Thread: Memory allocation for structures

Why do i get a Seg Fault if I run:

Code:

#include <stdio.h>

struct rec
{
    int i;
    float f;
    char c;
};

int main()
{
    struct rec *p;
    
    (*p).i=10;
    (*p).f=3.14;
    (*p).c='a';
    printf("%d %f %c\n",(*p).i,(*p).f,(*p).c);
    free(p);
    return 0;
}

but not

Code:

#include <stdio.h>

struct rec
{
    int i;
    float f;
    char c;
};

int main()
{
    struct rec *p;
    p=(struct rec *) malloc (sizeof(struct rec));
    (*p).i=10;
    (*p).f=3.14;
    (*p).c='a';
    printf("%d %f %c\n",(*p).i,(*p).f,(*p).c);
    free(p);
    return 0;
}

can someone explain why exactly.
and do I always have to do p=(struct rec *) malloc(sizeof(struct rec)); ?

Originally posted by Salem
struct rec *p;
p=(struct rec *) malloc (sizeof(struct rec));
(*p).i=10;
This on the other hand is better - you know exactly where that int is going - into the memory returned by malloc

Not necessarily better tho if malloc fails. Best do some error checking before using that pointer!

Code:

if ((p = (struct rec *) malloc (sizeof(struct rec))) == NULL)
{
/* malloc has failed, bail out or somthing */
}

thanks
yeah, I usually have ERROR checks in my programs
I am going to write a new function
void bail_out(char* error[]);