I'm running into an issue where my "TEST INSIDE BUILD" works but not "TEST OUTSIDE." A fragment of the code is here
command_t build_op_command(unsigned code, command_t comone, command_t comtwo)
{
commad_t s;
s=malloc(sizeof(*s));
switch(code)
{
case 5:
s->type=SEQUENCE_COMMAND;
...
}
s->status=-1;
s->input=NULL;
s->output=NULL;
s->u.command[0]=comone;
s->u.command[1]=comtwo;
printf("TEST INSIDE BUILD: %d and %s",s->u.command[0]->type, s->u.command[0]->u.word[0]);
s->u.word=NULL;
s->u.subshell_command=NULL; //not yet implemented
return s;
}
and
...
command_t op_command;
op_command=build_op_command(op_pop(op_s),comone,comtwo);
printf("TEST OUTSIDE: %d and %s",op_command->u.command[0]->type,op_command->u.command[0]->u.word[0]);
...
command_t is a pointer for struct command. I'm not quite sure why it would correctly inside the build function, but not work correctly outside of it. Any input would be greatly appreciated. I run into a segmentation fault, I've tried allocating space for s->u.word, but that didn't seem to help anything.
struct command
{
enum command_type type;
int status;
char *input;
char *output;
union
{
struct command *command[2];
char **word;
struct command *subshell_command;
} u;
};
typeder struct command *command_t;
You do not give enough information, post the definition of command_t.
u is probably a union:
s->u.command[0]=comone;
s->u.command[1]=comtwo;
printf("TEST INSIDE BUILD: %d and %s",s->u.command[0]->type, s->u.command[0]->u.word[0]);
After the first printf, you initialize other members of this union and override the command:
s->u.word=NULL;
s->u.subshell_command=NULL; //not yet implemented
The next printf report different contents.
All members of a union share the same location in memory, you cannot use more than one member at a time.
Related
I'm try to make simple kernel module with multithreading.
So I'm using linux/kthread.h, kernel v. 5.2.11
Problem: I can't passing the char array into thread: Segmentation fault.
That's what I'm doing:
typedef struct {
int num;
char origin[MAXSTR]; //part of input for current thread
struct completion wait_for_thread; //completion struct
} kthread_arg;
Then:
struct task_struct *task;
static kthread_arg kta_first_thread;
kta_first_thread.num = 1;
strncpy(kta_first_thread.origin, dat1, MAXSTR );
//Here I have normal char array 'origin'
init_completion(&kta_first_thread.wait_for_thread);
task = kthread_run(&thread_function, (void*)&kta_first_thread, "one");
And after that I have the error. Moreover, if you remove the array from struct, then everything works.
I'm sure doing something wrong?
The args passed to kernel_run must be kmalloced, your args is in stack. I have met the same problem, your code should like this:
struct your_struct* test=NULL;
struct task_struct* t=NULL;
test=(struct your_struct*)kmalloc(sizeof(struct your_struct),GFP_KERNEL);
t=kthread_run(your_function,(void*)test,name);
So, I'm working on Binary Trees and I need to move information from one node to another one (structs). The thing is that I have this function that uses strcpy to copy the names and surnames from one node to other node (both sent as pointers) and when I try to run that part of the code in the program, it crashes.
Here is the function that copies the info (It has to copy the info from nodo2 to nodo1):
void MoverDatos(Tarbol *nodo1, Tarbol *nodo2)
{
strcpy((*nodo1)->appat, (*nodo2)->appat);
strcpy((*nodo1)->apmat, (*nodo2)->apmat);
strcpy((*nodo1)->nombre, (*nodo2)->nombre);
(*nodo1)->matr=(*nodo2)->matr;
}
As it wasn't working that way, I tried to use this one instead:
void MoverDatos(Tarbol *nodo1, Tarbol *nodo2)
{
char cad1[20];
char cad2[20];
char cad3[20];
strcpy(cad1, (*nodo2)->appat);
strcpy(cad2, (*nodo2)->apmat);
strcpy(cad3, (*nodo2)->nombre);
strcpy((*nodo1)->appat, cad1);
strcpy((*nodo1)->apmat, cad2);
strcpy((*nodo1)->nombre, cad3);
(*nodo1)->matr=(*nodo2)->matr;
}
But it didn't work either. I don't know if I'm not setting the arguments the right way or if I need to use another function, so any help or sugestion would be great. Thanks in advance.
It does compile with those arguments. I'm not getting any error message, it just crash and says that "program.exe has stopped working".
This is the struct that I'm using:
typedef struct _tdato
{
long matr;
char nombre[20];
char appat[20];
char apmat[20];
struct _tdato *sig;
struct _tdato *ant;
struct _tdato *padre=NULL;
}Tdato;
typedef Tdato *Tarbol;
The crash comes when I trie to copy a char[ ] to nodo1.
I've got an issue with an assignment, but I'm not asking for help to do the assignment, just single problem.
My code is like this:
#include "linux/kernel.h"
#include "linux/unistd.h"
#include <linux/slab.h>
typedef _msg_t msg_t;
struct msg_t { /* members here */ };
static msg_t *bottom = NULL;
static msg_t *top = NULL;
int function_one (argA, argB) {
/* function is working, no need to show code*/
}
int function_two (argA, argB) {
/* function is working, so no need I guess to show the code*/
}
int main(int argc, char ** argv) {
char *in = "This is a testing message";
char msg[50];
int mlen;
function_one(in, strlen(in)+1);
mlen = function_two(msg, 50);
}
Here's the problem: When I do the make command from the directory, I get the error
/home/<username hidden by me>/dm510/linux-3.18.2/arch/um/os-linux/main.c:118:
multipli definition of 'main'
arch/um/kernel/built-in.o:
/home/<username hidden again>/dm510/linux-3.18.2/arch/um/kernel/file_i_created.c:60
first defined here"
What does this error mean? I only defined the main method one time in my own file
The message says you have (at least) two C files, main.c and file_i_created.c that are included in the build. Both have main() functions. (In C, the term is "function", not "method".) Remove one of those source files, or remove/rename the main() function in one of them.
You have multiple approaches here:
Usually there is only one main in a program. If so, decide, which is the actual main and rename the other one
If both mains are essential, you could try putting them in seperate namespaces
Really can't tell without seeing the file_i_created.c code though. Could be something else as well.
Im trying to assign a function inside a struct, so far I have this code:
typedef struct client_t client_t, *pno;
struct client_t
{
pid_t pid;
char password[TAM_MAX]; // -> 50 chars
pno next;
pno AddClient()
{
/* code */
}
};
int main()
{
client_t client;
// code ..
client.AddClient();
}
**Error**: *client.h:24:2: error: expected ‘:’, ‘,’, ‘;’, ‘}’ or ‘__attribute__’ before ‘{’ token.*
Which is the correct way to do it ?
It can't be done directly, but you can emulate the same thing using function pointers and explicitly passing the "this" parameter:
typedef struct client_t client_t, *pno;
struct client_t
{
pid_t pid;
char password[TAM_MAX]; // -> 50 chars
pno next;
pno (*AddClient)(client_t *);
};
pno client_t_AddClient(client_t *self) { /* code */ }
int main()
{
client_t client;
client.AddClient = client_t_AddClient; // probably really done in some init fn
//code ..
client.AddClient(&client);
}
It turns out that doing this, however, doesn't really buy you an awful lot. As such, you won't see many C APIs implemented in this style, since you may as well just call your external function and pass the instance.
As others have noted, embedding function pointers directly inside your structure is usually reserved for special purposes, like a callback function.
What you probably want is something more like a virtual method table.
typedef struct client_ops_t client_ops_t;
typedef struct client_t client_t, *pno;
struct client_t {
/* ... */
client_ops_t *ops;
};
struct client_ops_t {
pno (*AddClient)(client_t *);
pno (*RemoveClient)(client_t *);
};
pno AddClient (client_t *client) { return client->ops->AddClient(client); }
pno RemoveClient (client_t *client) { return client->ops->RemoveClient(client); }
Now, adding more operations does not change the size of the client_t structure. Now, this kind of flexibility is only useful if you need to define many kinds of clients, or want to allow users of your client_t interface to be able to augment how the operations behave.
This kind of structure does appear in real code. The OpenSSL BIO layer looks similar to this, and also UNIX device driver interfaces have a layer like this.
How about this?
#include <stdio.h>
typedef struct hello {
int (*someFunction)();
} hello;
int foo() {
return 0;
}
hello Hello() {
struct hello aHello;
aHello.someFunction = &foo;
return aHello;
}
int main()
{
struct hello aHello = Hello();
printf("Print hello: %d\n", aHello.someFunction());
return 0;
}
This will only work in C++. Functions in structs are not a feature of C.
Same goes for your client.AddClient(); call ... this is a call for a member function, which is object oriented programming, i.e. C++.
Convert your source to a .cpp file and make sure you are compiling accordingly.
If you need to stick to C, the code below is (sort of) the equivalent:
typedef struct client_t client_t, *pno;
struct client_t
{
pid_t pid;
char password[TAM_MAX]; // -> 50 chars
pno next;
};
pno AddClient(pno *pclient)
{
/* code */
}
int main()
{
client_t client;
//code ..
AddClient(client);
}
You are trying to group code according to struct.
C grouping is by file.
You put all the functions and internal variables in a header or
a header and a object ".o" file compiled from a c source file.
It is not necessary to reinvent object-orientation from scratch
for a C program, which is not an object oriented language.
I have seen this before.
It is a strange thing. Coders, some of them, have an aversion to passing an object they want to change into a function to change it, even though that is the standard way to do so.
I blame C++, because it hid the fact that the class object is always the first parameter in a member function, but it is hidden. So it looks like it is not passing the object into the function, even though it is.
Client.addClient(Client& c); // addClient first parameter is actually
// "this", a pointer to the Client object.
C is flexible and can take passing things by reference.
A C function often returns only a status byte or int and that is often ignored.
In your case a proper form might be
/* add client to struct, return 0 on success */
err = addClient( container_t cnt, client_t c);
if ( err != 0 )
{
fprintf(stderr, "could not add client (%d) \n", err );
}
addClient would be in Client.h or Client.c
You can pass the struct pointer to function as function argument.
It called pass by reference.
If you modify something inside that pointer, the others will be updated to.
Try like this:
typedef struct client_t client_t, *pno;
struct client_t
{
pid_t pid;
char password[TAM_MAX]; // -> 50 chars
pno next;
};
pno AddClient(client_t *client)
{
/* this will change the original client value */
client.password = "secret";
}
int main()
{
client_t client;
//code ..
AddClient(&client);
}
I am working on an embedded application where the device is controlled through a command interface. I mocked the command dispatcher in VC and had it working to my satisfaction; but when I then moved the code over to the embedded environment, I found out that the compiler has a broken implementation of pointer-to-func's.
Here's how I originally implemented the code (in VC):
/* Relevant parts of header file */
typedef struct command {
const char *code;
void *set_dispatcher;
void *get_dispatcher;
const char *_description;
} command_t;
#define COMMAND_ENTRY(label,dispatcher,description) {(const char*)label, &set_##dispatcher, &get_##dispatcher, (const char*)description}
/* Dispatcher data structure in the C file */
const command_t commands[] = {
COMMAND_ENTRY("DH", Dhcp, "DHCP (0=off, 1=on)"),
COMMAND_ENTRY("IP", Ip, "IP Address (192.168.1.205)"),
COMMAND_ENTRY("SM", Subnet, "Subunet Mask (255.255.255.0)"),
COMMAND_ENTRY("DR", DefaultRoute, "Default router (192.168.1.1)"),
COMMAND_ENTRY("UN", Username, "Web username"),
COMMAND_ENTRY("PW", Password, "Web password"),
...
}
/* After matching the received command string to the command "label", the command is dispatched */
if (pc->isGetter)
return ((get_fn_t)(commands[i].get_dispatcher))(pc);
else
return ((set_fn_t)(commands[i].set_dispatcher))(pc);
}
Without the use of function pointers, it seems like my only hope is to use switch()/case statements to call functions. But I'd like to avoid having to manually maintain a large switch() statement.
What I was thinking of doing is moving all the COMMAND_ENTRY lines into a separate include file. Then wraps that include file with varying #define and #undefines. Something like:
/* Create enum's labels */
#define COMMAND_ENTRY(label,dispatcher,description) SET_##dispatcher, GET_##dispatcher
typedef enum command_labels = {
#include "entries.cinc"
DUMMY_ENUM_ENTRY} command_labels_t;
#undefine COMMAND_ENTRY
/* Create command mapping table */
#define COMMAND_ENTRY(label,dispatcher,description) {(const char*)label, SET_##dispatcher, GET_##dispatcher, (const char*)description}
const command_t commands[] = {
#include "entries.cinc"
NULL /* dummy */ };
#undefine COMMAND_ENTRY
/*...*/
int command_dispatcher(command_labels_t dispatcher_id) {
/* Create dispatcher switch statement */
#define COMMAND_ENTRY(label,dispatcher,description) case SET_##dispatcher: return set_##dispatcher(pc); case GET_##dispatcher: return get_##dispatcher(pc);
switch(dispatcher_id) {
#include "entries.cinc"
default:
return NOT_FOUND;
}
#undefine COMMAND_ENTRY
}
Does anyone see a better way to handle this situation? Sadly, 'get another compiler' is not a viable option. :(
--- Edit to add:
Just to clarify, the particular embedded environment is broken in that the compiler is supposed to create a "function-pointer table" which is then used by the compiler to resolve calls to functions through a pointer. Unfortunately, the compiler is broken and doesn't generate a correct function-table.
So I don't have an easy way to extract the func address to invoke it.
--- Edit #2:
Ah, yes, the use of void *(set|get)_dispatcher was my attempt to see if the problem was with the typedefine of the func pointers. Originally, I had
typedef int (*set_fn_t)(cmdContext_t *pCmdCtx);
typedef int (*get_fn_t)(cmdContext_t *pCmdCtx);
typedef struct command {
const char *code;
set_fn_t set_dispatcher;
get_fn_t get_dispatcher;
const char *_description;
} command_t;
You should try changing your struct command so the function pointers have the actual type:
typedef struct command {
const char *code;
set_fn_t set_dispatcher;
get_fn_t get_dispatcher;
const char *_description;
} command_t;
Unfortunately, function pointers are not guaranteed to be able to convert to/from void pointers (that applies only to pointers to objects).
What's the embedded environment?
Given the information posted in the updates to the question, I see that it's really a bugged compiler.
I think that your proposed solution seems pretty reasonable - it's probably similar to what I would have come up with.
A function pointer isn't actually required to fit in a void*. You could check to make sure that the value you're calling is actually the address of the function. If not, use a function pointer type in the struct: either get_fn_t, or IIRC void(*)(void) is guaranteed to be compatible with any function pointer type.
Edit: OK, assuming that calling by value can't be made to work, I can't think of a neater way to do what you need than auto-generating the switch statement. You could maybe use an off-the-shelf ASP-style preprocessor mode for ruby/python/perl/php/whatever prior to the C preprocessor. Something like this:
switch(dispatcher_id) {
<% for c in commands %>
case SET_<% c.dispatcher %>: return set_<% c.dispatcher %>(pc);
case GET_<% c.dispatcher %>: return get_<% c.dispatcher %>(pc);
<% end %>
default:
return NOT_FOUND;
}
might be a bit more readable than the macro/include trick, but introducing a new tool and setting up the makefiles is probably not worth it for such a small amount of code. And the line numbers in the debug info won't relate to the file you think of as the source file unless you do extra work in your preprocessor to specify them.
Can you get the vendor to fix the compiler?
To what extent is the pointer-to-function broken?
If the compiler allows you to get the address of a function (I'm from C++, but &getenv is what I mean), you could wrap the calling convention stuff into assembler.
As said, I'm a C++ssie, but something in the way of
; function call
push [arg1]
push [arg2]
call [command+8] ; at the 4th location, the setter is stored
ret
If even that is broken, you could define an array of extern void* pointers which you define, again, in assembly.
try this syntax:
return (*((get_fn_t)commands[i].get_dispatcher))(pc);
It's been awhile since I've done C & function pointers, but I believe the original C syntax required the * when dereferencing function pointers but most compilers would let you get away without it.
Do you have access to the link map?
If so, maybe you can hack your way around the wonky function-pointer table:
unsigned long addr_get_dhcp = 0x1111111;
unsigned long addr_set_dhcp = 0x2222222; //make these unique numbers.
/* Relevant parts of header file */
typedef struct command {
const char *code;
unsigned long set_dispatcher;
unsigned long get_dispatcher;
const char *_description;
} command_t;
#define COMMAND_ENTRY(label,dispatcher,description) {(const char*)label,
addr_set_##dispatcher, addr_get_##dispatcher, (const char*)description}
Now compile, grab the relevant addresses from the link map, replace the constants, and recompile. Nothing should move, so the map ought to stay the same. (Making the original constants unique should prevent the compiler from collapsing identical values into one storage location. You may need a long long, depending on the architecture)
If the concept works, you could probably add a post-link step running a script to do the replacement automagically. Of course, this is just a theory, it may fail miserably.
Maybe, you need to look into the structure again:
typedef struct command {
const char *code;
void *set_dispatcher; //IMO, it does not look like a function pointer...
void *get_dispatcher; //more like a pointer to void
const char *_description;
} command_t;
Let say your dispatchers have the following similar function definition:
//a function pointer type definition
typedef int (*genericDispatcher)(int data);
Assume that the dispatchers are like below:
int set_DhcpDispatcher(int data) { return data; }
int get_DhcpDispatcher(int data) { return 2*data; }
So, the revised structure will be:
typedef struct command {
const char *code;
genericDispatcher set_dispatcher;
genericDispatcher get_dispatcher;
const char *_description;
} command_t;
Your macro will be:
#define COMMAND_ENTRY(label,dispatcher,description) \
{ (const char*)label, \
set_##dispatcher##Dispatcher, \
get_##dispatcher##Dispatcher, \
(const char*)description }
Then, you can set your array as usual:
int main(int argc, char **argv)
{
int value1 = 0, value2 = 0;
const command_t commands[] = {
COMMAND_ENTRY("DH", Dhcp, "DHCP (0=off, 1=on)")
};
value1 = commands[0].set_dispatcher(1);
value2 = commands[0].get_dispatcher(2);
printf("value1 = %d, value2 = %d", value1, value2);
return 0;
}
Correct me, if I am wrong somewhere... ;)