Originally Posted by shakunni

As part of my program, I have defined and initialized a structure. But when I compile it, gcc gives me the following error on all the initialization " initializer element is not constant". Anyone know why this is the case?

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C Syntax (Toggle Plain Text)

struct RECORD 
{
	char* username;
	char* password;
	char* type;
	struct RECORD *next;
};
 
struct RECORD *head=(struct RECORD*)malloc(sizeof(struct RECORD));
struct RECORD *temp=(struct RECORD*)malloc(sizeof(struct RECORD));
struct RECORD *linktemp=(struct RECORD*)malloc(sizeof(struct RECORD));
struct RECORD *pos=(struct RECORD*)malloc(sizeof(struct RECORD));

struct RECORD { char* username; char* password; char* type; struct RECORD *next; }; struct RECORD *head=(struct RECORD*)malloc(sizeof(struct RECORD)); struct RECORD *temp=(struct RECORD*)malloc(sizeof(struct RECORD)); struct RECORD *linktemp=(struct RECORD*)malloc(sizeof(struct RECORD)); struct RECORD *pos=(struct RECORD*)malloc(sizeof(struct RECORD));

I get an error on code lines 9-12.

Originally Posted by Aia

shakunni >Anyone know why this is the case?
A function must be called from another function block which could be the main function or one with a lineage to any main function.
Meaning that main() must have been called first at some point.
malloc() is a function and you are trying to execute it before main().

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c Syntax (Toggle Plain Text)

#include <stdio.h>
int c = getchar();   /* wrong */
 
int main (void)
{
    printf("%c\n", c);
    return 0;
}

#include <stdio.h> int c = getchar(); /* wrong */ int main (void) { printf("%c\n", c); return 0; }

Originally Posted by Aia

shakunni >Anyone know why this is the case?
A function must be called from another function block which could be the main function or one with a lineage to any main function.
Meaning that main() must have been called first at some point.
malloc() is a function and you are trying to execute it before main().

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c Syntax (Toggle Plain Text)

#include <stdio.h>
int c = getchar();   /* wrong */
 
int main (void)
{
    printf("%c\n", c);
    return 0;
}

#include <stdio.h> int c = getchar(); /* wrong */ int main (void) { printf("%c\n", c); return 0; }

Originally Posted by sbesch

I think that it might be useful to generalize this a bit more. The problem is that you didn't distinguish between what happens (or can happen) at compile time from what happens (or can happen) at run time - this is not always a simple thing to do. Nevertheless, since malloc is a function, it can't get called till the program runs, and as Aia pointed out, it can't get called if it's outside a function block.

You can allocate the structures at compile time if you want to. Outside of a function block:

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C Syntax (Toggle Plain Text)

      struct RECORD  {
        char* username;
        char* password;
        char* type;}
 
      struct RECORD next;                 //Allocates memory for structure.
      struct Record * pnext=&next;  //Provide a pointer reference to it.
      :
      : etc.

struct RECORD { char* username; char* password; char* type;} struct RECORD next; //Allocates memory for structure. struct Record * pnext=&next; //Provide a pointer reference to it. : : etc.

You still need to allocate the char* items at runtime, but space for the structures and the pointer to the structures will be done for you by the compiler. I would guess that this is what you actually want. Use malloc when you have an indeterminate (or perhaps very large) number of structures that must be generated at run time - that's what its for: to generate blocks of memory allocation that would be either inefficient or undesirable to include in the program's executable file. Use the compiler to allocate when you have only a few, definite structures.