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.
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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.)
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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.
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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.
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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 */
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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.
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