Let's say I have a text file composed like this
#####
typeofthread1
#####
typeofthread2
etc...
in my main I want to read that file, get the strings typeofthread1, typeofthread2 and create different threads using
pthread_t threads[NUM_THREADS];
for (i=0;i<NUM_THREADS;i++)
pthread_create(&threads[i], NULL, -> HERE <- , void * arg);
how can I put the just read typeofthread1, typeofthread2 strings into -> HERE <- making the main create two threads that point to two different thread prototype?
I want to do this because I want to create a program that creates different types of threads, depending on what I want to do, and choosing that from text file (sort of a configuration file)
any suggestion?
Map the string names to function pointers.
void * thread_type_1 ( void * );
void * thread_type_2 ( void * );
typedef void * (*start_routine_t)(void *);
typedef struct mapping_t {
const char * name;
start_routine_t function;
} mapping_t;
const mapping_t mappings[] = {
{"thread-type-1", &thread_type_1},
{"thread-type-2", &thread_type_2},
};
const size_t mapping_count =
sizeof(mappings)/sizeof(mappings[0]);
To select the proper thread function, loop over items in mappings and grab the function when the name matches.
start_routine_t get_start_routine ( const char * name )
{
size_t i;
for ( i=0; i < mapping_count; ++i )
{
if (strcmp(name,mappings[i].name) == 0) {
return mappings[i].function;
}
}
return NULL;
}
In wherever you launch the thread, you can use this as:
start_routine_t start_routine;
/* find start routine matching token from file. */
start_routine = get_start_routine(name);
if (start_routine == NULL) {
/* invalid type name, handle error. */
}
/* launch thread of the appropriate type. */
pthread_create(&threads[i], NULL, start_routine, (void*)arg);
A better approach would be to create a default thread_dispatch function that you launch all your pthreads with. This dispatch function would take a structure that contained a void* to a structure that contained the thread-specific data, and a string that specified the type of the thread-function you wanted to run. You could then, using a look-up table mapping the string to the function-pointer type created in a code-module, find the appropriate function pointer, and pass the thread-specific data to that function. So this would look something like the following:
typedef struct dispatch_data
{
char function_type[MAX_FUNCTION_LENGTH];
void* thread_specific_data;
} dispatch_data;
void* thread_dispatch(void* arg)
{
dispatch_data* data = (dispatch_data*)arg;
//... do look-up of function_pointer based on data->function_type string
return function_pointer(data->thread_specific_data);
}
Related
When creating a vfs using the tcl api how do you get the current filesystem in Tcl_Filesystem.pathInFilesystemProc
My code looks something like this:
typedef struct {
FILE* dbFile;
/*...*/
} FSBackend;
void createFS(const char* dbFile)
{
FSBackend* fsback = (FSBackend*)malloc(sizeof(FSBackend));
initDb(fsback,dbFile);
Tcl_Filesystem tfs;
tfs.typeName="Db Fs";
tfs.structureLength = sizeof(Tcl_Filesystem);
tfs.version = TCL_FILESYSTEM_VERSION_1;
tfs.pathInFilesystemProc = inFsProc;
/*...*/
Tcl_FSRegister((void*),tfs);
}
int inFsProc(Tcl_Obj* pathPtr,ClientData* cd)
{
/* How do I get my FSBackend struct here */
FSBackend* bk = /* ? */
int len;
const char* searchPath = Tcl_GetStringFromObj(pathPtr,&len);
char* foundPath = findFileInDb(searchPath,bk);
if (foundPath == 0) {
return -1;
}
cd = buildInternalRep(foundPath,bk);
return TCL_OK;
}
/**
...
*/
int main()
{
createFS("db1.db");
createFS("db2.db");
}
How do I, in inFsProc get back the struct I passed into Tcl_FSRegister?
The Tcl_FSData function says it can get it but I would then need to get a Tcl_Filesystem pointer
That's a weird one. The clientData handle there is not used to specify a mount point, but rather a separate capability of the filesystem type. Tcl's internal use of Tcl_FSRegister doesn't use it at all. The code which is as close as anything to a canonical use of it is the tclvfs package.
https://github.com/tcl-mirror/tclvfs/blob/master/generic/vfs.c#L385 shows us the use:
static void
Vfs_RegisterWithInterp(interp)
Tcl_Interp *interp;
{
ClientData vfsAlreadyRegistered;
/*
* We need to know if the interpreter is deleted, so we can
* remove all interp-specific mounts.
*/
Tcl_SetAssocData(interp, "vfs::inUse", (Tcl_InterpDeleteProc*)
Vfs_UnregisterWithInterp, (ClientData) 1);
/*
* Perform one-off registering of our filesystem if that
* has not happened before.
*/
vfsAlreadyRegistered = Tcl_FSData(&vfsFilesystem);
if (vfsAlreadyRegistered == NULL) {
Tcl_FSRegister((ClientData)1, &vfsFilesystem);
Tcl_CreateExitHandler(VfsExitProc, (ClientData)NULL);
Tcl_CreateThreadExitHandler(VfsThreadExitProc, NULL);
}
}
As you can see, the clientData there is really just being used as a marker so the code knows whether to do one-time initialisation.
To discover what the mount mapping is, you'll need to keep internal structures. You're strongly recommended to make the Tcl_Filesystem structure instance itself be global (or rather static at file scope) in your code.
I'm working on a network service that based on commands it receives over the network, it has workers perform different jobs. I want to have a log entry for every time a certain worker is tasked with doing some job.
I have a function (say function_caller) which, among other things, calls another function which it receives its pointer as an argument. I'd like to have my logger notify what kind of function function_caller calls.
Originally I wanted the function_caller to receive some enum instead of a function pointer, provide the enum to the logger, and then use a helper function which returns a suitable pointer based on the enum. However, function_caller is already deeply tangled in the codebase I'm working on, and it looks like it would be a lot of work to refactor all the functions that call function_caller to choose the right enum and use a new argument.
So my next idea was having a switch that for every function pointer will have some string representation of, but I've never stumbled upon something like that (and struggled to find anyone even mentioning such an idea on Google), so I have a feeling I might be missing some serious downsides to this option.
The only significant problem I see is that every developer that decides to pass a new kind of function pointer to function_caller will have to somehow know to update the switch, otherwise it will fail.
Am I missing anything else? Or maybe there's some other approach I should consider?
How about something like this? Instead of a switch, store a table of functions and their name strings. The table can even be kept dynamically updated, unlike a switch case. You will not need to walk along the edge of the standard as well!
#include <stdio.h>
typedef void (*callback_t) (void);
void first (void) { printf("%d", 1); };
void second (void) { printf("%d", 2); };
void third (void) { printf("%d", 3); };
typedef struct fntable_t
{
callback_t fn;
char *name;
} fntable_t;
fntable_t fntable[] =
{
{ first, "first" },
{ second, "second" },
{ third, "third" }
};
char* log_str(callback_t c)
{
for(int i = 0; i < sizeof(fntable) / sizeof(fntable_t); i++)
{
if(fntable[i].fn == c)
return fntable[i].name;
}
return "unknown";
}
void function_caller(callback_t c)
{
printf("%s",log_str(c));
c();
}
int main(void)
{
function_caller(first);
function_caller(second);
function_caller(third);
return 0;
}
You could replace function_caller with a wrapper macro of the same name that calls the renamed function function_caller_internal which gets an additional string argument. The wrapper macro can then pass an additional stringified function name.
This works only if function_caller is always called with a function name, not a function pointer variable.
Example:
#include <stdio.h>
static void funcA(void)
{
printf("This is funcA\n");
}
static void funcB(void)
{
printf("This is funcB\n");
}
/* renamed function gets an additional string argument */
static void function_caller_internal(void (*func)(void), const char *name)
{
printf("calling %s\n", name);
func();
}
/* wrapper macro stringifies the function name to pass it the additional argument */
#define function_caller(func) function_caller_internal(func, #func)
int main(void)
{
/* unchanged calls */
function_caller(funcA);
function_caller(funcB);
return 0;
}
This prints
calling funcA
This is funcA
calling funcB
This is funcB
If you can change the API of the functions, then consider using __func__ to get the textual name of each function. If you can have a function pointer type along the lines of this:
typedef void func_t (const char** name);
Then you can have each function return its name to the caller.
void foo (const char** name)
{
/* do foo stuff here */
*name = __func__;
}
void bar (const char** name)
{
/* do bar stuff here */
*name = __func__;
}
Example:
#include <stdio.h>
typedef void func_t (const char** name);
void foo (const char** name)
{
/* do foo stuff here */
*name = __func__;
}
void bar (const char** name)
{
/* do bar stuff here */
*name = __func__;
}
const char* function_caller (func_t* func, const char** name)
{
func(name);
return *name;
}
int main(void)
{
static func_t*const func [] =
{
foo,
bar,
};
const char* name;
for(size_t i=0; i<sizeof func/sizeof *func; i++)
{
puts( function_caller(func[i], &name) );
}
}
Assuming your codebase has sane variable names and function names, you can add a char * argument to your function caller:
void function_caller(char *name, int fpnt());
and then provide a macro:
#define function_caller_autoname(fpnt) function_caller(#fpnt, fpnt)
(Or, for spaghetti code, you can provide a macro with the same name as the function).
The #fpnt will be expanded by the proceprocessor to a string literal with the function name.
Then when your codebase called:
function_caller(some_function)
refactor it to:
function_caller_autoname(some_function)
# will be expanded to by the processor:
# function_caller("some_function", some_function)
or refactor it manually to provide the name/identificator/description of the function:
function_caller("Some function: ", some_function)
That way you can pass a custom string that describes the function along with the pointer. Also, each developer can pass a custom description string.
I have a function void startScanner(...) taking two function pointer as arguments: userType *vConfig(void) and void * vCallback(void). In this function i would like to create a thread and call vCallback() function in the function thread created. So i decided to pass vCallback as args to pthreadcreate.
The code of startScanner function :
void startScanner(tUsrStatus (*vConfig)(), void* (vCallback)()){
if(pthread_create(&scannerThread, NULL, scannerThreadFunc, vCallback))
{
printf("Thread creation fails!\n");
}
}
The scannerTread function:
static void *scannerThreadFunc(void *arg()){
void *funcptr(void) = arg;
while(1)
{
funcptr();
}
pthread_exit(NULL);
}
I get the following error:
error: function ‘funcptr’ is initialized like a variable
error: nested function ‘funcptr’ declared but never defined
How can i fix this?
Syntax errors aside (*) , it's impossible in standard C to pass a function pointer in a void *. There's a fundamental difference between pointers to functions and pointers to data, they can't be converted into each other. This is because there might be platforms where function and data pointers would differ even in size, or refer to different address spaces, or whatever.
But of course, there's a simple way to achieve what you want: Put your function pointer inside a struct and pass a pointer to that.
typedef (*callback)(void);
typedef struct threadargs
{
callback cb;
} threadargs;
void mycallback(void)
{
// ...
}
void *threadfunc(void *arg)
{
threadargs *ta = arg;
// call your callback:
ta->cb();
return ta; // or: return 0, or some pthread_exit(), ...
}
int main(void)
{
pthread_t thread;
threadargs ta = { mycallback };
pthread_create(&thread, 0, threadfunc, &ta);
// make sure "ta" lives for as long as the thread executes,
// here just wait until it exits:
pthread_join(&thread, 0);
}
add error checking etc.
(*) as for the concrete error you're getting, a function pointer needs parantheses around the identifier, so instead of
void *funcptr(void) = arg;
you'd have to write
void (*funcptr)(void) = arg;
To facilitate the usage of function pointers, it's common to typedef them, as seen in my example above. Anyways, as explained above, this wouldn't solve your problem here.
Suppose I have a function like this:
static int init_processing(char *buf, FILE *stream, enum operationMode mode) {
/* save index of `stream' in current operations */
/* start processing */
/* save some important variables for continue_processing */
off_t position;
enum operationMode _mode;
return num_processing_operations_left;
}
.. that I would be calling occasionally. And I have another function that does the actual processing I want:
static int continue_processing(FILE *stream) {
/* lookup the index of `stream' in current operations */
/* do some stuff */
/* save some static variables */
static off_t left = position;
static void *some_ptr;
return --num_processing_operations_left;
}
I also have a cleaning function to invoke when finishing up a certain operation:
static int end_processing(FILE *stream) {
/* check */
if (num_processing_operations_left)
return 1;
/* clean everything */
return 0;
}
As you can see, this related functions technique is very familiar that it is used by the standard library itself (e.g [malloc, free, realloc], [fdopen, fopen, fclose]).
What I want to achieve here, is how to share some variables across a bunch of functions ?
I thought of two solutions:
Put each set of functions in a file of their own, providing static variables valid only for the file itself.
Use only one function that takes an extra enum parameter as a mode and structure the function accordingly.
But these solutions aren't actually solutions, they're just workarounds to cope with the problem. So, is there any standard technique to share variables among functions ?
Create a context structure keeping all you need:
struct Context
{
FILE * stream;
off_t position;
unsigned num_processing_operations_left;
/* define some more important variables */
}
and pass it to all functions in question:
static int init_processing(struct Context * pctx, char *buf, enum operationMode mode);
static int continue_processing(struct Context * pctx);
static int end_processing(struct Context * pctx);
int main(void)
{
struct Context ctx = {0};
/* init ctx here */
int result = init_processing(&ctx, ...);
...
result = continue_processing(&ctx);
...
result = end_processing(&ctx);
...
}
I am passing queues like these between source files a.c and b.c
File : a.c
sq[a]=new_queue();
pthread_create(&st[a],NULL,sendPacket,sq[a]);
File : b.c
void *sendPacket(void *queue){
/* here i need to know which queue has come ,determine
the index of queue how can I do it? */
}
Create a more high-level representation of your queue. It seems the queue can be a void * (you're not showing its actual type, i.e. what does the new_queue() call return?), so embed that in a structure while adding the additional parameters:
struct queue_state {
void *queue;
int index;
};
Then instantiate a structure, and pass a pointer to it to the thread function:
struct queue_state qsa = malloc(sizeof *qsa);
if(qsa != NULL)
{
qsa->queue = new_queue();
qsa->index = 4711; /* or whatever */
pthread_create(&st[a], NULL, sendPacket, qsa);
}
Then the thread function can use the struct declaration to access all the fields. Of course, the declaration needs to be in a shared header (say queue.h) which is included from both C files.
Your question description is very rough. But at least from what I understand, you actually need to pass 2 parameters to your function: the (pointer to) queue (which seems an array for me), and the index within this queue.
You may not pack both your parameters in a single variable of type void*. What you may do is declare a struct with all the needed parameters, fill it, and pass a pointer to it to your thread.
Like this (error handling omitted):
struct Params
{
queue* m_Queue;
size_t m_Idx;
};
// ...
Params* pParams = new Params;
pParams->m_Queue = sq;
pParams->m_Idx = a;
pthread_create(&st[a],NULL,sendPacket, pParams);
void *sendPacket(void *pPtr)
{
Params* pParams = (Params*) pPtr;
// ...
delete pParams;
}
Probably it is easier if you just pass the index to the function:
void *sendPacket(int queue_idx) {
queue_t *queue = &sq[queue_idx];
}
If in b.c you have access to sq, you can just pass the index to the queue. Otherwise you can pass a struct containing the actual queue and the index