If size is zero, then the block pointed to by memblock is freed; the return value is NULL , and memblock is left pointing at a freed block. The return value points to a storage space that is suitably aligned for storage of any type of object. To get a pointer to a type other than void , use a type cast on the return value. The realloc function changes the size of an allocated memory block.
The memblock argument points to the beginning of the memory block. If memblock is NULL , realloc behaves the same way as malloc and allocates a new block of size bytes. If memblock is not NULL , it should be a pointer returned by a previous call to calloc , malloc , or realloc. The size argument gives the new size of the block, in bytes. To correct the defect, assign the result of the reallocation function to a temporary, and then replace the original pointer after successful reallocation.
This warning might generate noise if there is a live alias to the buffer-to-be-reallocated at the time of the assignment of the result of the reallocation function. Skip to main content. This browser is no longer supported. Find centralized, trusted content and collaborate around the technologies you use most. Connect and share knowledge within a single location that is structured and easy to search. I have this array of structs and this function takes a pointer to the pointer of the array.
The original size is 2, so whenever it reaches the size, I need to realloc and double the size. When this code runs, I get an invalid old size error from the realloc. What am I doing wrong? In your function, inv is presumably the address of a pointer variable. Its the value of that variable you want to pass to realloc.
Because you need to use realloc , you should use count to reference the array. You can only realloc data if the passed pointer is a valid allocated pointer, not a pointer within the allocated zone.
So you have to store your inv data so you can use realloc. Pointer on the current element must be a different variable. It will return you a pointer to memory address, say, 0x5, but it actually stores the number 13 right before it, in 0x4. That way when you call free on 0x5, it looks at the byte before it 0x4 , sees that it contains the number 13, and then it knows it has to free 13 bytes so it won't just free 0x5, it will also free 0x6, 0x7, 0x8, etc.
So if you realloc an uninitialized pointer which could point to anything , it will look at the byte right before it, and maybe it contains the value , who knows? After allocating the new memory, it will free bytes there because it assumes that's what you used to have allocated that you now want to realloc. With the specific code shown, there is no problem with using the null pointer initially. Jonathan Leffler Jonathan Leffler k gold badges silver badges bronze badges.
This was a really beautiful answer. Unfortunately I already accepted another also good one It's too bad there's no function for the purpose of shrinking an allocation without invalidating any pointers to it, since code could simply assume that such a function would always succeed even if the system can't Is there any danger in allocating memory using realloc using the initially NULL-valued ptr? No, that would exactly be like a malloc. Community Bot 1 1 1 silver badge. Shahbaz Shahbaz Sign up or log in Sign up using Google.
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