Pointers and arrays in functions

Question

Wrote part of the code from the book by B.Kernigan & D.Ritchie.

#include <cstdio>

using namespace std;

void strcopy(char *s, char *t);

int main()
{
    char *k = (char*)"TEXT1";
    char *g = (char*)"TEXT2";

    printf("%s\n", k);
    printf("%s\n", g);

    strcopy(k, g);

    printf("%s\n", k);
    printf("%s\n", g);

    return 0;
}

void strcopy(char *s, char *t)
{
    while ((*s = *t) != '\0') 
    {
        s++;
        t++;
    }     
}

When I try to compile, I get an error: "Exception thrown: write access violation."
What could be the problem?
I also noticed that many examples do not work directly in Visual Studio - here we can assume that the standard has changed and so on, but this example is a basic work with pointers. The authors do not use the char k = (char)"TEXT"; construct, instead they directly use char *k = "TEXT1"; but it doesn't work!
Are there actual books on the C language (not C++!)?

Answer 1

You are trying to write into memory occupied by a string literal. String literals are placed in a read-only memory segment, to prevent exactly this. Make it

char k[] = "TEXT1";

or something along these lines - a writable char array.

Answer 2

Found a solution: strcopy((char*)&k, (char*)&g);

Answer 3

Igor has explained why you get the exception and how to
overcome it. The following are observations about other
issues related to your example.

(1) You said:

When I try to compile, I get an error:
"Exception thrown: write access violation."

That's not exactly accurate. You do not get that error
when you compile, or link, or build, etc. Your sample
builds without errors and the exception occurs at
run time when your compiled code is executed.

This is not just a semantic quibble, it is important that
an accurate description of what happens and when it happens
be given when posting questions, so that an accurate
diagnosis can be made. In this case it is easy to tell
that it is not an error that happens at compile time,
but in many cases it is important to make a distinction.

I point this out just for your future consideration when
posting problems, and not to be critical.

(2) If you are going to try and learn and work with the
C language - and not the features of C as implemented
in C++ - then you should always work with the C compiler
and not with the C++ compiler. The easiest way to do
that is to ensure that your source code file has an
extension of .c and not .cpp - the former will use
the C compiler and rules by default, while the
latter will use the C++ compiler and rules by default.

In the sample you posted you have these lines:

#include <cstdio>

using namespace std;

These will only be accepted in C++ and are not valid
for C programs. So you probably have a file extension
of .cpp thus invoking the C++ compiler by default.

If you change the extension to .c you will need to
remove the using statement, and change the include
directive to

#include <stdio.h>

• Wayne

Answer 4

Do you know the literature on pure C? Even if there is
a hardcore explanation of all the details

Well, if you aspire to be a "language lawyer" then you
can spend a considerable amount of your future free
time reading the official ISO C Language Standard.
A draft copy of one version of this may be found here:

http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf

A later draft which reflects C11 changes may be found here:

http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf

The official final version of the Standard is not free
and must be purchased. However, late drafts often are
very close to the final or even identical.

Note that these Standards are primarily intended for the
use of language implementors, such as compiler writers,
and are not generally suited for learning the language.
They are always useful for clarifying what implementations
(compilers) should be doing.

For most aspiring C programmers, a combination
of contemporary textbooks and online tutorials
is usually a smoother and easier path.

• Wayne

Answer 5

There are a lot of superfluous things in C++ and C#.