Why malloc() Allocates Zero Physical RAM: Virtual Memory and Page Faults Explained

When a program calls malloc() to request memory in C or similar languages, the operating system does not immediately assign any physical RAM to the process. Instead, the kernel creates a virtual memory mapping by recording a new address range in the process's memory descriptor, leaving page table entries marked as invalid. Physical memory is only committed when the CPU actually tries to read or write a byte within that range, triggering a hardware event called a page fault. At that point, the CPU traps into the kernel, which walks a multi-level page table structure to locate or assign a real physical memory page. This lazy allocation strategy, managed through the Memory Management Unit and the kernel's virtual memory subsystem, allows operating systems to efficiently overcommit memory across many processes.