Tag: C/C++

|Design| Benefits of using Macro

What is the Macro?
If you write as “#define” in your code, that is “Macro” .

Macro will handle by the preprocessor.
You can think Macro as a method to replace the String which appears in you code..

pros:
– run faster than writing as function

cons:
– occupy memories

concepts:
– there is no the concept of pointer in macro.
just “replacing the string”

An question:
What is the output of the following code?

#define macro_to_func(pp) ({printf("in Macro ...\n"); minus ;})

void do_something(int *a, int *b){
    printf("do_something\n");
    int c;
    c = *a + *b;
    printf("do_something %d \n", c);
}

int minus(int *a, int *b){
    printf("minus\n");
    int c;
    c = *a - *b +1;
    printf("minus a = %d \n", *a);
    printf("minus b = %d \n", *b);
    printf("minus %d \n", c);
    return c;
}

int main()
{
    int (*do_something)(int* c, int* d);
    //printf("fptr 1 do_something %p\n", &do_something);

    int m = -1;
    int n = 7;

    printf("do_something = macro_to_func\n");
    do_something = macro_to_func(pp);
    //printf("fptr minus %p\n", &minus);
    //printf("fptr 2 do_something %p\n", &do_something);
    if(do_something){
        printf("in IF!!\n");
        do_something(&m, &n); --------------------------->???
    }
    return 0;
}

Answer: 

do_something = macro_to_func
in Macro …
in IF!!
minus
minus a = -1
minus b = 7
minus -7

Explanation:
“int (*do_something)” is a local function pointer.
In the macro, the macro_to_func will be replaced as “minus”, but do_something !!!

|Code| LLVM initial

One day, if you want to implement a module on the platform x86_64 for arm64, you will need to know the tool chain, “cross compiler”.
The famous modern compiler is LLVM.

Why should we understand the compiler?
– build for multi-platform
– improve the efficiency of your code
– …..

I have a conclusion for the developer whose library is under the user space.
Use suitable optimization provided by LLVM, and make your code readable.
Dazzle coding is meaningless.

你所不知道的 C 語言：編譯器和最佳化原理篇
[共筆] 不深不淺，帶你認識 LLVM
“編譯器 LLVM 淺淺玩” by Pokai Chang

|Design| Communicate with Kernel driver

User space can not exchange data with kernel space directly.
They need to use system call.
e.g. fopen, ioctl, write, read …… etc.

In Kernel Space ……
1. Register your device with “register_chrdev”, defined in linux/fs.h .
　https://ithelp.ithome.com.tw/articles/10159749
2. Implement driver functions, ioctl, open,…
　struct file_operations fops = {
　　.owner = THIS_MODULE,
　　.read = device1_read,
　　.write = device1_write,
　　.ioctl = device1_ioctl,
　　.open = device1_open,
　　.release = device1_release,
　};

　int ioctl(struct inode *, struct file *, unsigned int, unsigned long);
　is in linux/ioctl.h
　http://ccckmit.wikidot.com/lk:io
　開發 driver 需要的基礎知識
　user space/kernel space 的IO觀念及實作

Copy data ……
　Kernel Space to User Space: copy_to_user()
　User Space to Kernel Space: copy_from_user()

In User Space ……
Use system call to control kernel driver.
　fopen (open)
　write
　read
　close
　seek
　poll / select
　ioctl
　mmap
　fcntl


e.g. 

int main(int argc, char *argv[]){
　　int devfd;
　　int num = 0;

　　if (argc > 1) num = atoi(argv[1]);
　　if (num < 0) num = 0xff;

　　devfd = open("/dev/debug", O_RDONLY);
　　if (devfd == -1) {
　　　　printf("Can't open /dev/debug\n");
　　　　return -1;
　　}

　　printf("Write 0x%02x...\n", num);
　　ioctl(devfd, IOCTL_WRITE, num);
　　printf("Done. Wait 5 seconds...\n");
　　sleep(5);
　　close(devfd);

　　return 0;
}

|Code| Makefile -I -L -l

e.g.
gcc -o hello hello.c -I /home/hello/include -L /home/hello/lib -lworld


Means:
-I /home/hello/include
Find headers in sequence of /home/hello/include–>/usr/include–>/usr/local/include

-L /home/hello/lib:
Find librarys in sequence of /home/hello/lib–>/lib–>/usr/lib–>/usr/local/lib

-lworld:
Find the library “libworld.so” in the path of “-L /home/hello/lib “

|Code| How to use control flow by pthread_cond_timedwait()

There are three parameters related with pthread_cond_timedwait().

Definition:
int pthread_cond_timedwait(pthread_cond_t *restrict cond,
 pthread_mutex_t *restrict mutex,
 const struct timespec *restrict abstime);

Parameters:
pthread_cond_t *restrict cond : the condition you want to trigger this flow
pthread_mutex_t *restrict mutex : Mutual exclusion
const struct timespec *restrict abstime : absolutely waiting time ( = system time + waiting



【Step by Step】

Initial the mutex:
1. pthread_mutex_init(pthread_mutex_t *restrict  __mutex ,
              const pthread_mutexattr_t *restrict __mutex_attr);
　　 __mutex_attr can be NULL.
　　 __mutex_attr is the parameter about mutex.
　　If it is not NULL, go to “2.”
2. pthread_mutexattr_init(pthread_mutexattr_t * __mutex_attr );
　pthread_mutexattr_settype(pthread_mutexattr_t *attr, int type);
　　 type: PTHREAD_MUTEX_RECURSIVE,
　　　　　 PTHREAD_MUTEX_NORMAL ……
3. pthread_cond_init(pthread_cond_t *restrict __cond ,
 const pthread_condattr_t *restrict __cond_attr);

Start to wait for signal ” __cond “:
1. lock mutex: pthread_mutex_lock(&__mutex)
2. pthread_cond_timedwait(&__cond, &__mutex, &__abstime)
3. unlock mutex: pthread_mutex_unlock(&__mutex)

Start to send the signal ” __cond “:
1. lock mutex: pthread_mutex_lock(&__mutex)
2. pthread_cond_signal (&__cond)
3. unlock mutex: pthread_mutex_unlock(&__mutex)

Destroy the mutex:
1. pthread_mutex_destroy( pthread_mutex_t *restrict  __mutex );
2. pthread_mutexattr_destroy( pthread_mutexattr_t * __mutex_attr );
3. pthread_cond_destroy( pthread_cond_t *restrict __cond );

E.g.

// This thread will wait the signal.
void thread1(void *arg){
　int inArg = (int)arg;
　int ret = 0;
　struct timeval now;
　struct timespec outtime;

　pthread_mutex_lock(&g_mutex);

　gettimeofday(&now, NULL);
　outtime.tv_sec = now.tv_sec + 5;
　outtime.tv_nsec = now.tv_usec * 1000;
　ret = pthread_cond_timedwait(&g_cond, &g_mutex, &outtime);

　pthread_mutex_unlock(&g_mutex);
}


int main(void)
{
　pthread_t id1;
　int ret;

　// Initial mutex
　pthread_cond_init(&g_cond, NULL);
　pthread_mutex_init(&g_mutex, NULL);
　ret = pthread_create(&id1, NULL, (void *)thread1, (void *)1);

　if (0 != ret){
　　printf(“thread 1 create failed!\n”);
　　return 1;
　}

　printf(“Waiting %ds send the signal!\n”, SENDSIGTIME);
　sleep(SENDSIGTIME);

　// send the signal
　pthread_mutex_lock(&g_mutex);
　pthread_cond_signal(&g_cond);
　pthread_mutex_unlock(&g_mutex);
　pthread_join(id1, NULL);

　//Destroy the mutex
　pthread_cond_destroy(&g_cond);
　pthread_mutex_destroy(&g_mutex);
　return 0;
}

References:
https://blog.csdn.net/dead_g/article/details/73338960
https://linux.die.net/man/3/pthread_cond_timedwait
https://linux.die.net/man/3/pthread_cond_init
https://blog.csdn.net/yasi_xi/article/details/19112077

|Design| bit map

如果你的某個宣告有多重意義，
但你一次只想知道某的欄位的狀態，可以善加利用 bit map

舉例：

#define status1 (uint_8) 1<<0
#define status2 (uint_8) 1<<1
#define status3 (uint_8) 1<<2

uint_8 test_case = 2;
int test_hex_case = 0xFA;

If( test_case & status1 ){ // false; 00000000 <- 00000010 & 00000001
　　//do nothing
}
If( test_case & status2 ){ // true, not zero; 00000010 <- 00000010 & 00000010
　　//do something
}
If( test_hex_case & status2 ){ // true, not zero;
// 0xFA = 1111 1010
// 00000010 <- 1111 1010 & 00000010
　　// do something
}

|Design| C++ getInstance

如果你要獲取特定的物件

就可以使用 getInstance()

這樣比全域變數看起來舒心一點

e.g. ————————————————————————

// test.h
class SomeService
{
public:
static SomeService &GetInstance();
…
}

// test.cpp
SomeService& SomeService::GetInstance()
{
static SomeService instance;
return instance;
}

————————————————————————


Ref. https://blog.csdn.net/fanyunda1988/article/details/51516930

|RIL| 建了 data 之後 ……

有了 APN，Modem 建了 data 之後會發生什麼事呢？

會長出網卡 ~~
可以使用 ifconfig 查看

別忘了設定 default route
route add default gw {IP-ADDRESS} {INTERFACE-NAME}

以及 DNS，才能用網域找到 ip
sudo echo nameserver 8.8.8.8 > /etc/resolv.conf

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