Allocate memory inside function ??

[fx9200]

Hello I want to allocate memory for a void* pointer inside a function; i tried to do that by using malloc to the pointer passed as an argument ; the memory space was not kept after function return; how kan i do that thanks

[toxcct]

please show a brief piece of the code you wrote for that...

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[Cedric Moonen]

fx9200 wrote:

the memory space was not kept after function return;

You need to pass your pointer by reference. Otherwise it will still point at the previous memory location.

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Cédric Moonen Software developer
Charting control [v1.1]

[fx9200]

the code is too long but : #define STRUCT struct mystruct void function (void *input){ .... pointer = (STRUCT*) malloc (sizeof(STRUCT)); //filling in input attributes... input = pointer; } main(){ void *ptr; function (ptr); printf("%...", ptr->..........); } this work only if i allocate the memory for ptr in the main program

[toxcct]

fx9200 wrote:

pointer = (STRUCT*) malloc (sizeof(STRUCT));

why don't you directly write into input ?

input = (STRUCT*) malloc (sizeof(STRUCT));

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[fx9200]

i tried it i had the same result in all cases

[prasad_som]

Change this to,

void function (void **input){
....
*pointer = (STRUCT*) malloc (sizeof(STRUCT));

//filling in input attributes...

input = pointer;
}

main(){
void *ptr;
function (&ptr);
printf("%...", ptr->..........);
}

Prasad Notifier using ATL | Operator new[],delete[][^]

[fx9200]

how to pass reference(pointer) by reference ? please give example

[toxcct]

he seems to doing C, not C++... so no reference available (in the sense of C++ understands it). but pointers remain good

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[fx9200]

:rolleyes: yes so ?

[toxcct]

fx9200 wrote:

yes so ?

so, prasad_som[^] gave you an answer... hve you read it ?

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[fx9200]

I tried this it works , but i cannot fix the allocated emory size by a no-static way void fill(void *input); struct mystruct { char *ch; int n; }; #define ARG_SIZE sizeof(struct mystruct) int main(){ void *ptr; ptr = (void*) malloc (ARG_SIZE); fill(ptr); printf ("%i",((struct mystruct*)ptr)->n); printf ("--%s",((struct mystruct*)ptr) ->ch); return 0; } void fill(void *input){ struct mystruct* st; st = (struct mystruct*) input; st ->n = 3;//operation is ok st->ch = (char *) malloc (3*sizeof(char)); strcpy(st->ch,"OK"); input = (void*) st; }

[prasad_som]

Though, its not applicable in C context. You need to understand theory behind Cedric's suggesstion. In you original code, you was passing pointer argument. Though its a pointer, it will be passed by value, and inside that function, memory allocated will be not at address you expected. In that case you need to pass pointer to pointer, as shown in my previous reply.

Prasad Notifier using ATL | Operator new[],delete[][^]

[fx9200]

i'm trying thanks

[fx9200]

but how the code below works; i allocated memory for chr* inside function

[prasad_som]

Which code you are talking about ?

Prasad Notifier using ATL | Operator new[],delete[][^]

[fx9200]

void fill(void *input); struct mystruct { char *ch; int n; }; #define ARG_SIZE sizeof(struct mystruct) int main(){ void *ptr; ptr = (void*) malloc (ARG_SIZE); fill(ptr); printf ("%i",((struct mystruct*)ptr)->n); printf ("--%s",((struct mystruct*)ptr) ->ch); return 0; } void fill(void *input){ struct mystruct* st; st = (struct mystruct*) input; st ->n = 3;//operation is ok st->ch = (char *) malloc (3*sizeof(char)); strcpy(st->ch,"OK"); input = (void*) st;

[fx9200]

void fill(void *input); struct mystruct { char *ch; int n; }; #define ARG_SIZE sizeof(struct mystruct) int main(){ void *ptr; ptr = (void*) malloc (ARG_SIZE); fill(ptr); printf ("%i",((struct mystruct*)ptr)->n); printf ("--%s",((struct mystruct*)ptr) ->ch); return 0; } void fill(void *input){ struct mystruct* st; st = (struct mystruct*) input; st ->n = 3;//operation is ok st->ch = (char *) malloc (3*sizeof(char)); strcpy(st->ch,"OK"); input = (void*) st; }

[prasad_som]

Here, though you are allocating memory for ch member of mystruct inside the function, you have passed mystruct* as parameter and , for that memory is already allocated, before calling this function. And bitwise(shallow) copy happens in this case. p.s. Dont's create multiple threads for same code.

Prasad Notifier using ATL | Operator new[],delete[][^]

[fx9200]

ok thanks