HOWTO: Initilize Large Character Arrays

ID: Q38728

The information in this article applies to:

Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax
Microsoft C/C++ for MS-DOS, version 7.0
Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52
Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0

SUMMARY

A common problem in C programming is initializing a large character array. There are several ways of doing this, as well as several potential problems.

MORE INFORMATION

Method 1 -- String Literals

One method of initializing character arrays is to use a character string literal. The minimum limit allowed by ANSI for a character string literal after concatenation is 509 characters. The limit in early versions of Microsoft C/C++ was between 512-2048 characters depending on the specific version of the compiler. Because of the limit on the length of a string literal, you cannot initialize character arrays longer than these limits with this method. (These limits include the final null character of a C "string." Thus the statement char a[] = "12"; results in a 3-element array.)

Because there is also a limit on the line length in most editors, you normally cannot directly put this many characters in a string literal. The compiler will concatenate a series of quoted strings into a single string, however, so the declaration


char a[] = "a"
           "b";

is the same as:


char a[] = "ab";

This allows placing large literal initializers into the code as shown below. This method runs into the compiler limit.


char stuff[] =
   "xxx...xxx"
       ...
   "xxx...xxx";

(The ANSI standard states that strings separated only by white space are automatically concatenated.)

Method 2 -- Character Initializers

The following can be used:


char stuff [] =
   { 'a', ...
          ...
     ... 'z' };

However, such an initializer is tedious to type. If using this method, write a program that will read a data file and output the proper initializer.

Method 3 -- Multidimensional Arrays


char stuff[][10] =  {
   "0123456789",
   ...
   "0123456789"  };

The value 10 is not important EXCEPT that it must match the actual length of the string constants. If any of the constants are shorter than the length specified, the end of that row will be padded out with zero bytes. If any are longer, the extra characters will be thrown away. This results in a two dimensional array. Another pointer can be used to access the following in almost any method desired:


   char *stuffptr = (char *) stuff;

This method seems to be the most convenient. The big problem with using a pointer to try and address the array as a single dimensional array is that the extra null characters make counting difficult, particularly if all the initializer strings are not the same length. Thus stuffptr[97] may not access the element you expect unless you count very carefully.

Method 4 -- Assembly Modules

The array can also be defined in MASM and linked to your C program. In MASM, once the correct segment and public definitions are done, write the following:


stuff   db      "abcdefghijkl"
        db      "morestuff"
        ...
        db      "laststuff"

In C, access the array with the following:


extern char stuff[];   /*    char * stuff;   will NOT work    */

Method 5 -- Read from a File

Another method is to read the values into the array at run time from a data file. If the file is read in large blocks (for example, using read or fread), the I/O will be quite fast. This method also has the advantage that the initialization string can be changed without having to change and recompile the code.

Additional query words:


Keywords          : kbLangC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 
Version           : MS-DOS:6.0,6.00a,6.00ax,7.0; WINDOWS:1.0,1.5,1.51,1.52; WINDOWS NT:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS NT WINDOWS 
Issue type        : kbhowto

Last Reviewed: July 5, 1999