struct values are changing from one function to other in unexpected manner

Question

#include <stdio.h>
#include <limits.h>
#include <string.h>
#include <stdlib.h>

typedef struct
{
    unsigned char a[3];
    unsigned short data[3];
    unsigned int d1;
    unsigned int d2;
} abc; 

void f1(abc *ptr)
{
    printf("values are :0x%x  \t0x%x  \t0%x",ptr->data[0],ptr->data[1],ptr->data[2]);
    //Hex map of ptr  is given below (taken by  gdb --> x/20b ptr )
    //ptr-->0x01 0x00 0x00 0x00 0x44 0x29 0xff 0xff
    //      0xff 0xff        
}
void f()
{
    abc req;
    req.a[0]=1;
    req.a[1]=0;
    req.a[2]=0;
    req.data[0]=0x2944;
    req.data[1]=0xffff;
    req.data[2]=0xffff;
    req.d1= 0xffff;
    req.d2= 0xffff;
    f1(&req);   

    //Hex map of req  is given below (taken by  gdb --> x/20b req )
    // req--> 0x01 0x00 0x00 0x00 0x44 0x29 0xff 0xff
    //        0xff 0xff    
}
int main()
{
    f();
    return 0;
}

I am working on the code sample given above . filling values in one struct in 1 function and passing struct to other function. But values of struct are changed in the caller function. When I checked with gdb, struct byte representation is same. But some how values are changed. Can any one explain what happened and how to overcome it gcc version is 4.9.3 and gdb version is 7.7.1 and OS is ubuntu 14.

Below is the output from gdb.

req values in f() --> 
(gdb) p/x req
$1 = {
  a = {0x1, 0x0, 0x0},
  data = {0x2944, 0xffff, 0xffff},
  d1 = 0xffff,
  d2 = 0xffff

(gdb) x/20b &req
0xffffcb80:     0x01    0x00    0x00    0x00    0x44    0x29    0xff    0xff


ptr values in f1()---->
(gdb) p/x *ptr
$1 = {
  a = {0x1, 0x0, 0x0},
  data = {0x4400, 0xff29, 0xffff},

  (gdb) x/20b ptr
0xffffcb80:     0x01    0x00    0x00    0x00    0x44    0x29    0xff    0xff

My guess is that `p/x *ptr` includes the *padding* between members. Related question: [Why isn't sizeof for a struct equal to the sum of sizeof of each member?](https://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member) — Some programmer dude, Jul 11 '18 at 14:15
Note there is one padding byte after the `char[3]`. Nothing is changed. — Paul Ogilvie, Jul 11 '18 at 14:18
I ran your code through gcc version 4.8.1 and also through an online gcc compiler and the code works fine. The output is what you desire. — Prakash Bansal, Jul 11 '18 at 14:48
print is also not printing correct values.It looks padding is doing something wrong here. Same code when I run in home computer it is working fine. — shekhar singh, Jul 11 '18 at 15:09
@Aconcagua `short` in variadic argumens is promoted to `int` anyway — Eugene Sh., Jul 11 '18 at 15:13
Somehow I tend not to believe you.... Are you sure you are running the exact same code you have posted? — Eugene Sh., Jul 11 '18 at 15:17
Eugene Sh :- Yes . same code is not working in office. Even I discussed this with my 4 to 5 colleague. None of them was able to explain the reason. — shekhar singh, Jul 11 '18 at 15:20
Then it sounds like some compiler bug or, which is more likely, you are doing something wrong. Like not compiling after the change or something. — Eugene Sh., Jul 11 '18 at 15:29
Yes it could be compiler bug. I will check again and come back — shekhar singh, Jul 11 '18 at 15:56

score 0 · Answer 1 · answered Jul 12 '18 at 03:23

Can any one explain what happened

The most likely explanation is that

the functions f and f1 are in different translation units, and
the definition of struct abc is different between them (for example, in one translation unit the struct has __attribute__((packed)), while in the other one it doesn't.

If so, this is an ODR (one definition rule) violation; your program is malformed, no compiler diagnostic required.

Printing sizeof(struct abc) in both f and f1 is one way to confirm or disprove this guess.

If the sizes are different, then saving pre-processed output (gcc -E) and looking inside for how struct abc is defined will likely be illuminating.

@shekharsingh Rearranged in what way? Did you find the problem? — Employed Russian, Jul 13 '18 at 14:48

struct values are changing from one function to other in unexpected manner

1 Answers1