rs232 string comparison in C

Question

I am trying to make a programm which can read commands from a RS232 port and use them for the next action.

I am using a string compare command to compare the desired 'action' string with the RS232 string. Something goes wrong with the string conversions somewhere. I used a putstr commando to see what my microcontroller is getting from my computer but it doesn't work propperly. It returns the last two chars of my string with a dot or a 'd' in the middle. (I have absolutely no clue where the dot/d come from..)

So this is my main code:

int length;
char *str[20];
while(1)
{
    delayms(1000);
    length = 5; //maximum length per string
    getstr(*str, length); //get string from the RS232
    putstr(*str); //return the string to the computer by RS232 for debugging
    if (strncmp (*str,"prox",strlen("prox")) == 0) //check wether four letters in the string are the same as the word "prox"
    {
        LCD_clearscreen(0xF00F);
        printf ("prox detected");
    }
    else if (strncmp (*str,"AA",strlen("AA")) == 0) //check wether two letters in the string are the same as the chars "AA"
    {
        LCD_clearscreen(0x0F0F);
        printf ("AA detected");
    }
}

These are the used RS232 functions:

/*
 * p u t s t r
 *
 *  Send a string towards the RS232 port
 */
void putstr(char *s)
{
    while(*s != '\0')
    {
            putch(*s);
            s++;
    }
}

/*
 * p u t c h
 *
 *  Send a character towards the RS232 port
 */
void putch(char c)
{
    while(U1STAbits.UTXBF);     // Wait for space in the transmit buffer
    U1TXREG=c;
    if (debug) LCD_putc(c);
}

/*
 * g e t c
 *
 *  Receive a character of the RS232 port
 */
char getch(void)
{
    while(!has_c());    // Wait till data is available in the receive buffer
    return(U1RXREG);
}

/*
 * g e t s t r
 *
 * Receive a line with a maximum amount of characters
 * the line is closed with '\0'
 * the amount of received characters is returned
 */
 int getstr(char *buf, int size)
 {
    int i;

    for (i = 0 ; i < size-1 ; i++)
    {
        if ((buf[i++] = getch()) == '\n') break;
    }
    buf[i] = '\0';

    return(i);
}

When I use this programm with my Microchip hooked up to a terminal I get something like this:

What I send:
abcdefgh

What I get back (in sets of 3 characters):
adbc.de.fg.h

score 3 · Answer 1 · answered Jun 07 '12 at 13:07

3

The problem is how you declare your string. As it is now, you declare an array of 20 char pointers. I think you should probably declare it as a normal char array:

char str[20];

When you then pass the array to the functions, just use e.g. getstr(str, length);.

answered Jun 07 '12 at 13:07

Some programmer dude

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Thanks for your super fast response! I have just changed my code and now I have the same problem as I used to have (before I got the problem I posted over here). When I use my terminal now I can write something and it only returns the first letter in sets of two. So when I write this: abcdef it returns this: ace – user1442205 Jun 07 '12 at 13:26
Problem solved!! My getstring function had i++ 2 times! int getstr(char *buf, int size) { int i; for (i = 0 ; i < size-1 ; i++) { if ((buf[i] = getch()) == '\n') break; } buf[i] = '\0'; return(i); } – user1442205 Jun 07 '12 at 14:49

score 2 · Answer 2 · answered Jun 07 '12 at 13:22

As far as i know the strcmp function works when you pass the pointer to the string,not the string itself.

When you Use

char *str[20];

You are declaring an array of pointers named "str",not an array of char.

Your problem is that you are passing an array of pointers to the strcmp function. You can solve it by declaring your string as:

 char string[20];

If for some strange reason you need to use char *,The following declaration is equivalent:

   char * str = malloc(20*sizeof(int))

Hope That helps.

rs232 string comparison in C

2 Answers2