My script has an asynchronous conversation that polls a queue for new messages, while the user performs other tasks. I've put the web_reg_async_attributes() into init, my callbacks are in asynccallbacks.c, and my main logic is in action.c
The async polls every 5s, checking the message queue. When there is a message, I would like the callback to set a flag that the action.c has access to so that it can execute logic conditionally. I've tried using a global variable, declared in init, but it is not visible in asynccallbacks.c.
Is there a way to accomplish this? (I don't want to use files because I'm measuring activities that take less than a second and if I put the file system into the picture, my response times won't be representative).
In the first file (asynccallbacks.h) :
// Explicit definition, this actually allocates
// as well as describing
int Global_Variable;
// Function prototype (declaration), assumes
// defined elsewhere, normally from include file.
void SomeFunction(void);
int main(void) {
Global_Variable = 1;
SomeFunction();
return 0;
}
In the second file (action.c) :
// Implicit declaration, this only describes and
// assumes allocated elsewhere, normally from include
extern int Global_Variable;
// Function header (definition)
void SomeFunction(void) {
++Global_Variable;
}
In this example, the variable Global_Variable is defined in asynccallbacks.h. In order to utilize the same variable in action.h, it must be declared. Regardless of the number of files, a global variable is only defined once; however, it must be declared in any file outside of the one containing the definition.
I know its basic but I am not familiar with C and I couldn't understand the answers here in the subject .
Inside a C file I have this functions :
void uart_event_handle(app_uart_evt_t * p_event)
{
}
static void uart_init(void)
{
}
void initialize()
{
uart_init();
}
The static function uart_init() was inside some example program main.c , i am trying to put it inside another C file ( this one)
The error occurs only when I call : uart_init(); . Otherwise it will not happen.
Declaring a function static makes it invisible outside the translation unit. This is similar to declaring fields private in a class, because static "hides" the function from all other files.
This lets you define a new function with the same name in some other file without worrying about name collisions. At the same time, static makes it impossible to call the function from outside the .c file where it is defined.
Your example provides a use case for making static functions: initialize becomes part of the "public" interface of the library, while uart_init remains hidden.
My understanding is that a local static variable has "program scope"; that is, once defined it lives until the program terminates. Furthermore, my understanding is that local statics are also accessible outside the current module/C-file they are defined inside of:
void doSomething() {
static int myVar = 5; // Program scope & accessible to other modules
}
My understanding is that global static variables also have program scope, but that they are only visible to the current module/C-file:
static int myVar = 5; // Program scope but "module private"
void doSomething() {
// Whatever here...
}
Isn't this a bit backwards? I would have expected it to be the other way around. So I ask: is my understanding above correct? If so, then what is the motivation for local statics to be accessible outside their current module, but not global statics?
You messed up all. Static local variable is a place to hide data from other functions but retain data for future calls of that function.
Static global variables have
static storage duration (Retain data for future call)
internal linkage (variable is restricted to single file) and
file scope (Only in the current file the variable can be referenced)
Static local variables have
static storage duration
no linkage
block scope
Nope. Local Static variables are accessible with in given module.
While Global static variables are accessible with in given C-file.
Local static variable has limited scope to the function only.As you can see, following code gives compilation error because it tries to access "prox" variable from main even though it is static.
int main(void)
{
test();
printf("The PROX is : %d", prox);
}
test()
{
static prox;
prox=prox+5;
printf("INSIDE prox[%d]",prox);
}
I am looking at a crash dump, and an important clue to how this crash occurred may be inside the value of a static variable (an integer in this case) inside a function. The problem is that the function with that static variable is is not in my call stack where the crash occurred so I can't just look at it directly. Is there a way to view the contents of this variable from the debugger from outside the function that declares it?
Edit:
Sample code has been requested
int funcitonWithStaticVar()
{
static int iRetVal;
if (iRetVal == 0)
{
iRetVal = initializeValue();
}
return iRetVal
}
void functionThatCrashes()
{
// Crash occurs in this function. The
// static variable in the other function
// may hold an important clue as to why
}
int foo()
{
functionWithStaticVar();
functionThatCrashes();
}
You can determine the address of the static variable by viewing the disassembly of the function that accesses it.
I am writing a large C program for embedded use. Every module in this program has an init() function (like a constructor) to set up its static variables.
The problem is that I have to remember to call all of these init functions from main(). I also have to remember to put them back if I have commented them out for some reason.
Is there anything clever I do to make sure that all of these functions are getting called? Something along the lines of putting a macro in each init function that, when you call a check_inited() function later, sends a warning to STDOUT if not all the functions are called.
I could increment a counter, but I'd have to maintain the correct number of init functions somewhere and that is also prone to error.
Thoughts?
The following is the solution I decided on, with input from several people in this thread
My goal is to make sure that all my init functions are actually being called. I want to do
this without maintaining lists or counts of modules across several files. I can't call
them automatically as Nick D suggested because they need to be called in a certain order.
To accomplish this, a macro included in every module uses the gcc constructor attribute to
add the init function name to a global list.
Another macro included in the body of the init function updates the global list to make a
note that the function was actually called.
Finally, a check function is called in main() after all of the inits are done.
Notes:
I chose to copy the strings into an array. This not strictly necessary because the
function names passed will always be static strings in normal usage. If memory was short
you could just store a pointer to the string that was passed in.
My reusable library of utility functions is called "nx_lib". Thus all the 'nxl' designations.
This isn't the most efficient code in the world but it's only called a boot time so that
doesn't matter for me.
There are two lines of code that need to be added to each module. If either is omitted,
the check function will let you know.
you might be able to make the constructor function static, which would avoid the need to give it a name that is unique across the project.
this code is only lightly tested and it's really late so please check carefully before trusting it.
Thank you to:
pierr who introduced me to the constructor attribute.
Nick D for demonstrating the ## preprocessor trick and giving me the framework.
tod frye for a clever linker-based approach that will work with many compilers.
Everyone else for helping out and sharing useful tidbits.
nx_lib_public.h
This is the relevant fragment of my library header file
#define NX_FUNC_RUN_CHECK_NAME_SIZE 20
typedef struct _nxl_function_element{
char func[NX_FUNC_RUN_CHECK_NAME_SIZE];
BOOL called;
} nxl_function_element;
void nxl_func_run_check_add(char *func_name);
BOOL nxl_func_run_check(void);
void nxl_func_run_check_hit(char *func_name);
#define NXL_FUNC_RUN_CHECK_ADD(function_name) \
void cons_ ## function_name() __attribute__((constructor)); \
void cons_ ## function_name() { nxl_func_run_check_add(#function_name); }
nxl_func_run_check.c
This is the libary code that is called to add function names and check them later.
#define MAX_CHECKED_FUNCTIONS 100
static nxl_function_element m_functions[MAX_CHECKED_FUNCTIONS];
static int m_func_cnt = 0;
// call automatically before main runs to register a function name.
void nxl_func_run_check_add(char *func_name)
{
// fail and complain if no more room.
if (m_func_cnt >= MAX_CHECKED_FUNCTIONS) {
print ("nxl_func_run_check_add failed, out of space\r\n");
return;
}
strncpy (m_functions[m_func_cnt].func, func_name,
NX_FUNC_RUN_CHECK_NAME_SIZE);
m_functions[m_func_cnt].func[NX_FUNC_RUN_CHECK_NAME_SIZE-1] = 0;
m_functions[m_func_cnt++].called = FALSE;
}
// call from inside the init function
void nxl_func_run_check_hit(char *func_name)
{
int i;
for (i=0; i< m_func_cnt; i++) {
if (! strncmp(m_functions[i].func, func_name,
NX_FUNC_RUN_CHECK_NAME_SIZE)) {
m_functions[i].called = TRUE;
return;
}
}
print("nxl_func_run_check_hit(): error, unregistered function was hit\r\n");
}
// checks that all registered functions were called
BOOL nxl_func_run_check(void) {
int i;
BOOL success=TRUE;
for (i=0; i< m_func_cnt; i++) {
if (m_functions[i].called == FALSE) {
success = FALSE;
xil_printf("nxl_func_run_check error: %s() not called\r\n",
m_functions[i].func);
}
}
return success;
}
solo.c
This is an example of a module that needs initialization
#include "nx_lib_public.h"
NXL_FUNC_RUN_CHECK_ADD(solo_init)
void solo_init(void)
{
nxl_func_run_check_hit((char *) __func__);
/* do module initialization here */
}
You can use gcc's extension __attribute__((constructor)) if gcc is ok for your project.
#include <stdio.h>
void func1() __attribute__((constructor));
void func2() __attribute__((constructor));
void func1()
{
printf("%s\n",__func__);
}
void func2()
{
printf("%s\n",__func__);
}
int main()
{
printf("main\n");
return 0;
}
//the output
func2
func1
main
I don't know how ugly the following looks but I post it anyway :-)
(The basic idea is to register function pointers, like what atexit function does.
Of course atexit implementation is different)
In the main module we can have something like this:
typedef int (*function_t)(void);
static function_t vfunctions[100]; // we can store max 100 function pointers
static int vcnt = 0; // count the registered function pointers
int add2init(function_t f)
{
// todo: error checks
vfunctions[vcnt++] = f;
return 0;
}
...
int main(void) {
...
// iterate vfunctions[] and call the functions
...
}
... and in some other module:
typedef int (*function_t)(void);
extern int add2init(function_t f);
#define M_add2init(function_name) static int int_ ## function_name = add2init(function_name)
int foo(void)
{
printf("foo\n");
return 0;
}
M_add2init(foo); // <--- register foo function
Why not write a post processing script to do the checking for you. Then run that script as part of your build process... Or better yet, make it one of your tests. You are writing tests, right? :)
For example, if each of your modules has a header file, modX.c. And if the signature of your init() function is "void init()"...
Have your script grep through all your .h files, and create a list of module names that need to be init()ed. Then have the script check that init() is indeed called on each module in main().
If your single module represents "class" entity and has instance constructor, you can use following construction:
static inline void init(void) { ... }
static int initialized = 0;
#define INIT if (__predict_false(!initialized)) { init(); initialized = 1; }
struct Foo *
foo_create(void)
{
INIT;
...
}
where "__predict_false" is your compiler's branch prediction hint. When first object is created, module is auto-initialized (for once).
Splint (and probably other Lint variants) can give a warning about functions that are defined but not called.
It's interesting that most compilers will warn you about unused variables, but not unused functions.
Larger running time is not a problem
You can conceivably implement a kind of "state-machine" for each module, wherein the actions of a function depend on the state the module is in. This state can be set to BEFORE_INIT or INITIALIZED.
For example, let's say we have module A with functions foo and bar.
The actual logic of the functions (i.e., what they actually do) would be declared like so:
void foo_logic();
void bar_logic();
Or whatever the signature is.
Then, the actual functions of the module (i.e., the actual function declared foo()) will perform a run-time check of the condition of the module, and decide what to do:
void foo() {
if (module_state == BEFORE_INIT) {
handle_not_initialized_error();
}
foo_logic();
}
This logic is repeated for all functions.
A few things to note:
This will obviously incur a huge penalty performance-wise, so is
probably not a good idea (I posted
anyway because you said runtime is
not a problem).
This is not a real state-machine, since there are only two states which are checked using a basic if, without some kind of smart general logic.
This kind of "design-pattern" works great when you're using separate threads/tasks, and the functions you're calling are actually called using some kind of IPC.
A state machine can be nicely implemented in C++, might be worth reading up on it. The same kind of idea can conceivably be coded in C with arrays of function pointers, but it's almost certainly not worth your time.
you can do something along these lines with a linker section. whenever you define an init function, place a pointer to it in a linker section just for init function pointers. then you can at least find out how many init functions have been compiled.
and if it does not matter what order the init functions are called, and the all have the same prototype, you can just call them all in a loop from main.
the exact details elude my memory, but it works soemthing like this::
in the module file...
//this is the syntax in GCC..(or would be if the underscores came through in this text editor)
initFuncPtr thisInit __attribute((section(.myinits)))__= &moduleInit;
void moduleInit(void)
{
// so init here
}
this places a pointer to the module init function in the .myinits section, but leaves the code in the .code section. so the .myinits section is nothing but pointers. you can think of this as a variable length array that module files can add to.
then you can access the section start and end address from the main. and go from there.
if the init functions all have the same protoytpe, you can just iterate over this section, calling them all.
this, in effect, is creating your own static constructor system in C.
if you are doing a large project and your linker is not at least this fully featured, you may have a problem...
Can I put up an answer to my question?
My idea was to have each function add it's name to a global list of functions, like Nick D's solution.
Then I would run through the symbol table produced by -gstab, and look for any functions named init_* that had not been called.
This is an embedded app so I have the elf image handy in flash memory.
However I don't like this idea because it means I always have to include debugging info in the binary.