munmap

munmap is a system call in POSIX-compliant Unix-like systems that removes a memory region from a process’s address space. The call takes two arguments: a starting address and a length, and disables any mappings that cover that range. The pages within the specified region become available for other mappings or memory allocations. For anonymous mappings, the content is discarded; for file-backed mappings, the kernel may write back dirty pages as part of its normal paging behavior. If the unmapping range only partially overlaps an existing mapping, the overlapping portion is unmapped and the remaining parts remain mapped as separate regions.

The address must be aligned to the system’s page size, and the length must be non-zero and also aligned to the page size on most systems. The range should refer to mappings present in the process’s address space; attempting to unmap an invalid or non-mapped region may fail. Unmapping a region that contains mappings with different properties (such as read or write permissions) results in those mappings being adjusted or removed accordingly. After a successful unmap, any references to the now-unmapped region become invalid and accessing that region typically results in a segmentation fault.

Return value and errors: munmap returns 0 on success and -1 on error, with errno set to indicate the problem. Common error codes include EINVAL for invalid address alignment, zero length, or a range not properly aligned; EFAULT when addr points outside the process’s address space or to an invalid region.

See also: mmap, mprotect, mremap, msync.