What's the correct way to do pointer arithmetic in this example ?
Say I store a struct in a buffer at some offset and then want to get a pointer to it. e.g.
Is the pointerCode:int offset; sometype mystruct, *ptr; char buff[32768]; memcpy(buff[offset], mystruct, sizeof(mystruct));
ptr = (mystruct *)buffer[offset];
or
ptr = (mystruct *)buffer + offset;
or
ptr = (mystruct *)((&buffer[0]) + offset);
or something else ? I'm getting confused - and coredumps ...
thks
orCode:int offset; sometype mystruct, *ptr; char buff[32768]; memcpy(&buff[offset], &mystruct, sizeof(sometype ));
Code:int offset; sometype mystruct, *ptr; char buff[32768]; memcpy(bufd + offset, &mystruct, sizeof(sometype ));
Sorry - didn't make myself clear enough.
Assume you can't change the code I posted, but I need a pointer to the place in the buffer where the stored struct starts.
Is this legal ??
ptr = (sometype *)buffer[offset];
Should I then be able to write something like
printf("%d\n", ptr->some_int_member);
??
TIA
>Assume you can't change the code I postedBut it's got bugs.Code:memcpy(buff[offset], mystruct, sizeof(mystruct));You see, buff[offset] is a char, not a pointer; and mystruct is a struct, not a pointer. Since memcpy takes pointers for its first two arguments, if we leave it the way you posted, we'd likely to be writing to wherever from la-la land.Code:memcpy(&buff[offset], &mystruct, sizeof(mystruct));Code:#include <stdio.h> #include <string.h> struct mytype { char name[20]; int i; double d; }; int main(void) { char buffer[128]; struct mytype myvar = { "name", -1, 3.14 }, *ptr; int offset = 24; ptr = (struct mytype*)&buffer[offset]; memcpy(ptr, &myvar, sizeof(myvar)); printf("ptr->name = \"%s\"\n", ptr->name); printf("ptr->i = %d\n", ptr->i); printf("ptr->d = %lf\n", ptr->d); return 0; } /* my output ptr->name = "name" ptr->i = -1 ptr->d = 3.140000 */
7. It is easier to write an incorrect program than understand a correct one.
40. There are two ways to write error-free programs; only the third one works.*
Thanks to all for the suggestions and apologies for posting c**p code. I didn't want to post hundreds of lines of real code and came up with a buggy example :-(
The alignment problem looks to be the most probable cause of my core dumps, so I'll have to find some other way of doing this.
D
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