What function is critical in ensuring the security integrity of copied data to a buffer?

Length checking is critical in ensuring the security integrity of copied data to a buffer because it helps prevent buffer overflow vulnerabilities. When data is copied to a buffer, if the size of the incoming data exceeds the buffer's allocated size, it can overwrite adjacent memory locations. This can lead to unpredictable behavior, including the potential for executing arbitrary code, which poses serious security risks.

By implementing length checking, the program verifies that the size of the incoming data does not exceed the buffer's capacity before performing the copy operation. This acts as a safeguard to ensure that the only valid and intended data is processed, thereby enhancing the integrity and security of the application.