How does mmap work?

Question

I am working on programs in Linux which needs mmap file from harddrive, but i have a question, what can make it fail. Like if all the memories are fragmented, which has only

Answer 1

mmap works by manipulating your process's page table, a data structure your CPU uses to map address spaces. The CPU will translate "virtual" addresses to "physical" ones, and does so according to the page table set up by your kernel.

When you access the mapped memory for the first time, your CPU generates a page fault. The OS kernel can then jump in, "fix up" the invalid memory access by allocating memory and doing file I/O in that newly allocated buffer, then continue your program's execution as if nothing happened.

mmap can fail if your process is out of address space, something to watch out for these days for 32-bit code, where all usable address can be mapped pretty quickly with large data sets. It can also fail for any of the things mentioned in the "Errors" section of the manpage.

Accessing memory inside a mapped region can also fail if the kernel has issues allocating memory or doing I/O. In that case your process will get a SIGBUS signal.