C - allocate memory correctly for a structure containing an array of another structure

Question

I want to allocate memory for a structure that contains an array of another structure called table . I found that when assigning pointers to functions at the end, the variables in the linkedObjects array linkedObjects corrupted, so I believe that my dynamic memory processing is incorrect.

Here's how I do it now:

 typedef struct Object { void *key; struct Object *top; struct Object *next; } Object; typedef struct Table{ Object *linkedObjects; size_t size, originalSize; HashFcn hfun; PrintFcn pfun; ComparisonFcn fcomp; } Table; TableP CreateTable(size_t tableSize, HashFcn hfun, PrintFcn pfun, ComparisonFcn fcomp) { int i; struct Table *table = malloc(sizeof(table)); if (table==NULL) { ReportError(MEM_OUT); return NULL; } table->linkedObjects = NULL; table->linkedObjects = malloc(tableSize * sizeof(Object)); for(i=0;i<tableSize;i++) { table->linkedObjects[i].next = malloc( MAX_IN_LIST*sizeof(Object) ); table->linkedObjects[i].top = malloc( MAX_IN_LIST*sizeof(Object) ); table->linkedObjects[i].key = NULL; table->linkedObjects[i].top->key = NULL; table->linkedObjects[i].next->key = NULL; if (table->linkedObjects[i].next == NULL) { ReportError(MEM_OUT); return NULL; } } table->size = tableSize; table->originalSize = tableSize; table->hfun = hfun; table->pfun = pfun; table->fcomp = fcomp; return table; } 

Edit: I edited the function code to reflect the answers:

 TableP CreateTable(size_t tableSize, HashFcn hfun, PrintFcn pfun, ComparisonFcn fcomp) { int i; struct Table *table = malloc(sizeof(table)); if (table==NULL) { ReportError(MEM_OUT); return NULL; } table->linkedObjects = NULL; table->linkedObjects = malloc(tableSize * sizeof(Object)); if (table->linkedObjects == NULL) { ReportError(MEM_OUT); return NULL; } for(i=0;i<tableSize;i++) { table->linkedObjects[i].next = NULL; table->linkedObjects[i].top = NULL; table->linkedObjects[i].key = NULL; } table->size = tableSize; table->originalSize = tableSize; table->hfun = hfun; table->pfun = pfun; table->fcomp = fcomp; //printf("%p\n", table->hfun); return table; } 

but still, when I get to the destination at the end, table->linkedObjects[0].key , which is null and the value 0x0 , will go into the value 0x8048cc0 . This happens when this line is executed:

 table->originalSize = tableSize; 

Other Edit: Confirmed that this happens randomly in recent calls (not just in the line above):

 table->size = tableSize; table->originalSize = tableSize; table->hfun = hfun; table->pfun = pfun; table->fcomp = fcomp; 

Answer 1

struct Table *table = malloc(sizeof(table));

it should be

struct Table *table = malloc(sizeof(table));

I sometimes like C.

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Answer 2

As usual, get rid of the habit of using type names under sizeof . This is how your memory allocations should have looked

 Table *table = malloc(sizeof *table); ... table->linkedObjects = malloc(tableSize * sizeof *table->linkedObjects); 

This will also fix the typo error in the first distribution.

Answer 3

 table->linkedObjects[i].next = malloc( MAX_IN_LIST*sizeof(Object) ); table->linkedObjects[i].top = malloc( MAX_IN_LIST*sizeof(Object) ); 

This does not seem to make sense. When I see next or top with collections, I expect one pointer to one Object (to the next element in the collection or to the first element in the collection).

You would like to do the following:

 for(i=0;i < (tableSize-1);i++) { table->linkedObjects[i].top = table->linkedObjects[0]; table->linkedObjects[i].next = table->linkedObjects[i+1]; table->linkedObjects[i].key = NULL; } 

This will allocate memory for each Object and after that sets the pointers.