define WPAS_DBUS_IFACE_INTERFACE WPAS_DBUS_INTERFACE ".Interface"
#define WPAS_DBUS_NETWORKS_PART "Networks"
#define WPAS_DBUS_IFACE_NETWORK WPAS_DBUS_INTERFACE ".Network"
#define WPAS_DBUS_BSSIDS_PART "BSSIDs"
#define WPAS_DBUS_IFACE_BSSID WPAS_DBUS_INTERFACE ".BSSID"
int running = 1;
void stopLoop(int sig)
{
running = 0;
}
void sendScan()
{
// TODO !
}
void loop(DBusConnection* conn)
{
DBusMessage* msg;
DBusMessageIter args;
DBusMessageIter subArgs;
int argType;
int i;
int buffSize = 1024;
char strValue[buffSize];
const char* member = 0;
sendScan();
while (running)
{
// non blocking read of the next available message
dbus_connection_read_write(conn, 0);
msg = dbus_connection_pop_message(conn);
// loop again if we haven't read a message
if (!msg)
{
printf("No message received, waiting a little ...\n");
sleep(1);
continue;
}
else printf("Got a message, will analyze it ...\n");
// Print the message member
printf("Got message for interface %s\n",
dbus_message_get_interface(msg));
member = dbus_message_get_member(msg);
if(member) printf("Got message member %s\n", member);
// Check has argument
if (!dbus_message_iter_init(msg, &args))
{
printf("Message has no argument\n");
continue;
}
else
{
// Go through arguments
while(1)
{
argType = dbus_message_iter_get_arg_type(&args);
if (argType == DBUS_TYPE_STRING)
{
printf("Got string argument, extracting ...\n");
/* FIXME : got weird characters
dbus_message_iter_get_basic(&args, &strValue);
*/
/* FIXME : segmentation fault !
dbus_message_iter_get_fixed_array(
&args, &strValue, buffSize);
*/
/* FIXME : segmentation fault !
dbus_message_iter_recurse(&args, &subArgs);
*/
/* FIXME : deprecated!
if(dbus_message_iter_get_array_len(&args) > buffSize)
printf("message content to big for local buffer!");
*/
//printf("String value was %s\n", strValue);
}
else
printf("Arg type not implemented yet !\n");
if(dbus_message_iter_has_next(&args))
dbus_message_iter_next(&args);
else break;
}
printf("No more arguments!\n");
}
// free the message
dbus_message_unref(msg);
}
}
int main(int argc, char* argv[])
{
DBusError err;
DBusConnection* conn;
int ret;
char signalDesc[1024]; // Signal description as string
// Signal handling
signal(SIGKILL, stopLoop);
signal(SIGTERM, stopLoop);
// Initialize err struct
dbus_error_init(&err);
// connect to the bus
conn = dbus_bus_get(DBUS_BUS_SYSTEM, &err);
if (dbus_error_is_set(&err))
{
fprintf(stderr, "Connection Error (%s)\n", err.message);
dbus_error_free(&err);
}
if (!conn)
{
exit(1);
}
// request a name on the bus
ret = dbus_bus_request_name(conn, WPAS_DBUS_SERVICE, 0, &err);
if (dbus_error_is_set(&err))
{
fprintf(stderr, "Name Error (%s)\n", err.message);
dbus_error_free(&err);
}
/* Connect to signal */
// Interface signal ..
sprintf(signalDesc, "type='signal',interface='%s'",
WPAS_DBUS_IFACE_INTERFACE);
dbus_bus_add_match(conn, signalDesc, &err);
dbus_connection_flush(conn);
if (dbus_error_is_set(&err))
{
fprintf(stderr, "Match Error (%s)\n", err.message);
exit(1);
}
// Network signal ..
sprintf(signalDesc, "type='signal',interface='%s'",
WPAS_DBUS_IFACE_NETWORK);
dbus_bus_add_match(conn, signalDesc, &err);
dbus_connection_flush(conn);
if (dbus_error_is_set(&err))
{
fprintf(stderr, "Match Error (%s)\n", err.message);
exit(1);
}
// Bssid signal ..
sprintf(signalDesc, "type='signal',interface='%s'",
WPAS_DBUS_IFACE_BSSID);
dbus_bus_add_match(conn, signalDesc, &err);
dbus_connection_flush(conn);
if (dbus_error_is_set(&err))
{
fprintf(stderr, "Match Error (%s)\n", err.message);
exit(1);
}
// Do main loop
loop(conn);
// Main loop exited
printf("Main loop stopped, exiting ...\n");
dbus_connection_close(conn);
return 0;
}
Any pointer to any nice, complete, low-level C tutorial is strongly appreciated! I'm also planning to do some remote method call, so if the tutorial covers this subject it would be great! Saying I'm not very smart because I don't get it with the official tutorial is also appreciated :-p!
Or is there another way to communicate with wpa_supplicant (except using wpa_cli)?
EDIT 1:
Using 'qdbusviewer' and the introspection capabilty, this helped me a lot discovering what and how wpa_supplicant works using dbus. Hopping that this would help someone else!
Edit 2:
Will probably come when I'll find a way to read string values on D-Bus!
You have given up the tools that would help you to learn D-Bus more easily and are using the low level libdbus implementation, so maybe you deserve to be in pain. BTW, are you talking about ARM, like a cell phone ARM ? With maybe 500 Mhz and 256 MB RAM ? In this case the processor is well suited to using glib, Qt or even python. And D-Bus is most useful when you're writing asynchronous event driven code, with an integrated main loop, for example from glib, even when you're using the low level libdbus (it has functions to connect to the glib main loop, for example).
Since you're using the low level library, then documentation is what you already have:
http://dbus.freedesktop.org/doc/api/html/index.html
Also, libdbus source code is also part of the documentation:
http://dbus.freedesktop.org/doc/api/html/files.html
The main entry point for the documentation is the Modules page (in particular, the public API section):
http://dbus.freedesktop.org/doc/api/html/modules.html
For message handling, the section DBusMessage is the relevant one:
DBusMessage
There you have the documentation for functions that parse item values. In your case, you started with a dbus_message_iter_get_basic. As described in the docs, retrieving the string requires a const char ** variable, since the returned value will point to the pre-allocated string in the received message:
So for int32 it should be a "dbus_int32_t*" and for string a "const char**". The returned value is by reference and should not be freed.
So you can't define an array, because libdbus won't copy the text to your array. If you need to save the string, first get the constant string reference, then strcpy to your own array.
Then you tried to get a fixed array without moving the iterator. You need a call to the next iterator (dbus_message_iter_next) between the basic string and the fixed array. Same right before recursing into the sub iterator.
Finally, you don't call get_array_len to get the number of elements on the array. From the docs, it only returns byte counts. Instead you loop over the sub iterator using iter_next the same way you should have done with the main iterator. After you have iterated past the end of the array, dbus_message_iter_get_arg_type will return DBUS_TYPE_INVALID.
For more info, read the reference manual, don't look for a tutorial. Or just use a reasonable d-bus implementation:
https://developer.gnome.org/gio/2.36/gdbus-codegen.html
GIO's GDBus automatically creates wrappers for your d-bus calls.
http://qt-project.org/doc/qt-4.8/intro-to-dbus.html
http://dbus.freedesktop.org/doc/dbus-python/doc/tutorial.html
etc.
You don't need to use/understand working of dbus If you just need to write a C program to communicate with wpa_supplicant. I reverse engineered the wpa_cli's source code. Went through its implementation and used functions provided in wpa_ctrl.h/c. This implementation takes care of everything. You can use/modify whatever you want, build your executable and you're done!
Here's the official link to wpa_supplicant's ctrl_interface:
http://hostap.epitest.fi/wpa_supplicant/devel/ctrl_iface_page.html
I doubt this answer will still be relevant to the author of this question,
but for anybody who stumbles upon this like I did:
The situation is now better than all those years ago if you don't want to include GTK/QT in your project to access dbus.
There is dbus API in Embedded Linux Library by Intel (weird I remember it being open, maybe it is just for registered users now?)
and systemd sd-bus library now offers public API. You probably run systemd anyway unless you have a really constrained embedded system.
I have worked with GDbus, dbus-cpp and sd-bus and although I wanted a C++ library,
I found sd-bus to be the simplest and the least problematic experience.
I did not try its C++ bindings but they also look nice
#include <stdio.h>
#include <systemd/sd-bus.h>
#include <stdlib.h>
const char* wpa_service = "fi.w1.wpa_supplicant1";
const char* wpa_root_obj_path = "/fi/w1/wpa_supplicant1";
const char* wpa_root_iface = "fi.w1.wpa_supplicant1";
sd_bus_error error = SD_BUS_ERROR_NULL;
sd_bus* system_bus = NULL;
sd_event* loop = NULL;
sd_bus_message* reply = NULL;
void cleanup() {
sd_event_unref(loop);
sd_bus_unref(system_bus);
sd_bus_message_unref(reply);
sd_bus_error_free(&error);
}
void print_error(const char* msg, int code) {
fprintf(stderr, "%s %s\n", msg, strerror(-code));
exit(EXIT_FAILURE);
}
const char* get_interface(const char* iface) {
int res = sd_bus_call_method(system_bus,
wpa_service,
wpa_root_obj_path,
wpa_root_iface,
"GetInterface",
&error,
&reply,
"s",
"Ifname", "s", iface,
"Driver", "s", "nl80211");
if (res < 0) {
fprintf(stderr, "(get) error response: %s\n", error.message);
return NULL;
}
const char* iface_path;
/*
* an object path was returned in reply
* this works like an iterator, if a method returns (osu), you could call message_read_basic in succession
* with arguments SD_BUS_TYPE_OBJECT_PATH, SD_BUS_TYPE_STRING, SD_BUS_TYPE_UINT32 or you could
* call sd_bus_message_read() and provides the signature + arguments in one call
* */
res = sd_bus_message_read_basic(reply, SD_BUS_TYPE_OBJECT_PATH, &iface_path);
if (res < 0) {
print_error("getIface: ", res);
return NULL;
}
return iface_path;
}
const char* create_interface(const char* iface) {
int res = sd_bus_call_method(system_bus,
wpa_service,
wpa_root_obj_path,
wpa_root_iface,
"CreateInterface",
&error,
&reply,
"a{sv}", 2, //pass array of str:variant (dbus dictionary) with 2
//entries to CreateInterface
"Ifname", "s", iface, // "s" variant parameter contains string, then pass the value
"Driver", "s", "nl80211");
if (res < 0) {
fprintf(stderr, "(create) error response: %s\n", error.message);
return NULL;
}
const char* iface_path;
res = sd_bus_message_read_basic(reply, SD_BUS_TYPE_OBJECT_PATH, &iface_path);
if (res < 0) {
print_error("createIface: ", res);
}
return iface_path;
}
int main() {
int res;
const char* iface_path;
//open connection to system bus - default either opens or reuses existing connection as necessary
res = sd_bus_default_system(&system_bus);
if (res < 0) {
print_error("open: ", res);
}
//associate connection with event loop, again default either creates or reuses existing
res = sd_event_default(&loop);
if (res < 0) {
print_error("event: ", res);
}
// get obj. path to the wireless interface on dbus so you can call methods on it
// this is a wireless interface (e.g. your wifi dongle) NOT the dbus interface
// if you don't know the interface name in advance, you will have to read the Interfaces property of
// wpa_supplicants root interface — call Get method on org.freedesktop.DBus properties interface,
// while some libraries expose some kind of get_property convenience function sd-bus does not
const char* ifaceName = "wlp32s0f3u2";
if (!(iface_path = get_interface(ifaceName))) { //substitute your wireless iface here
// sometimes the HW is present and listed in "ip l" but dbus does not reflect that, this fixes it
if (!(iface_path = create_interface(ifaceName))) {
fprintf(stderr, "can't create iface: %s" , ifaceName);
cleanup();
return EXIT_FAILURE;
}
}
/*
call methods with obj. path iface_path and dbus interface of your choice
this will likely be "fi.w1.wpa_supplicant1.Interface", register for signals etc...
you will need the following to receive those signals
*/
int runForUsec = 1000000; //usec, not msec!
sd_event_run(loop, runForUsec); //or sd_event_loop(loop) if you want to loop forever
cleanup();
printf("Finished OK\n");
return 0;
}
I apologize if the example above does not work perfectly. It is an excerpt from an old project I rewrote to C from C++ (I think it's C(-ish), compiler does not protest and you asked for C) but I can't test it as all my dongles refuse to work with my desktop right now. It should give you a general idea though.
Note that you will likely encounter several magical or semi-magical issues.
To ensure smooth developing/testing do the following:
make sure other network management applications are disabled (networkmanager, connman...)
restart the wpa_supplicant service
make sure the wireless interface is UP in ip link
Also, because is not that well-documented right now:
You can access arrays and inner variant values by sd_bus_message_enter_container
and _exit counterpart. sd_bus_message_peek_type might come handy while doing that.
Or sd_bus_message_read_array for a homogenous array.
The below snippet works for me
if (argType == DBUS_TYPE_STRING)
{
printf("Got string argument, extracting ...\n");
char* strBuffer = NULL;
dbus_message_iter_get_basic(&args, &strBuffer);
printf("Received string: \n %s \n",strBuffer);
}