What's wrong with this code?

Code:

/*
#include<stdio.h>

struct msg{
  char *p1;
  char *p2;
  char *p3;
}mymsg;

mymsg.p1="Teach yourself C in 21 days!";
mymsg.p2="By SAMS publishing.";

int main(void)
{
  printf("mymsg.p1 points to %s\n",mymsg.p1);
  printf("mymsg.p2 points to %s\n",mymsg.p2);

  /* p3 points to the same string p1 points to */
  mymsg.p3=mymsg.p1;
  printf("mymsg.p3 points to the string p1 points to %s\n",mymsg.p3);

  /* p3 points to the same string p2 points to */
  mymsg.p3=mymsg.p2;
  printf("mymsg.p3 points to the string p2 points to  %s\n",mymsg.p3);

  return 0;
}
*/

My compiler tells me:

"stru_array3.c", line 9: warning: old-style declaration or incorrect type for: mymsg
"stru_array3.c", line 9: identifier redeclared: mymsg
current : int
previous: struct msg {pointer to char p1, pointer to char p1, pointer to char p2, pointer to char p3} : "stru_array3.c", line 7

You can't make assignments to the structure members in global scope. Initialization is allowed though:

Code:

#include<stdio.h>

struct msg{
    char *p1;
    char *p2;
    char *p3;
}mymsg = {
    "Teach yourself C in 21 days!",
    "By SAMS publishing."
};

int main(void)
{
    printf("mymsg.p1 points to %s\n",mymsg.p1);
    printf("mymsg.p2 points to %s\n",mymsg.p2);
    
    /* p3 points to the same string p1 points to */
    mymsg.p3=mymsg.p1;
    printf("mymsg.p3 points to the string p1 points to %s\n",mymsg.p3);
    
    /* p3 points to the same string p2 points to */
    mymsg.p3=mymsg.p2;
    printf("mymsg.p3 points to the string p2 points to  %s\n",mymsg.p3);
    
    return 0;
}

m not sure but dont you think it would be better if you declared your structure in the main function ??

Secondly

char *p1 is only one byte of memory space. This can store just one alphabet! therefore i dont think that it is possible to have a whole string like "teach your self.....21 days"

Originally posted by ylzhang

Code:

/*
#include<stdio.h>

struct msg{
  char *p1;
  char *p2;
  char *p3;
}mymsg;

mymsg.p1="Teach yourself C in 21 days!";
mymsg.p2="By SAMS publishing.";

int main(void)
{
  printf("mymsg.p1 points to %s\n",mymsg.p1);
  printf("mymsg.p2 points to %s\n",mymsg.p2);

  /* p3 points to the same string p1 points to */
  mymsg.p3=mymsg.p1;
  printf("mymsg.p3 points to the string p1 points to %s\n",mymsg.p3);

  /* p3 points to the same string p2 points to */
  mymsg.p3=mymsg.p2;
  printf("mymsg.p3 points to the string p2 points to  %s\n",mymsg.p3);

  return 0;
}
*/

My compiler tells me:

"stru_array3.c", line 9: warning: old-style declaration or incorrect type for: mymsg
"stru_array3.c", line 9: identifier redeclared: mymsg
current : int
previous: struct msg {pointer to char p1, pointer to char p1, pointer to char p2, pointer to char p3} : "stru_array3.c", line 7

>dont you think it would be better if you declared your structure in the main function ??
It depends on the programmer, but 97% (made up number) of structures are used in multiple functions, so declaring it to be local to main wouldn't be the best of ideas in most cases.

>char *p1 is only one byte of memory space.
Actually, a pointer is more likely to be two or four bytes of memory, but this size can vary depending on the machine. A char is one byte, a pointer to char is completely different.

>This can store just one alphabet!
It can store the address of a single location in memory that happens to contain just one character.

>therefore i dont think that it is possible to have a whole string like "teach your self.....21 days"
Of course it can, a string literal is a pointer to the first character in a sequence of contiguous characters. If you have the first address, it's a simple matter of pointer arithmetic:

Code:

#include <stdio.h>

int main(void)
{
    char *p = "A sequence of characters\n";

    while (*p)
        putchar(*p++);

    return 0;
}

Of course, the standard library saves us from actually doing this in a serious program:

Code:

#include <stdio.h>

int main(void)
{
    char *p = "A sequence of characters\n";

    fputs(p, stdout);

    return 0;
}