mmap(2) - Manual pages

NAME

mmap — map files or devices into memory

SYNOPSIS

#include <sys/mman.h>

void *
mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);

The mmap function causes the pages starting at addr and continuing for at most len bytes to be mapped from the object described by fd, starting at byte offset offset. If len is not a multiple of the pagesize, the mapped region may extend past the specified range. Any such extension beyond the end of the mapped object will be zero-filled.

If addr is non-zero, it is used as a hint to the system. (As a convenience to the system, the actual address of the region may differ from the address supplied.) If addr is zero, an address will be selected by the system. The actual starting address of the region is returned. A successful mmap deletes any previous mapping in the allocated address range.

The protections (region accessibility) are specified in the prot argument by OR'ing the following values:

PROT_EXEC: Pages may be executed.
PROT_READ: Pages may be read.
PROT_WRITE: Pages may be written.
PROT_NONE: Placeholder when requesting no access permission.

As a NetBSD extension, the PROT_MPROTECT macro can be used to request additional permissions for later use with mprotect(2). For example PROT_MPROTECT(PROT_READ) requests that future PROT_READ mappings are allowed and can be enabled using mprotect(2), but does not currently grant read mappings to the returned memory segment. This is necessary for switching pages between writable and executable when PaX MPROTECT restrictions are in place. See mremap(2) for a sample use case.

Note that, due to hardware limitations, on some platforms PROT_WRITE may imply PROT_READ, and PROT_READ may imply PROT_EXEC. Portable programs should not rely on these flags being separately enforceable.

The flags parameter specifies the type of the mapped object, mapping options and whether modifications made to the mapped copy of the page are private to the process or are to be shared with other references. Note that either MAP_SHARED or MAP_PRIVATE must be specified. Sharing, mapping type and options are specified in the flags argument by OR'ing the following values:

MAP_ALIGNED(n)

Request that the allocation be aligned to the given boundary. The parameter n should be the base 2 logarithm of the desired alignment (e.g., to request alignment to 16K, use 14 as the value for n). The alignment must be equal to or greater than the platform's page size as returned by sysconf(3) with the _SC_PAGESIZE request. The following constants are defined for convenience:

MAP_ALIGNMENT_64KB
MAP_ALIGNMENT_16MB
MAP_ALIGNMENT_4GB
MAP_ALIGNMENT_1TB
MAP_ALIGNMENT_256TB
MAP_ALIGNMENT_64PB

MAP_ANON

Map anonymous memory not associated with any specific file. The file descriptor is not used for creating MAP_ANON regions, and must be specified as -1. The mapped memory will be zero filled.

MAP_FILE

Mapped from a regular file or character-special device memory. Read accesses beyond the end of of the file or device but less than the current page size will be zero-filled. Write accesses beyond the end of the file or device but less than the current page size will not affect the file or device. References beyond the end of file that are beyond the current page size will result in the delivery of SIGBUS signal.

MAP_FIXED

Do not permit the system to select a different address than the one specified. If the specified address cannot be used, mmap will fail. If MAP_FIXED is specified, addr must be a multiple of the pagesize. Use of this option is discouraged.

MAP_HASSEMAPHORE

Notify the kernel that the region may contain semaphores and that special handling may be necessary.

MAP_INHERIT

Permit regions to be inherited across execve(2) system calls.

MAP_NORESERVE

Only reserve address space, but do not reserve swap space or any other resources for this mapping. Access to the address space is not guaranteed and may result in a segmentation violation. Unimplemented.

MAP_REMAPDUP

Only valid for mremap(2).

MAP_RENAME

Assign the referenced private pages to the file descriptor provided. Unimplemented.

MAP_STACK

Allocate a memory segment that can be used either for a process or thread stack. This currently has no effect, but its use is reserved for architectures that might require special treatment of that address space. Unimplemented.

MAP_TRYFIXED

Attempt to use the address addr even if it falls within the normally protected process data or text segment memory regions. If the requested region of memory is actually present in the memory map, a different address will be selected as if MAP_TRYFIXED had not been specified. If addr is NULL, this flag is ignored and the system will select a mapping address.

MAP_WIRED

Lock the mapped region into memory as with mlock(2).

MAP_PRIVATE

Modifications made by this process are private, however modifications made by other processes using MAP_SHARED will be seen.

MAP_SHARED

Modifications are shared.

The close(2) function does not unmap pages, see munmap(2) for further information.

The current design does not allow a process to specify the location of swap space. In the future we may define an additional mapping type, MAP_SWAP, in which the file descriptor argument specifies a file or device to which swapping should be done.

If MAP_FIXED is not specified, the system will attempt to place the mapping in an unused portion of the address space chosen to minimize possible collision between mapped regions and the heap.

RETURN VALUES

Upon successful completion, mmap returns a pointer to the mapped region. Otherwise, a value of MAP_FAILED is returned and errno is set to indicate the error. The symbol MAP_FAILED is defined in the header <sys/mman.h>. No successful return from mmap() will return the value MAP_FAILED.