Expression syntax in function main (Turbo-C)

Question

I'm using a DOS virtual machine to bypass a segfault, yet Turbo-C doesn't want to compile this code:

#include <stdio.h>

int main() {
       FILE *fp = fopen("somefile.txt", "w");
       if(fp == NULL) {
               fprintf(stderr, "NaF");
               return -1;
       }
       char /*1*/ *ch = NULL;
       while(ch /*2*/ < (char *) 209* /*3*/1024) {
                fprintf(fp, "& - %p, * - %c\n", ch, *ch/*4*/);
                ch++;
       }
       fclose(fp);
       return 0;
}

Error list:

1. Expression syntax in function main
2. Undefined symbol 'ch' in function main
3. Illegal use of pointer in function main
4. Invalid indirection in function main

I assume this must be some kind of ancient C, because I'm positive this code would compile on a modern compiler. I am aware it would produce a segfault, hence why I'm writing in a DOS environment.

Answer 1

1) and 2) Turbo-C used the C90 version of the standard. It did not allow variable declarations in the middle of a { } body, but only at the top. Therefore char* ch has to be moved:

int main (void)
{
  char* ch = NULL
  ...

3) You try to multiply a pointer (char *) 209. This simply isn't allowed in C and will not compile on modern compilers either.

And finally, pointer arithmetic ch++ used on a pointer which doesn't point at an allocated object isn't well-defined by any version of C. It probably worked in Turbo C, but no guarantees.

I think this program was supposed to grab a memory dump of RAM and store it to a text file. MS DOS allowed direct access of memory. However, the code was already questionable even back in 1989.

Using char for accessing raw memory is a bad idea, since it is a type with implementation-defined signedness. Use unsigned char or uint8_t instead.