Creating a structure on a heap?

Question

I was instructed to write the strdup model by creating a String structure on the heap where the copy of the source is stored. I think I successfully encoded strdup, but I'm not sure if I created Struct on the heap ...

typedef 
struct String {
    int length;
    int capacity;
    unsigned check;
    char ptr[0];
} String;

char* modelstrdup(char* src){
    int capacity =0, length=0, i = 0 ;
    char *string;
    while ( src[length] != '\0'){
        length++;
    }
    capacity = length;
    string = malloc(sizeof(String) + capacity + 1);
    while ( i < length ){
        string[i] = src[i];
        i++;
    }
    string[i+1] = '\0';

    return string;
}   

Answer 1

Yes, you created a structure on the heap. You didn’t populate it correctly, and you will have problems with deleting it - I’m not sure whether this covered homework or not. Be that as it may, you are more likely to get a memory corruption or, with luck, a memory leak, than to release one of these lines.

Code that works with standard C89 and C99

Your code, slightly corrected ...

typedef 
struct String {
    int length;
    int capacity;
    char *ptr;
} String;

char* modelstrdup(char* src){
    int length = strlen(src);
    char *space = malloc(sizeof(String) + length + 1);
    //String *string = space;  // Original code - compilers are not keen on it
    String *string = (String *)space;
    assert(space != 0);
    string->ptr = space + sizeof(String);  // or sizeof(*string)
    string->length = length;
    string->capacity = length + 1;
    strcpy(string->ptr, src);
    return string->ptr;
}

C89, C99 ( C99/++). , "struct hack" ( , C99). . . , - , .

, , , . .

---

, C99

C99 6.7.2.1 16 " ":

; . , . -, , . ¹⁰⁶⁾ -, a. ( → ) , ( ) , , ( ), , ; , . , , , undefined, - .

¹⁰⁶ , , .

"struct hack" " ", :

typedef 
struct String {
    int length;
    int capacity;
    char ptr[];
} String;

char* modelstrdup(char* src){
    int length = strlen(src);
    String *string = malloc(sizeof(String) + length + 1);
    assert(string != 0);
    string->length = length;
    string->capacity = length + 1;
    strcpy(string->ptr, src);
    return string->ptr;
}

GCC 4.0.1, ( "-Wall -Wextra" ). String * string = (String *); , , ; , .

---

, C89 C99

GCC, - ISO C . , , "gcc -Wall -Wextra -std = c99 -pedantic":

#include <assert.h>
#include <stdlib.h>
#include <string.h>

typedef
struct String {
    int length;
    int capacity;
    char ptr[0];
} String;

char* modelstrdup(char* src){
    int length = strlen(src);
    String *string = malloc(sizeof(String) + length + 1);
    assert(string != 0);
    string->length = length;
    string->capacity = length + 1;
    strcpy(string->ptr, src);
    return string->ptr;
}

, C , C. ; , , , , . , , . C , , .

Answer 2

, , . string char *, struct String. , strdup() , .

. , malloc() . strdup() , .

Answer 3

You have. Malloc, new, etc. Everyone uses a bunch.

Answer 4

Yes, it mallocreturns memory to the heap.