CODEWITHSUNDEEP
cmemorymemory-managementheap-memoryrealloc
user13507084
user13507084

Reputation:

Can I realloc() an unallocated pointer?

Normally, realloc() is used to reallocate a previously allocated pointer:

int *DynamicArray = malloc(sizeof(int)*SomeArbitraryValue);
// Some rando code where DynamicArray is used
DynamicArray = realloc(DynamicArray, sizeof(int)*SomeOtherArbitraryValue)

But can realloc() be used to directly allocate memory? As in

int *DynamicArray = realloc(/*...*/);

Can realloc() handle non-preallocated pointers?

Upvotes: 1

Views: 1185

Answers (2)

R.. GitHub STOP HELPING ICE
R.. GitHub STOP HELPING ICE

Reputation: 215317

As the answer by @ZhangBoyang tells you, yes, assuming one possible interpretation of your question. However the way you've worded your question ("non-preallocated pointers") suggests you may have misunderstanding of some of the concepts involved. malloc does not "allocate pointers". It allocates objects, and pointers are values that point to objects. The lifetime of those objects are not connected to the lifetime of any particular pointer pointing to them.

Passing a pointer to realloc doesn't "do anything to" the pointer. It does something to the object pointed to by it. If the pointer is uninitialized or invalid, the call has undefined behavior and bad things will happen. If the pointer is a null pointer, however, realloc(ptr, n) will behave exactly as malloc(n).

Upvotes: 2

Zhang Boyang
Zhang Boyang

Reputation: 91

Yes, just pass NULL to it's first argument.

The manpage of realloc(3) says ...

The realloc() function changes the size of the memory block pointed to by ptr to size bytes. The contents will be unchanged in the range from the start of the region up to the minimum of the old and new sizes. If the new size is larger than the old size, the added memory will not be initialized. If ptr is NULL, then the call is equivalent to malloc(size), for all values of size; if size is equal to zero, and ptr is not NULL, then the call is equivalent to free(ptr). Unless ptr is NULL, it must have been returned by an earlier call to malloc(), calloc(), or realloc(). If the area pointed to was moved, a free(ptr) is done.

Upvotes: 5

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