How to calculate the start address of a structure from offset of a field with only that specific field pointer

Question

I have following code. structure emp_ has five elements and all of its first four members could be printed with print_emp_details() function which takes a pointer to a emp_ struct. Given only a offset and a pointer to a particular field, start_address(glthread_node_t *node_t) should return (after casting) the start address of the struct emp_ which is then pass into print_emp_details(). I am getting gibberish printf output from print_emp_details() and couldn't find what I am doing wrong. The offsetValue is only thing I am getting right.

#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <string.h>
#include "glthread.h"


#define glthread_node_init(glnode)  \
    glnode->left = NULL;            \
    glnode->right = NULL;


#define offset(struct_name, fld_name) \
  (unsigned int)&(((struct_name *)0)->fld_name)

typedef struct glthread_node_ {
    struct glthread_node_ *left;
    struct glthread_node_ *right;
} glthread_node_t; 

typedef struct glthread_ {
    glthread_node_t *head;
    unsigned int offset;
} glthread_t;



typedef struct emp_ {
    char name[30];
    unsigned int salary;
    char designation[30];
    unsigned int emp_id;
    glthread_node_t glnode;
} emp_t;


struct emp_ *
start_address(glthread_node_t *node_t){
  unsigned int offsetValue;
  offsetValue=offset(emp_t,glnode);
  printf("offset = %d\n", offsetValue);
  return (struct emp_*)(node_t-offsetValue);
}

void
print_emp_details(emp_t *emp){

    printf("Employee name = %s\n", emp->name);
    printf("salary = %u\n", emp->salary);
    printf("designation = %s\n", emp->designation);
    printf("emp_id = %u\n", emp->emp_id);
}

void
init_glthread(glthread_t *glthread, unsigned int offset){

    glthread->head = NULL;
    glthread->offset = offset;
}

int
main(int argc, char **argv){
  emp_t *emp1=calloc(1,sizeof(emp_t));
  strncpy(emp1->name,"Harris", strlen("Harris"));
  emp1->salary= 100;
  strncpy(emp1->designation,"HR",strlen("HR"));
  emp1->emp_id=13;
  glthread_node_init((&emp1->glnode));
  print_emp_details(start_address(&emp1->glnode));
  return 0;
}

Output:

offset = 72
Employee name = ?T??
salary = 0
designation = 
emp_id = 1920169263

Could you add the exact program you ran to get that output? I don't see how the `offset = 72` line is getting printed, for example. — Fricative Melon, Feb 19 '21 at 03:10
You need to show the definition of glthread_node_t in the header. Pointer arithmetic in C works on increments of sizeof(). — BadZen, Feb 19 '21 at 03:12
Try `(char*)node_t - offsetValue` for it to have any chance to work. And you should use the standard [`offsetof`](https://en.cppreference.com/w/c/types/offsetof) instead. — dxiv, Feb 19 '21 at 03:12
Does this answer your question? [How can one calculate a pointer to the beginning of a struct from a pointer pointing at a member of the struct in a portable way?](https://stackoverflow.com/questions/53895524/how-can-one-calculate-a-pointer-to-the-beginning-of-a-struct-from-a-pointer-poin) — dxiv, Feb 19 '21 at 03:19
`strncpy(emp1->name,"Harris", strlen("Harris"));` --> living on the edge of failing to form a _stirng_ should `"Harris"`'s replacement be length 30. — chux - Reinstate Monica, Feb 19 '21 at 03:28
`(char*)node_t - offsetValue` didn't work. Similar output @dxiv — H.Jamil, Feb 19 '21 at 03:43
@H.Jamil It works fine [here](https://godbolt.org/z/7Eoorc). — dxiv, Feb 19 '21 at 03:46
@H.Jamil Step 1, use `offsetof` as commented above, not `offset`. 2) Save time, enable all warnings to fix the annoying small issues.. — chux - Reinstate Monica, Feb 19 '21 at 03:47

score 2 · Accepted Answer · answered Feb 19 '21 at 03:42

The problem is at the end of start_address:

return (struct emp_*)(node_t-offsetValue);

When you add or subtract a value from a pointer, it results in a pointer increased/decreated by that value times the size of the type the pointer points to. It is done this way to make array indexing work.

You need to cast the pointer to char * to add/subtract single byte values:

return (struct emp_*)((char *)node_t-offsetValue);

How to calculate the start address of a structure from offset of a field with only that specific field pointer

1 Answers1