CODEWITHSUNDEEP
c
Zaphry
Zaphry

Reputation: 11

strcpy in a specific memory address returns value plus garbage

I'm required to save data, which happens to be a char array of 4096 characters tops, in a specified memory address.

So here i get the memory address from a uint32_t into the void* ptrAddress 

uint32_t number = 1268;
void* ptrAddress = &number;

and here i try to copy the array of As, which works! but not really, because i get some garbage in the middle

char tryArray[4096];

for(int i = 0; i < 4095; i++ ){

    tryArray[i] = 'A';
}
//EDIT: added the null terminator (forgot to do that)
tryArray[4095] = '\0';

char* copy = strcpy(ptrAddress, (char*)tryArray);

printf("lets see: %s\n", copy);

output (imagine 4096 As and 'garbage' as some garbage character):

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA'garbage''garbage''garbage'AAAAAAAAAAAAAAAAA

And after that i get a seg fault

What am i doing wrong? If there's anything more you need to know from the code or my intentions with it, please tell me!

Upvotes: 0

Views: 242

Answers (1)

Carl Norum
Carl Norum

Reputation: 225032

You have a couple of problems.

First, this code:

uint32_t number = 1268;
void* ptrAddress = &number;

Doesn't make a pointer to memory address 1268, like you seem to indicate you want it to. It makes a pointer to the integer where that 1268 is stored. You then overrun that storage by a lot, causing undefined behaviour, so after that it's game over.

If you want a pointer to a specific memory address:

void *ptrAddress = (void *)0x1268;

Make sure that address is legit in your environment/address space, though!

Second, strcpy works on null-terminated strings. You should probably use memcpy if you plan to work with a 4096 byte (non-null terminated) buffer. Note that means you can't print using printf (at least the way you're trying).

printf("%-4096s");

Should do it though.

Upvotes: 4

Related Questions