GENERAL: STRING TABLES IN C

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Information in this article applies to:

• C166 Version 3.12
• C166 Version 4.01
• C251 Version 2.14
• C51 Version 5.50

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QUESTION

I have an application that I have to port to various foreign languages. I would like to create a string table and then change the strings for each language that I will support. What's the best way to encapsulate my strings in a single file and access them from my C program?

ANSWER

There are a number of good ways to do this. The following suggestion is only one of them. However, it is relatively easy to implement and understand. It uses 3 concepts that you must introduce into your program.

1. Use an array of pointers to the message strings.
2. Use a list of manifest constants to index into the pointer array.
3. Use a single macro to access any string.

The first step is to create an assembly file (a very simple one) that contains the messages and a table of pointers to the messages. Since you will probably want to locate the table at a fixed address, this is best way to do that. For example:

;The following is an array of pointers to the messages

cseg      at    08000  ; *** Change this for your assembler ***

hstrtab:  dw    str1
          dw    str2
          dw    str3
          dw    str4

; Here are the messages
str1:     db    'Bon Jour', 0
str2:     db    'Adios', 0
str3:     db    'Merci', 0
str4:     db    'Please', 0

end

Next, create a manifest constant (using #define or enum) for each message in the table. You will use these to index into the array. For example:

enum message_strings
  {
  MSG_HELLO = 0,
  MSG_GOODBYE,
  MSG_THANKS,
  MSG_PLEASE
  };

Finally, you need a macro that lets you reference strtab in your C program:

#define MSGTAB ((char code * code *) 0x8000)

The only trick here is that the addresses (0x8000 in the C program and 8000h in the assembly file) must match.

Now, you are ready to access the strings in your C program.

void main (void)
{
SCON  = 0x50;   /* SCON: mode 1, 8-bit UART, enable rcvr */
TMOD |= 0x20;   /* TMOD: timer 1, mode 2, 8-bit reload */
TH1   = 221;    /* TH1:  reload value for 1200 baud @ 16MHz */
TR1   = 1;      /* TR1:  timer 1 run */
TI    = 1;      /* TI:   set TI to send first char of UART */

printf ("%s\n", MSGTAB [MSG_HELLO]);
printf ("%s\n", MSGTAB [MSG_GOODBYE]);
printf ("%s\n", MSGTAB [MSG_THANKS]);
printf ("%s\n", MSGTAB [MSG_PLEASE]);
}

Last Reviewed: Saturday, July 09, 2005

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