Q47764: Ambiguous Documentation of Enter ASCII (EA) Command

Article: Q47764
Product(s): See article
Version(s): 2.10 2.20 | 2.10 2.20
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_basic
Last Modified: 16-AUG-1989

In the "Microsoft C 5.1 Optimizing Compiler" manual, the "CodeView and
Utilities" section does not thoroughly document the EA command for
CodeView. The following is a more complete explanation:

   EA address [list]

   The Enter ASCII (EA) command modifies array to the value of [list].
   Address refers to what you want to modify. [list] refers to a
   string literal such as "hello". EA will copy [list] to array. Thus,
   if you specify address by giving an array, such as an array of
   characters, the array will have [list] copied into it. However, if
   you were to give a pointer to a character, then the bytes that make
   up the pointer will be directly modified, not the block of memory
   the pointer is pointing at.

Consider the following examples:

char array[] = "Hello";
char *ptr    = "Hello";

>EA array "Hi"
>EA ptr   "Hi"

The result is that the array would contain "Hillo", and ptr will be
pointing to some unknown memory location.

The array is considered a constant pointer to a block of memory, and
as such, any operations on it can only affect the block of memory.
However, ptr is a variable; therefore, any operations on it affect the
value of ptr, which happens to be a two- or four-byte number. That
number happens to refer to a location in memory.

If you want to changed the block of memory that ptr points to, you
must do the following:

   >EA &ptr[0] "Hi"

Essentially, this takes the address of the character that ptr is
pointing at, gives a constant address, and thus modifies the block of
memory there. To clarify, imagine you have a character located some
place in memory. To move a character, you must make a copy of it to
some other memory location; it is not possible to take its address and
modify the address directly to change where the character is located.
Thus, the address of that character is always a constant value. ptr[0]
happens to be a character located someplace in memory. As such, the
address of that specific character is a constant value. Since a
constant cannot be modified, EA changes the block of memory starting
at where the constant pointer is pointing.

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