# Avoiding double allocation for structs and data

Let’s say you want to allocate a struct that contains a pointer to other data that also needs allocation. The natural way to do it is calling malloc twice, one for the container and one for the data.

 `````` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 `````` ``````/* The natural way to allocate a struct and its data */ struct sample_type { float A; uint32_t B; uint8_t C; bool D; bool E; bool F; bool G; }; struct sample_struct { uint32_t Count; sample_type *Data; }; uint32_t Count = 5; sample_struct *Container = (sample_struct*)malloc(sizeof(sample_struct)); Container->Count = Count; Container->Data = (sample_type*)malloc(sizof(sample_type) * Count); // Do something to the data for(int i = 0; i < Container->Count; i++) { Container->Data[i] = {}; } ``````

Something doesn’t feel right, these two allocations are going to be used together, one after the other but there is no guarantee they will be contiguous in memory, a cache miss is likely to happen everytime you access the data.

Here is one way to use just a single allocation for the container and its data.

 `````` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 `````` ``````/* Using a single allocation */ struct sample_type { float A; uint32_t B; uint8_t C; bool D; bool E; bool F; bool G; }; struct sample_struct { uint32_t Count; sample_type *Data; }; uint32_t Count = 5; sample_struct *Container = (sample_struct*)malloc( sizeof(sample_struct) + sizeof(sample_type) * Count); Container->Count = Count; Container->Data = (sample_type*)(Container + 1); // Do something to the data for(int i = 0; i < Container->Count; i++) { Container->Data[i] = {}; } ``````

If you ask me the second version is better, a single call to malloc and things that belong together are adjacent in memory.

There is something you need to be aware of while using this trick, Data should always be the last member of the struct.

 `````` 1 2 3 4 5 6 7 8 9 10 11 12 13 `````` ``````struct sample_struct { uint32_t Count; sample_type *Data; float Other; // WRONG, writing to this variable will overwrite values in the Data array and vice versa. }; struct sample_struct { uint32_t Count; float Other; // CORRECT, Remember to add struct members before the Data array. sample_type *Data; }; ``````