Updating string in C

Question

I have a function on a small micro that sends one character at a time out the UART port.

USART_0_write('x');

This causes a character x to go out the serial port.

I have created this function that works:

char source[] = "Test Text.";
for (uint8_t i=0; i < strlen(source); i++){
    USART_0_write(source[i]);
}

All I want to do is simply dynamically update what string I'm sending, without creating a different array for each thing I send.

I basically want to do what I assume strcpy does, but I can't get that function to work. I got a version using strcpy to compile once, but it must have been a memory leak because when I ran it, all the I/O ports on the chip went screwy.

strcpy(source,"Different String");

I want to do something like this then call my first function again and have it print the new updated string out the serial port.

I understand the concept of pointers, but everything I read the syntax explanation I can't understand. I have tried a ton of different combinations, putting starts before and after everything. No matter what it's a total fail.

I read over this great explanation, however, like most explanations out there, the thing stops JUST SHORT of actually providing the single line of code that actually makes it all work at the bottom when it talks about dynamically updating a string: https://www.cs.bu.edu/teaching/cpp/string/array-vs-ptr/

Thank you.

ADDITIONAL INFORMATION:

(I edited my post and added this, since you can't add a new post below without "answering" your own question, and the comment reply section doesn't allow for any formatting and limited to 500 characters, which is dumb).

I am trying to create a menu by doing the following:

my_string = "Menu item 1";
sendString(my_string); //function to iterate through characters sending them
my_string = "Menu item 2";
sendString(my_string); //function to iterate through characters sending them
my_string = "Menu item 3";
sendString(my_string); //function to iterate through characters sending them

Now there is probably a way to do this that requires a masters degree. I'm just trying to get this stupid menu working. I'm literally creating individual functions that create a new character array for each line, which is the way wrong way to do it, but it's the only thing I can figure out after fighting with this for hours.

Answer 1

Assume what you wrote is literally what you tried....

source[] is a 10-byte array.

"Different String" is 17 bytes long.

You overflowed source[] and clobbered who-knows-what.

Answer 2

Based on your last comment, it seems like you need to store the contents of menus in an array of strings. In C, this would be a char **, e.g. char **menu;.

If you have 3 items in your menu, then you can allocate the memory for the menu like so: char **menu = (char **)malloc(3 * sizeof(char *));

Then you need to fill in the items in the menu, e.g.:

menu[0] = (char *)malloc(64 * sizeof(char)); // 64 as an example
menu[1] = (char *)malloc(64 * sizeof(char));
...
strcpy(menu[0], "Item 1");
strcpy(menu[1], "Item 2");
...

and so on. Then you can iterate over the three items in your menu, and send each one to your function.

for(int i = 0; i < 3; i++) {
    sendString(menu[i]);
}

Answer 3

This is NOT an "answer" but it's the only way I can trick stackoverflow into letting me post code.

void makeMenu(void){
    char **menu = (char **)malloc(3 * sizeof(char *));

    menu[0] = (char *)malloc(64 * sizeof(char)); // 64 as an example
    menu[1] = (char *)malloc(64 * sizeof(char));
    menu[2] = (char *)malloc(64 * sizeof(char));

    strcpy(menu[0], "Welcome to the menu. Make selection:");
    strcpy(menu[1], "1: Turn on light");
    strcpy(menu[1], "2: Turn off light");

}

void sendMenu(**menu){
    for (uint8_t i=0; i < 3; i++){
        sendSingleLine(menu[i]);
    }
}

void sendSingleLine(*string_to_send){
    for (uint8_t i=0; i < strlen(string_to_send); i++){
        USART_0_write(string_to_send[i]);
    }
}

int main(void){
makeMenu();
    sendMenu();
}

^^^ tried this, won't compile.

Answer 4

Thanks for the help everyone but I'm out of time. Defeated on this one. I've been at this for a while - I've written low level drivers and radio systems, etc but never have to deal with text. This is the sort of thing they teach on first day Python coding. lol.

Here's how I worked around it. Ugly but it works. I just call uart_menu() and it prints. Done. If I can figure out how to send my arrays to the function without having to re-write it each time that would be helpful.

void uart_menu(void){
    char line1[] = "Test Menu:\n";
    for (uint8_t i=0; i < strlen(line1); i++){
        USART_0_write(line1[i]);
    }

    char line2[] = "1: Turn on 0xA2\n";
    for (uint8_t i=0; i < strlen(line2); i++){
        USART_0_write(line2[i]);
    }

    char line3[] = "2: Turn off 0xA2\n";
    for (uint8_t i=0; i < strlen(line3); i++){
        USART_0_write(line3[i]);
    }

    char line4[] = "3: Turn on All Addresses\n";
    for (uint8_t i=0; i < strlen(line4); i++){
        USART_0_write(line4[i]);
    }

    char line5[] = "4: Turn off All Addresses\n";
    for (uint8_t i=0; i < strlen(line5); i++){
        USART_0_write(line5[i]);
    }

}