malloc() of struct array with varying size structs

Question

How does one malloc an array of structs correctly if each struct contains an array of strings which vary in size?

So each struct might have a different size and would make it impossible to

realloc(numberOfStructs * sizeof(structName))

after

malloc(initialSize * sizeof(structName)

How does one allocate memory for this and keep track of what is going on?

Answer 1

If your structure has a char *, it takes up the size of one pointer. If it has a char[200], it takes up two hundred bytes.

Answer 2

I am making some guesses here, based on the information you have provided. The only reason I can see for wanting to realloc an array of structs is if you want to add more structs to that array. That's cool. There are plenty of reasons to want that kind of dynamic storage. The best way to handle it, especially if the structures are themselves dynamic, is to keep an array of pointers to these structures. Example:

1. Data structure:

typedef struct {
    int numberOfStrings;
    char ** strings;
}
stringHolder;

typedef struct {
    int numberOfStructs;
    stringHolder ** structs;
}
structList;

2. Managing dynamic arrays of strings:

void createNewStringHolder(stringHolder ** holder) {
    (*holder) = malloc(sizeof(stringHolder));
    (*holder)->numberOfStrings = 0;
    (*holder)->strings = NULL;
}

void destroyStringHolder(stringHolder ** holder) {
    // first, free each individual string
    int stringIndex;
    for (stringIndex = 0; stringIndex < (*holder)->numberOfStrings; stringIndex++)
    { free((*holder)->strings[stringIndex]); }
    // next, free the strings[] array
    free((*holder)->strings);
    // finally, free the holder itself
    free((*holder));
}

void addStringToHolder(stringHolder * holder, const char * string) {
    int newStringCount = holder->numberOfStrings + 1;
    char ** newStrings = realloc(holder->strings, newStringCount * sizeof(char *));
    if (newStrings != NULL) {
        holder->numberOfStrings = newStringCount;
        holder->strings = newStrings;
        newStrings[newStringCount - 1] = malloc((strlen(string) + 1) * sizeof(char));
        strcpy(newStrings[newStringCount - 1], string);
    }
}

3. Managing a dynamic array of structures:

void createNewStructList(structList ** list, int initialSize) {
    // create a new list
    (*list) = malloc(sizeof(structList));
    // create a new list of struct pointers
    (*list)->numberOfStructs = initialSize;
    (*list)->structs = malloc(initialSize * sizeof(stringHolder *));
    // initialize new structs
    int structIndex;
    for (structIndex = 0; structIndex < initialSize; structIndex++)
    { createNewStringHolder(&((*list)->structs[structIndex])); }
}

void destroyStructList(structList ** list) {
    // destroy each struct in the list
    int structIndex;
    for (structIndex = 0; structIndex < (*list)->numberOfStructs; structIndex++)
    { destroyStringHolder(&((*list)->structs[structIndex])); }
    // destroy the list itself
    free((*list));
}

stringHolder * addNewStructToList(structList * list) {
    int newStructCount = list->numberOfStructs + 1;
    size_t newSize = newStructCount * sizeof(stringHolder *);
    stringHolder ** newList = realloc(list->structs, newSize);
    if (newList != NULL) {
        list->numberOfStructs = newStructCount;
        list->structs = newList;
        createNewStringHolder(&(newList[newStructCount - 1]));
        return newList[newStructCount - 1];
    }
    return NULL;
}

4. Main program:

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

int main (int argc, char * argv[]) {
    structList * allHolders;
    createNewStructList(&allHolders, 10);

    addStringToHolder(allHolders->structs[4], "The wind took it");
    addStringToHolder(allHolders->structs[4], "Am I not merciful?");
    addStringToHolder(allHolders->structs[7], "Aziz, Light!");

    printf("%s\n", allHolders->structs[4]->strings[0]);  // The wind took it
    printf("%s\n", allHolders->structs[4]->strings[1]);  // Am I not merciful?
    printf("%s\n", allHolders->structs[7]->strings[0]);  // Aziz, Light!

    stringHolder * newHolder = addNewStructToList(allHolders);
    addStringToHolder(newHolder, "You shall not pass!");
    printf("%s\n", newHolder->strings[0]);               // You shall not pass!
    printf("%s\n", allHolders->structs[10]->strings[0]); // You shall not pass!

    destroyStructList(&allHolders);
    return 0;
}

Answer 3

You don't, generally. There are two reasons you might want to do this:

1. So that a single free() will release the entire block of memory.
2. To avoid internal memory fragmentation.

But unless you have an exceptional situation, neither are very compelling, because there is crippling drawback to this approach:

If you do this, then block[i] is meaningless. You have not allocated an array. There is no way to tell where your next struct starts without either examining the struct or having outside information about the size/position of your structs in the block.

Answer 4

It is not so clear how your struct type is declared. C99 has a special construct for such things, called flexible array member of a struct :

As a special case, the last element of a structure with more than one named member may have an incomplete array type; this is called a flexible array member.

You could do something like

typedef struct myString myString;
struct myString { size_t len; char c[]; };

You may then allocate such a beast with

size_t x = 35;
myString* s = malloc(sizeof(myString) + x);
s->len = x;

and reallocate it with

size_t y = 350;
{
  myString* tmp = realloc(s, sizeof(myString) + y);
  if (!tmp) abort(); // or whatever
  tmp->len = y;
}
s = tmp;

To use this more comfortably you'd probably better wrap this into macros or inline functions.