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problem with character pointer

Mohit Gupta

Hello,

1) I am declaring a pointer to a string as
Xdata Unsigned char *sTemperature [4] = {"NIL","25 ","30 ","37 "};

This is shown in the .lst file as a array occupying only 12 bytes where as it should take 16 bytes according to me (4 bytes for NULL characters also) Why is it so?

Also if I give the statement:
STemperature[2][2] = 'C';
'C' is not appended after 25 in the program. What is wrong?

2) If I declare a array as:
Unsigned char display[40];
Unsigned char sBlankString = " "; /* say a string of 40 blank characters */

And give a statement
Strncpy(&display[400],sBlankString,strlen(sBlankString));

Shouldn't the compiler give a warning message as we are copying at display[400] where as the
Display is defined only for 40 characters?


Regards,

Mohit

  • 0

    1) I am declaring a pointer to a string as
    Xdata Unsigned char *sTemperature [4] = {"NIL","25 ","30 ","37 "};

    This is shown in the .lst file as a array occupying only 12 bytes where as it should take 16 bytes according to me (4 bytes for NULL characters also) Why is it so?
    You see the size of an array of pointers to characters. The compiler places 4 pointers in the array, each with a size of 3 bytes (generic pointer).

    Also if I give the statement:
    STemperature[2][2] = 'C';
    'C' is not appended after 25 in the program. What is wrong?

    STemperature[2] selects the 3rd pointer in the list which points to the first char of "30". The 'C' is placed behind the '0'. This overwrites the terminating 0x0 of the string which may give you strange results. There is no room for appending characters. You have to copy the string elsewhere and then append characters. Example:

    char cString [16]; /* this allows 15 characters + 0x0 */
strcpy ( cString, STemperature[1] ); /* copy "25" */
strcat ( cString, "C" ); /* Add 'C' to "25" */

    2) If I declare a array as:
    Unsigned char display[40];
    Unsigned char sBlankString = " "; /* say a string of 40 blank characters */

    And give a statement
    Strncpy(&display[400],sBlankString,strlen(sBlankString));


    &display[400] is a pointer to an unsigned char which is passed as an argument to a function. I don't think the compiler has to check the contents of a parameter. Your variable can be preset with
     memset ( display, ' ', sizeof(display)); /* fill with spaces */
    This works always, independent of the size of the variable.

    HHK

  • 0

    Shouldn't the compiler give a warning message as we are copying at display[400] where as the Display is defined only for 40 characters?

    The 'C' language does not do array bounds checking!

    You need something like lint if you want to check you code for problems like this.

    Technically, [] is an operator in 'C'; it simply indexes relative to a pointer - any pointer - it is not constrained to arrays!

    Thus display[400] simply refers to the location 400 bytes after the location pointed-to by display; the compiler just assumes that you knew what you were doing when you asked it to do that