The function definition provided in the source for redisAsyncCommand( ) is:
static int __redisAsyncCommand(redisAsyncContext *ac, redisCallbackFn *fn, void *privdata, const char *cmd, size_t len) { ... }
What is the purpose of the void *privdata argument? In what cases would it be useful?
As I understand by reading the code on gihub, the purpose of privdata is to send your callback some predefined data (which can be anything; that is why void* is used). In your callback (fn pointer to redisCallbackFn) you will recieve that privdata as parameter (for example look at cb->fn(ac,reply,cb->privdata); in func __redisRunCallback file async.c)
For example (simplified pseudo code for something similar) is bellow. In this example there are 3 successive calls to __redisAsyncCommandSimplified and only one handler (callback). In callback I have used privdata to determine behavior of
callback. Your callback can also use that privdata data for something else (like parameter for another function call, logging, structure creation/population, etc)...
#include <stdio.h>
#include <string.h>
typedef void (*Callback)(int, void*);
int __redisAsyncCommandSimplified(Callback call, void* privdata) {
call(1, privdata);
return 1;
}
void myHandler(int status, void* privdata) {
char* str = (char*)privdata;
printf("%s = ", str);
if (strcmp (str, "john") == 0) {
printf("lennon");
}
else if (strcmp(str, "ringo") == 0) {
printf("star");
}
else if (strcmp(str, "ringo") == 0) {
printf("star");
}
else if (strcmp(str, "paul") == 0) {
printf("mccartney");
}
else if (strcmp(str, "george")) {
printf("harrison");
}
else {
printf("who?!?");
}
printf("\n");
}
int main()
{
char c[20];
strcpy(c, "john");
__redisAsyncCommandSimplified(myHandler, c);
strcpy(c, "paul");
__redisAsyncCommandSimplified(myHandler, c);
strcpy(c, "someone else");
__redisAsyncCommandSimplified(myHandler, c);
return 0;
}
Related
I have a function to print debug logs which has to be toggled depending on the environment variable. Instead of checking the env var each time the print_trace() is called, what should be the best method to store it and reuse that value?
void print_trace(const char* msg)
{
const char* s = getenv("DEBUG_TRACE");
if(!strcmp(s,"ON"))
printf(msg);
}
There is no main() as this is a shared library.
You could save the result of the decision in a static variable.
void print_trace(const char* msg)
{
static int debug_on = -1; // -1 == not yet set
if (debug_on == -1) {
const char* s = getenv("DEBUG_TRACE");
debug_on = s && (strcmp(s, "ON") == 0);
}
if(debug_on)
printf("%s", msg);
}
You could use the thread safe call_once feature that was added in C11.
Example:
#include <threads.h>
static bool debug_mode; // your debug mode flag
void set_debug_mode(void) { // this is only called once
const char *s = getenv("DEBUG_TRACE");
debug_mode = s && !strcmp(s, "ON");
}
void print_trace(const char* msg) {
static once_flag flag = ONCE_FLAG_INIT;
call_once(&flag, set_debug_mode); // called once to set debug_mode
if(debug_mode)
printf(msg);
}
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Question:
I want to create a reusable function, because in my code much line that use same code structure
The code using for example if { if { `Only here's the different` } }. Of course the pattern not same as this, this using as an example.
I've been code using framework such as Laravel, there's a directive called as SLOT
Is there any way I can inject code in the middle of for loop? Or anything same as SLOT inside C programming
Sample code:
void functionname() {
for (int i=0; i < total_count; i++) {
SELECT THE ITEM (i)
if (a == b) return;
if (c) {
CODE INJECT HERE
}
}
}
Forget to mention before, a, b, c and so on from the coding above is getting from ITEM (i)
You should use a callback. i.e. you should send a function pointer (i.e. the address of the function you want to execute) and use that pointer to execute that function inside your loop.
In the example below, p is a pointer to a function taking a const char * for a parameter and returning an int.
int (*p)(const char *s) ;
NB: all functions passed as parameter, to be used as callback must have the same prototype (which is why such functions are often declared taking a generic pointer parameter void * to accept whatever you've got to send to the function).
So with your example and with functions taking void * as a parameter and returning void *, and with param defining a parameter that you want to feed to your function, this gives us the following code:
void functionname(void *(*func)(void *)) {
for (int i=0; i < total_count; i++) {
SELECT THE ITEM (i)
if (a == b) return;
if (c) {
func(¶m);
}
}
}
you can call your function wiht whatever function respecting the prototype... For instance:
void *my_custom_function(void *param) {
...
}
...
functionname(my_custom_function);
...
As suggested in the comment by KamilCik, use function pointers:
void functionname(void *fx)(void)) {
for (int i=0; i < total_count; i++) {
SELECT THE ITEM (i)
if (a == b) return;
if (c) {
//CODE INJECT HERE
fx();
}
}
}
And use it like
void foo(void) { puts("foo() called"); }
void bar(void) { puts("bar() called"); }
int main(void) {
functionname(foo);
functionname(bar);
}
For a concrete example:
#include <stdio.h>
int a = 1;
int b = 2;
typedef void (*selector)(int, int *);
typedef void (*injector)(void);
void select1(int x, int *c) { printf("%s: %d\n", __func__, *c = x); }
void select2(int x, int *c) { printf("%s: %d\n", __func__, *c = x); }
void inject1(void) { printf("%s\n", __func__); }
void inject2(void) { printf("%s\n", __func__); }
void
functionname(size_t total_count, selector SELECT_THE_ITEM,
injector CODE_INJECT_HERE )
{
for (size_t i=0; i < total_count; i++) {
int c;
SELECT_THE_ITEM (i, &c);
if (a == b) return;
if (c) {
CODE_INJECT_HERE();
}
}
}
int
main(void)
{
functionname(2, select1, inject1);
functionname(3, select2, inject2);
}
You can do what you ask by defining your "CODE INJECT HERE" as the body of a function, and passing a pointer to that function:
void functionname(void (*inject)(void)) {
for (int i=0; i < total_count; i++) {
SELECT THE ITEM (i)
if (a == b) return;
if (c) {
inject();
}
}
}
void do_something(void) {
CODE INJECT HERE
}
void do_something_else(void) {
OTHER CODE INJECT HERE
}
int main(void) {
functionname(do_something));
functionname(do_something_else));
}
Do note, however, that this is not simple code injection in the same sense as a macro would provide. In particular, the executions of do_something() and do_something_else() will not see the local variables of main() or of functionname(), and the do_* functions can return only from themselves, not from a caller further up the chain. The former can be mitigated to some extent by passing parameters to the do_* functions (which they must be prepared to accept).
Another alternative would be to use a macro instead of a function to provide the common framework. It would look something like this:
#define frame_it(x) do { \
for (int i=0; i < total_count; i++) { \
SELECT THE ITEM (i) \
if (a == b) return; \
if (c) { \
x \
} \
} \
} while (0)
int main(void) {
frame_it(
CODE INJECT HERE
);
frame_it(
OTHER CODE INJECT HERE
);
}
That keeps the CODE INJECT HERE code in the function using it, which might be advantageous if in fact each such piece of code is used in only one place. It also allows both that code and the framing code to access the local variables of the function in which they appear, and to return from that function if desired.
However, macro programming has earned a mostly-deserved reputation for being error prone and difficult to read and debug. Your particular need may be one that is well served by this approach, but do not choose this direction lightly.
Function pointers are great for this. You can typedef the function signatures you'd like to support. Example:
/* A signature for condition checking functions, taking a "void*" argument
and returning true or false */
typedef bool(*cond_check_t)(void*);
/* A signature for functions to execute if a condition is met. This takes a
"void*" argument but you decide what you need */
typedef void(*exec_t)(void*);
You can package these two in a struct to form a nice pair:
typedef struct {
cond_check_t checker;
exec_t executor;
} check_exec_t;
And with that, another struct to keep a bunch of these condition and executor pairs:
typedef struct {
size_t size;
size_t capacity;
check_exec_t *conditionals;
} cond_pack_t;
You then create support functions for adding checkers and executors and a function to processes one of these packaged checkers and executors.
cond_pack_t* cond_pack_create(size_t capacity) {
cond_pack_t* cp = malloc(sizeof(*cp));
if(cp) {
cp->conditionals = malloc(sizeof(*cp->conditionals) * capacity);
if(cp->conditionals) {
cp->size = 0;
cp->capacity = capacity;
} else {
free(cp);
cp = NULL;
}
}
return cp;
}
void cond_pack_destroy(cond_pack_t *cp) {
free(cp->conditionals);
free(cp);
}
bool cond_pack_add(cond_pack_t *cp, cond_check_t checker, exec_t executor) {
if(cp->size == cp->capacity) return false;
cp->conditionals[cp->size].checker = checker;
cp->conditionals[cp->size].executor = executor;
++cp->size;
return true;
}
void cond_pack_process(cond_pack_t *cp) {
for(size_t i = 0; i < cp->size; ++i) {
if(cp->conditionals[i].checker(NULL)) { /* execute checker */
cp->conditionals[i].executor(NULL); /* execute executor */
}
}
}
With that, a usage example could look like this
//---
bool some_check(void *foo) {
return true;
}
void some_executor(void *foo) {
printf("some_executor\n");
}
bool some_other_check(void *foo) {
return false;
}
void some_other_executor(void *foo) {
printf("some_other_executor\n");
}
int main() {
cond_pack_t *cp = cond_pack_create(10);
if(cp) {
cond_pack_add(cp, some_check, some_executor);
cond_pack_add(cp, some_other_check, some_other_executor);
cond_pack_process(cp); /* run all the checkers / executors */
cond_pack_destroy(cp);
}
}
Demo
I'm building a program that takes input as if it is a bare MAC address and turn it into a binary string. I'm doing this on a embedded system so there is no STD. I have been trying something similar to this question but after 2 days I haven't achieved anything, I'm really bad with these kind of things.
What I wanted is output to be equal to goal, take this into consideration:
#include <stdio.h>
int main() {
const char* goal = "\xaa\xbb\xcc\xdd\xee\xff";
printf("Goal: %s\n", goal);
char* input = "aabbccddeeff";
printf("Input: %s\n", input);
char* output = NULL;
// Magic code here
if (output == goal) {
printf("Did work! Yay!");
} else {
printf("Did not work, keep trying");
}
}
Thanks, this is for a personal project and I really want to finish it
First, your comparison should use strcmp else it'll be always wrong.
Then, I would read the string 2-char by 2-char and convert each "digit" to its value (0-15), then compose the result with shifting
#include <stdio.h>
#include <string.h>
// helper function to convert a char 0-9 or a-f to its decimal value (0-16)
// if something else is passed returns 0...
int a2v(char c)
{
if ((c>='0')&&(c<='9'))
{
return c-'0';
}
if ((c>='a')&&(c<='f'))
{
return c-'a'+10;
}
else return 0;
}
int main() {
const char* goal = "\xaa\xbb\xcc\xdd\xee\xff";
printf("Goal: %s\n", goal);
const char* input = "aabbccddeeff";
int i;
char output[strlen(input)/2 + 1];
char *ptr = output;
for (i=0;i<strlen(input);i+=2)
{
*ptr++ = (a2v(input[i])<<4) + a2v(input[i]);
}
*ptr = '\0';
printf("Goal: %s\n", output);
if (strcmp(output,goal)==0) {
printf("Did work! Yay!");
} else {
printf("Did not work, keep trying");
}
}
#include <stdio.h>
int limit;
char alp[26]={'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','w','x','y','z'};
void encode(char message[21],char enc_message[21],int key);
void decode(char enc_message[21],char dec_message[21],int key);
main()
{
int key,i=0,j=0;
char message[21];
char enc_message[21];
char dec_message[21];
char encrypted[21];
char a='\0';
printf("Input the characters to encrypt\n");
while(i<21 && a!='\n')
{
scanf("%c",&a);
message[i]=a;
i=i+1;
}
for(i=0;;i++) /*custom strlen*/
{
if( message[i]= '\0')
{
limit=i;
break;
}
}
printf("Input the key");
scanf("%d",key);
for(i=0;i<21;i++)
{
enc_message[i]=message[i];
}
encode(message[21],enc_message[21],key);
for(i=0;i<21;i++)
{
dec_message[i]=enc_message[i];
}
for(i=0;i<limit;i++)
{
printf("%c",enc_message[i]);
}
printf("\n\n");
decode(enc_message[21],dec_message[21],key);
for(i=0;i<limit;i++)
{
printf("%c",dec_message[i]);
}
}
void encode(char message[21],char enc_message[21],int key)
{
/*char temp[21];*/
int x,y;
for(x=0;x<limit;x++) /* message check */
{
for(y=0;y<26;y++) /* <----- alphabet check */
{
if (enc_message[x]==alp[y]) enc_message[x]=alp[y+key];
}
}
}
/*------------------------------------------------------------------------*/
void decode(char enc_message[21],char dec_message[21],int key)
{
int x,y;
for (x=0;x<limit;x++)
{
for(y=0;y<26;y++)
{
if (dec_message[x]==alp[y+key]) dec_message[x]=alp[y];
}
}
}
The compiler says,the mistake has to do with the way I call functions(and write them)and says: passing argument1 of 'encode' makes pointer from integer without a cast ,and that is for argument 2 of 'encode' and the exact same for 'decode'
Thanks in advance!
You are passing a single element and it's not even a valid element, try
decode(enc_message, dec_message, key);
Also, format your code so it's readable that is really important, and looping to compute the length of the string to use it in another loop is not a very smart thing, print it in a loop like
for (int i = 0 ; enc_message[i] != '\0' ; ++i) ...
also, don't over use break, just think about the logical condition for the loop, it's the same one where you break. Code is much more readable if the condition appears in the right place.
I want to get the string from USART1 of STM32VLDiscovery (STM32F100X4) and write an AT Command Parser from the string received from USART1.
Below are the concept that I have developed but I am not sure whether it's correct or not.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "dosomethinga.h"
void dosomethingB();
void GET_AT_COMMAND(char*);
void takecommand(char *, char *);
int quit;
int main()
{ char buff[15];
char command = '\0';
quit = 0;
while(!quit)
{
printf("Enter your command: ");
scanf("%s", &buff);
if (buff[0] == 'A' && buff[1] == 'T' && buff[2] == '+')
{
GET_AT_COMMAND(buff);
}
}
}
void dosomethingB()
{
printf("dosomethingB called \n");
}
void GET_AT_COMMAND(char *text)
{
int command;
char temp[10] = "";
/*if(text[3] == 'A')
command = 1;
else if(text[3] == 'B')
command = 2;
else if(text[3] == 'Z')
command = 3;
*/
takecommand(text,temp);
if (strcmp(temp, "CALLA") == 0)
command = 1;
if (strcmp(temp, "CALLB") == 0)
command = 2;
if (strcmp(temp, "Z") == 0)
command = 3;
switch(command)
{
case 1:
dosomethingA();
break;
case 2:
printf("herehere.... \n");
dosomethingB();
break;
case 3:
printf("Exiting program.... \n");
quit = 1;
break;
default:
printf("Nothing to do here \n");
}
}
void takecommand(char *mycmd, char *hold)
{
int i;
for(i = 0; i < 10 ; i++)
{
hold[i] = mycmd[i+3];
}
}
Can anyone explain on the steps that I should do? Thanks.
Basicly you should wait an attention "AT" from the input and ignore anything before it.
For example inputs "XYZATZ\r" and "AaatZ\r" should be both handled as a "ATZ" command.
There can also be short pause between 'A' and 'T' (and all other chars of commands too), because human may type those commands.
By the default all commands end to "\r" character.
See more about AT commands from ITU-T documentation. For example from V.250 standard.
There are probably many alternative ways to implement that. The best alternative depends on your needs. If you are going to implement all AT-commands of mobile-terminal, then you should spend more time for the parser. If you want make some test application for few commands, then your implementation could be simple as your provided one.
I have developed this AT command parser, this could be use for reference.
When you get data from UART1 just call this method at_wait_msg() to parse the AT message
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
static const char *AT_HEADER = "AT";
static const char *AT_DEVICE_PROFILE = "DR";
static const char *AT_SET_DEVICE = "SD";
static const char AT_EOF = '\r';
typedef enum {
DeviceProfile,
SetDevice,
Error
} AT_Msg_Type;
typedef struct {
char header[3];
char command[3];
char data[128];
} AT_Msg_Data;
static void at_wait_msg(char text);
static void at_wait_msg_complete(char *text);
static void at_parse_msg(AT_Msg_Data *data);
static AT_Msg_Type at_check_format(AT_Msg_Data *data);
static char _rx_data[512];
static uint16_t _rx_index = 0;
int main()
{
//example data getting from UART1
char text[] = "ATDR\rATSD123456abchelloworld\r1123ATssa\r";
for (int i = 0; i < strlen(text) + 1; i++) {
//to simulate getting data from UART1 byte per byte
at_wait_msg(text[i]);
}
return 0;
}
static void at_wait_msg(char text)
{
_rx_data[_rx_index++] = text;
if (text == AT_EOF) {
at_wait_msg_complete(_rx_data);
_rx_index = 0;
}
}
static void at_wait_msg_complete(char *text)
{
AT_Msg_Data data;
int result = sscanf_s(_rx_data, "%2s%2s%s\r",
data.header, sizeof(data.header),
data.command, sizeof(data.command),
data.data, sizeof(data.data));
if (result >= 2) {
at_parse_msg(&data);
}
}
static void at_parse_msg(AT_Msg_Data *data)
{
AT_Msg_Type type = at_check_format(data);
switch (type) {
case DeviceProfile:
printf("device profile\r\n");
break;
case SetDevice:
printf("settings %s\r\n", data->data);
break;
case Error:
default:
printf("Error\r\n");
break;
}
}
static AT_Msg_Type at_check_format(AT_Msg_Data *data)
{
if (strcmp(data->header, AT_HEADER) != 0) {
return Error;
}
if (strcmp(data->command, AT_DEVICE_PROFILE) == 0) {
return DeviceProfile;
}
if (strcmp(data->command, AT_SET_DEVICE) == 0) {
return SetDevice;
}
return Error;
}