Thread: MALLOC function usage prob

How is my usage of malloc wrong??

Code:

typedef struct stack {
	char data[SIZE_MAX]; // SIZE_MAX #defined as 100
	int size;
} stack;

stack *s=(stack*)malloc(sizeof(stack));  //line# 11

output:

Code:

prg1.c:11: error: initializer element is not constant

Give us more code. I believe the above error could occur if you call malloc in the global scope (outside main and/or another function). Is that the case? The global variables are initialized in compile time, not at run time, so you cannot use malloc to intialize

here's the code

Code:

...
#define SIZE_MAX 100

typedef struct stack {
	char data[SIZE_MAX];
	int size;
} stack;

stack *s=(stack*)malloc(sizeof(stack));

void init_stack() {
	s->size=0;
	return;
}

void push(char c) {
	if(s->size<SIZE_MAX) {
		s->data[(s->size)++]=c;
	}
	else printf("\nStack Full!\n");
	return;
}

char pop() {
	if(s->size==0) {
		printf("\nStack Empty!\n");
		return('~');
	}
	else return s->data[--(s->size)];
}
int main(void) {
	int t,i,j=0;
	char expressions[100][400],ch;
	printf("no. of expressions:");
	scanf("&#37;d",&t);
	printf("enter the expressions");
	for(i=0;i<t;i++) {
		printf("expr[%d]:",i+1);
		scanf("%s",expressions[i]);
	}

	for(i=0;i<t;i++) {
		init_stack();
		while(j<strlen(expressions[i])) {
			ch=expressions[i][j++];
			if(isalpha(ch)) {
				printf("%c",ch);
			}
			else if(ch==')') {
				ch=pop();
				if(ch!='~') printf("%c",ch);
			}
			else if(ch=='+'||ch=='-'||ch=='*'||ch=='/'||ch=='^') {
				push(ch);
			}
		}
	}
	return 0;
}

So, the correct way to dynamically allocate memory for a global variable is this:

Code:

....
stack *s;

int main(void) {
        ...
        s = (stack *)malloc(sizeof(stack));
        ...
}

But, as matsp mentioned you could just do this:

Code:

....
stack s;

int main(void) {
    ...
}

and replace of course "s->" with "s.". There is typically a difference in the two ways, but in your program there is no need to dynamically allocate memory. It would make sense only if you where allocating memory for the "stack.data[]" array based on a user input, for example, and not a fixed SIZE_MAX number.

Originally Posted by Elysia

The correct way is not to cast the return of malloc when compiling as C.

Why is that? I though you would get the same result, but it was typically better to cast the return value of malloc()

Yes, as matp and others said malloc should not be used in global scope.
I've re-arranged your code to make it work...!

Code:

...
#define SIZE_MAX 100

typedef struct stack 
{
	char data[SIZE_MAX];
	int size;
} stack;

stack *s;

void init_stack() 
{
	s->size=0;
	return;
}

void push(char c) 
{
	if(s->size<SIZE_MAX) 
	{
		s->data[(s->size)++]=c;
	}
	else 
		printf("\nStack Full!\n");
	return;
}

char pop() 
{
	if(s->size==0) 
	{
		printf("\nStack Empty!\n");
		return('~');
	}
	else 
		return s->data[--(s->size)];
}
int main(void) 
{
	int t,i,j=0;
	char expressions[100][400],ch;

	s = (stack *) malloc(sizeof(stack));
	printf("no. of expressions:");
	scanf("%d",&t);
	printf("enter the expressions");
	for(i=0;i<t;i++) 
	{
		printf("expr[%d]:",i+1);
		scanf("%s",expressions[i]);
	}

	for(i=0;i<t;i++) 
	{
		init_stack();
		while(j<strlen(expressions[i])) 
		{
			ch=expressions[i][j++];
			if(isalpha(ch)) 
			{
				printf("%c",ch);
			}
			else if(ch==')') 
			{
				ch=pop();
				if(ch!='~') printf("%c",ch);
			}
			else if(ch=='+'||ch=='-'||ch=='*'||ch=='/'||ch=='^') 
			{
				push(ch);
			}
		}
	}
	return 0;
}

First, you cast the return of malloc. Do not do this. This was explained earlier!

Could you tell in detail for not to cast the malloc result.
I feel we should, since the malloc returns void pointer.
I've been doing this and it works well with C...!

Originally Posted by patil.vishwa

Could you tell in detail for not to cast the malloc result.
I feel we should, since the malloc returns void pointer.
I've been doing this and it works well with C...!

And it will continue to do so, until the day when it suddenly stops. But: all those things in red here are links -- so when people say "It's in the FAQ", and the word FAQ is red, that means you can click on it to go to that entry in the FAQ.