In Solidity, the free memory pointer is a critical concept when working directly with low-level memory.
Memory in Solidity is contiguous and linear, with a section reserved for dynamic storage, starting from the address stored in the free memory pointer. This pointer is stored at a specific location in memory (0x40) and tells the EVM where the next free slot in memory begins, allowing for efficient dynamic memory allocation.
The free memory pointer:
Helps manage memory usage efficiently by keeping track of where unallocated memory begins.
Is automatically adjusted by Solidity for standard high-level operations but needs to be manually managed when using inline assembly, especially if temporary overrides are necessary.
Let’s explore a more detailed example to understand how to manage the free memory pointer manually, including the importance of restoring it.
Example of Manually Managing the Free Memory Pointer in Assembly
In the following example, we create a function that:
- Caches the free memory pointer.
- Temporarily overrides it by storing three 32-byte values at that location.
- Executes some placeholder operation (using inline assembly).
- Restores the free memory pointer to its original value to avoid potential issues with memory corruption.
Memory in Solidity is contiguous and linear, with a section reserved for dynamic storage, starting from the address stored in the free memory pointer. This pointer is stored at a specific location in memory (0x40) and tells the EVM where the next free slot in memory begins, allowing for efficient dynamic memory allocation.
The free memory pointer:
Helps manage memory usage efficiently by keeping track of where unallocated memory begins.
Is automatically adjusted by Solidity for standard high-level operations but needs to be manually managed when using inline assembly, especially if temporary overrides are necessary.
Let’s explore a more detailed example to understand how to manage the free memory pointer manually, including the importance of restoring it.
Example of Manually Managing the Free Memory Pointer in Assembly
In the following example, we create a function that:
- Caches the free memory pointer.
- Temporarily overrides it by storing three 32-byte values at that location.
- Executes some placeholder operation (using inline assembly).
- Restores the free memory pointer to its original value to avoid potential issues with memory corruption.
If you do not restore the free memory pointer and any other logic is executed after this function, the free memory pointer remains corrupted which will result in issues if any operation attempts to store values in memory (which now attempts to store it at arg3 instead of 0x80)
