Thread: traverse struct using pointer calculus

Hi All

The original idea was posted struct-property name is in char* by matsp

So, to summarize the problem: Suppose you have the following struct

Code:

struct CONFIG {
  char *my_val
  char *other_val
}

And you have a function that returns one of the two strings of this struct as follows:

Code:

char* function get_config_property(char* prop_name) {
  // prop_name contains "my_val" or "other_val"
  // -> access propterty using 'prop_name' and return it
}

Below is the code I wrote to accomplish this (using matsp idea):

Code:

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

char* getProperty(char *name ) ;
char* setProperty(char *name, char *value ) ;

struct CONFIG
 {
    char *abc ;
 } *config ;

struct propentry
 {
    ptrdiff_t offset;
    const char *name;
 };


#define PE(a) { offsetof(struct CONFIG, a), #a }
struct propentry proptable[] =
 {
    PE(abc),
    //PE(def)
 };

int main(void) {
    config = (struct CONFIG*)malloc(sizeof(config)) ;
    setProperty("abc", "hello") ;
    printf("MAIN TEST1, abc=%d\n", config->abc) ;
    printf("MAIN TEST2, abc=%d\n", getProperty("abc")) ;
    return 0;
}

char* setProperty(char *name, char *value )
{
    int i;
    for(i = 0; i < sizeof(proptable)/sizeof(proptable[0]); i++)
    {
        if (strcmp(proptable[i].name, name) == 0)
        {
           char *ptr = (char *)(((char *)config) + proptable[i].offset);
           ptr = value;
        }
    }
    return NULL;
}

char* getProperty(char *name )
{
    int i;
    for(i = 0; i < sizeof(proptable)/sizeof(proptable[0]); i++)
    {
        if (strcmp(proptable[i].name, name) == 0)
        {
             char *ptr = (char *)(((char *)config) + proptable[i].offset);
             return ptr;
        }
    }
    return NULL;
}

This code compiles and executes, but doesn't produce the correct output. I have the feeling that I'm very close to the solution, but I probably have to do something with malloc, to initialize the fields of the struct, but where ?
Furthermore, I don't understand the difference between

Code:

char *ptr = (char *)(((char *)config) + proptable[i].offset);

and

Code:

char *ptr = (char *)((config + proptable[i].offset);

I would say the first one is incorrect, but when the struct only contains int properties, everything works fine!

Any comments ?

thnx a lot
LuCa

I tried (main):

Code:

.....
config->abc = (char*)malloc(sizeof(char)*10) ;
setProperty("abc", "hello") ;
.....

and I tried (inside setProperty):

Code:

....
char *ptr = (char *)(((char *)config) + proptable[i].offset);
ptr = (char*)malloc(10 * sizeof(char)) ;
//ptr = value; --> doesn't feel right!
*ptr = *value; 
....

This last example would be preferred, but both modifications still give incorrect output!!!

setProperty(config->abc,"hello");

This is cheating! I think I've not been clear enough.
The struct can have many propteries. Here I have only one to keep things simple!
So the setProptery function only knows 3 things:

• the name of the struct
• the name of the property (char *name)
• the value to assign to this proptery (char *value)

So, this can easily be solved as follows by replacing setProperty (ignoring getProptery) with:

Code:

char* setProperty(char *name, char *value )
{
    if ( strcmp("abc", name) == 0 ) {
      config->abc = value
    }  
    else if ( strcmp("def", name) == 0 ) { 
       config->def = value ; // as an example!
    }
    else if ( ... ) { ... }
    .....
 return NULL;
}

But I don't prefer this way!
Actually, my struct will eventually contains chars*, ints, floats, etc

what if you read the property name and its value from file ? Then you cannot do setProperty(config->abc, value) ; but only setProperty(prop_name, value);

Here is a working example, but the struct only has int properties

Code:

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

int getProperty(char *name ) ;
int setProperty(char *name, char *value ) ;

struct CONFIG
 {
    int abc ;
    int def ;
 } *config ;

struct propentry
 {
    ptrdiff_t offset;
    const char *name;
 };


#define PE(a) { offsetof(struct CONFIG, a), #a }
struct propentry proptable[] =
 {
    PE(abc),
    PE(def)
 };

int main(void) {
    config = (struct CONFIG*)malloc(sizeof(config)) ;

    setProperty("abc", "20") ;
    printf("MAIN TEST1, abc=%d\n", config->abc) ; // test config->abc
    setProperty("def", "30") ;
    printf("MAIN TEST2, abc=%d\n", getProperty("abc")) ; // returns config->abc
    printf("MAIN TEST2, def=%d\n", getProperty("def")) ; // returns config->def
    return 0;
}

int setProperty(char *name, char *value )
{
    int i;
    for(i = 0; i < sizeof(proptable)/sizeof(proptable[0]); i++)
    {

        if (strcmp(proptable[i].name, name) == 0)
        {
           int *ptr = (int *)(((char *)config) + proptable[i].offset);
           *ptr = (int)strtol(value, NULL, 10) ;
        }
    }
    return -1;
}

int getProperty(char *name )
{
    int i;
    for(i = 0; i < sizeof(proptable)/sizeof(proptable[0]); i++)
    {
        if (strcmp(proptable[i].name, name) == 0)
        {
             int *ptr = (int *)(((char *)config) + proptable[i].offset);
             //int *ptr = (int *)(config + proptable[i].offset);
             return *ptr;
        }
    }
    return -1;
}

thnx a lot, I have the char* example working!!

In both the examples I allocate config

Code:

config = (struct CONFIG*)malloc(sizeof(config)) ;

so why is the second example crashing ?

Is there an alternative to offsetof ?

Furthermore, I have made my struct more complex (real situation) and tried the following

Code:

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

void* getProperty(char *name ) ;
void setProperty(char *name, char *value ) ;

struct CONFIG
 {
    char *abc ;
    int *def ;
 } *config ;

struct propentry
 {
    ptrdiff_t offset;
    const char *name;
 };


#define PE(a) { offsetof(struct CONFIG, a), #a }
struct propentry proptable[] =
 {
    PE(abc),
    PE(def)
 };

int main(void) {
    config = (struct CONFIG*)malloc(sizeof(config)) ;
    //config->abc = malloc(10*sizeof(char)) ;
    setProperty("abc", "hello") ;
    setProperty("def", "20") ;
    printf("MAIN TEST1, abc=%s\n", config->abc) ;
    printf("MAIN TEST2, abc=%s\n", (char*)getProperty("abc")) ;
    printf("MAIN TEST3, def=%d\n", config->def) ;
    printf("MAIN TEST4, def=%d\n", (int*)getProperty("def")) ;
    return 0;
}

void setProperty(char *name, char *value )
{
    int i;
    for(i = 0; i < sizeof(proptable)/sizeof(proptable[0]); i++)
    {

        if (strcmp(proptable[i].name, name) == 0)
        {
          char* bytes = (char*)config;
          bytes += proptable[i].offset;
          void** prop = (void**)bytes;
          *prop = (void*)value ;
        }
    }
}

void* getProperty(char *name )
{
    int i;
    for(i = 0; i < sizeof(proptable)/sizeof(proptable[0]); i++)
    {
        if (strcmp(proptable[i].name, name) == 0)
        {
          char* bytes = (char*)config;
          bytes += proptable[i].offset;
          void** prop = (void**)bytes;
          return *prop ;

I'm not sure if void* can be used like this, however, the output is almost correct. Any comments ?

cheers
LuCa

Ok, its not the correct way of doing things. However I'm interested in whats going on and how far I can get with pointers, so lets consider it an exercise then! You're right about my problem with pointers, and as you can see all my questions here are about or have to do with pointers!!