Creating a string from a character array

RE: Creating a string from a character array

Milan — Mon, 19 Jan 2015 09:16:02 GMT

Honestly I tried to assign a NULL values into the buffer and it did not work. Now, I have done it properly using memset and it seems to be working fine.

memset(p_data, 0, length);

Thank you.

RE: Creating a string from a character array

John — Fri, 16 Jan 2015 14:51:43 GMT

Are you making sure to clear the buffer pointed to by p_data after each comparison? If you are reusing the same buffer and not clearing it, that would explain your behavior as the "rate" string is already in the buffer.

RE: Creating a string from a character array

Milan — Fri, 16 Jan 2015 10:48:25 GMT

Thank you all for contribution I think the strcmp() is clear now however my problem still persists. I will try to explain so hopefully somebody will be able to help.

I am using the nRF51-dk and in this case original unmodified experimental_ble_app_uart.

The only change that I have made to the code is the following to trigger LED when string matches:

void nus_data_handler(ble_nus_t * p_nus, uint8_t * p_data, uint16_t length)
{
  for (int i = 0; i < length; i++)
  {
      simple_uart_put(p_data[i]);
  }

simple_uart_put('\n');

if (strcmp("rate", (const char*)p_data) == 0){
  LEDS_INVERT(BSP_LED_1_MASK);
  }
}

What I am getting is rather surprising.

1.When I type in "rat" the LED is not inverted

2.Then typing in "rate" LED is inverted

3.Then typing "rat" LED is surprisingly inverted

4.At this point LED is inverted every time "r" "ra" "rat" "rate" is sent

5.1. After typing any 4 letters etc. "abcd" LED is not inverted but after that only "rate" can invert LED

5.2 After typing any 5 letters nothing can invert LED not even "rate"

Anyone could give an advise how to make this robust?

Regards

Milan

RE: Creating a string from a character array

Matt Barr — Fri, 16 Jan 2015 00:37:32 GMT

Yes, I wish I had a krone for every time I've done that, although in net worth I'd still be well behind Nordic Semiconductor's CEO! :-)

RE: Creating a string from a character array

John — Thu, 15 Jan 2015 23:25:16 GMT

Good point on the *s1 == *s2 in the for loop. I was a bit glib when I looked at the code.

RE: Creating a string from a character array

Matt Barr — Thu, 15 Jan 2015 22:14:34 GMT

Hi Milan, perhaps your C platform is buggy, or maybe there's something else going on in your test case, but strcmp should not tell you a string matches an initial sub-string.

Given a compliant strcmp implementation (as in John's example, or the standard C library), the code snippet:

char buf[BUFSZ];

strcpy (buf, "rat");
(void) printf ("%s %s rate\n", buf, (strcmp (buf, "rate")) ? "!=" : "==");
strcpy (buf, "rate");
(void) printf ("%s %s rate\n", buf, (strcmp (buf, "rate")) ? "!=" : "==");
strcpy (buf, "rated");
(void) printf ("%s %s rate\n", buf, (strcmp (buf, "rate")) ? "!=" : "==");

should produce:

rat != rate
rate == rate
rated != rate

The return 0 in the strcmp example John posted is hit only when *s1 == *s2 and *s1 == 0, so *s2 == 0 as well. The for loop exits on the first difference and returns non-zero, so regardless of operand order that code will return 0 only when the two strings are identical.

RE: Creating a string from a character array

Nguyen Hoan Hoang — Thu, 15 Jan 2015 18:00:31 GMT

ah! you're right. I've forgotten about it.

RE: Creating a string from a character array

John — Thu, 15 Jan 2015 17:56:48 GMT

Milan may have a point. An implementation of strcmp attributed to PJ Plauger is:

int strcmp (const char * s1, const char * s2)
{
    for(; *s1 == *s2; ++s1, ++s2)
        if(*s1 == 0)
            return 0;
    return *(unsigned char *)s1 < *(unsigned char *)s2 ? -1 : 1;
}

So if the null terminator of s1 is encountered before a difference is detected with s2, the strcmp will return 0. So, flipping the order of the arguments to strcmp might be one solution.

The web page I found this on is here: strcmp

RE: Creating a string from a character array

Nguyen Hoan Hoang — Thu, 15 Jan 2015 16:47:55 GMT

the "do something" is executed only if the first 4 char of your p_data is equal to "rate"

RE: Creating a string from a character array

Milan — Thu, 15 Jan 2015 16:39:10 GMT

Let say I do this:

if (strcmp((char*)p_data, "rate") == 0)
{do something}

The function is executed in case I type all of these: r, ra, rat, rate

How can I avoid it? I wrote a comparison fuction that works however I expected strcmp to work as well.

Thanks for your entry.

Milan

RE: Creating a string from a character array

Nguyen Hoan Hoang — Thu, 15 Jan 2015 16:27:10 GMT

type string is a C++ class. It does not exists in C. A char array is a string already.

strcmp(arr, "what ever") == 0 ... that is how it's done. strcmp is a C function.