Need help processing CHAR strings with printf

Question

I'm using softserial to communicate with a bluetooth modem and I am pushing strings to the serial by using the following code:

char bt_string = "test";
bluetooth_println(bt_string);

I need to be able to replace the string with

printf("  Error: cmd=%02hX, res=%02hX\n", CMD_SEND_CID, res);

I have tried the following code

char bt_string;
sprintf(bt_string, "  Error: cmd=%02hX, res=%02hX\n", CMD_SEND_CID, res);
bluetooth_println(bt_string);

But it fails to output anything. I'm obviously misunderstanding something. Thanks for any help.

Answer 1

You need to provide a buffer for your string.

char bt_string[256]; // <-- or any size that you are sure will be enough for what you will put in.

eventually, for safety you can use snpritf to avoid any buffer overflow:

#define MAX_BT_STRING 256
char bt_string[MAX_BT_STRING];
snprintf(bt_string, MAX_BT_STRING,"  Error: cmd=%02hX, res=%02hX\n", CMD_SEND_CID, res);
bluetooth_println(bt_string);

Answer 2

char *str and char str[] are distinctly different. Check this question for more details.

In your problem, you declared bt_string as const char *bt_string = "test", where bt_string is pointer which points to the first char in string "test". This string has a size of 5 bytes(don't forget the terminator \0);

In the next step:

sprintf(bt_string, "  Error: cmd=%02hX, res=%02hX\n", CMD_SEND_CID, res);

You dump more than 5 bytes to bt_string which only has 5 bytes available space. The parts beyond 5 bytes will overwrite the contents after bt_string, which may lead to some serious situation or nothing, it depends on what is followed.

To settle this problem, you have to allocate enough memory space:

1. allocate on stack as A.S.H answered. the content is determined after function finished.
2. allocate via malloc;
3. use static key word to force the string stored either in BSS section or DATA section.