mmap

Maps file system object into virtual memory.

Format

#include  <types.h>

#include  <mman.h>

void mmap  (void *addr, size_t len, int
           prot, int flags, int filesdes,
           off_t off);

Arguments

addr

The starting address of the new region (truncated to a page boundary).

len

The length in bytes of the new region (rounded up to a page boundary).

prot

Access permission, as defined in the <mman.h> header file. Specify either PROT_NONE, PROT_READ, or PROT_WRITE.

flags

Attributes of the mapped region as the results of a bitwise- inclusive OR operation on any combination of the following:

• MAP_FILE or MAP_ANONYMOUS

• MAP_VARIABLE or MAP_FIXED

• MAP_SHARED or MAP_PRIVATE

filedes

The file that you want to map to the new mapped file region returned by the open function.

off

The offset into the file that gets mapped at address addr.

Description

The addr and len arguments specify the requested starting address and length in bytes for the new region. The address is a multiple of the page size returned by sysconf(_SC_PAGE_SIZE).

If the len argument is not a multiple of the page size returned by sysconf(_SC_PAGE_SIZE), then the result of any reference to an address between the end of the region and the end of the page containing the end of the region is undefined.

The flags argument specifies attributes of the mapped region. Values for flags are constructed by a bitwise- inclusive OR operation on the flags from the following list of symbolic names defined in the <mman.h> header file:

MAP_FILE	Create a mapped file region.
MAP_ANONYMOUS	Create an unnamed memory region.
MAP_VARIABLE	Place region at the computed address.
MAP_FIXED	Place region at fixed address.
MAP_ SHARED	Share changes.
MAP_ PRIVATE	Changes are private.

The MAP_FILE and MAP_ANONYMOUS flags control whether the region you want to map is a mapped file region or an anonymous shared memory region. One of these flags must be selected.

If MAP_FILE is set in the flags argument:

• A new mapped file region is created, mapping the file associated with the filedes argument.

• The off argument specifies the file byte offset where the mapping starts. This offset must be a multiple of the page size returned by sysconf(_SC_PAGE_SIZE).

• If the end of the mapped file region is beyond the end of the file, the result of any reference to an address in the mapped file region corresponding to an offset beyond the end of the file is unspecified.

If MAP_ANONYMOUS is set in the flags argument:

• A new memory region is created and initialized to all zeros.

• If the filedes argument is not -1, the mmap function fails.

The new region is placed at the requested address if the requested address is not null and it is possible to place the region at this address. When the requested address is null or the region cannot be placed at the requested address, the MAP_VARIABLE and MAP_FIXED flags control the placement of the region. One of these flags must be selected.

If MAP_VARIABLE is set in the flags argument:

• If the requested address is null or if it is not possible for the system to place the region at the requested address, the region is placed at an address selected by the system.

If MAP_FIXED is set in the flags argument:

• If the requested address is not null, the mmap function succeeds even if the requested address is already part of another region. (If the address is within an existing region, the effect on the pages within that region and within the area of the overlap produced by the two regions is the same as if they were unmapped. In other words, whatever is mapped between addr and addr + len is unmapped.)

• If the requested address is null and MAP_FIXED is specified, the results are undefined.

The MAP_PRIVATE and MAP_SHARED flags control the visibility of modifications to the mapped file or shared memory region. One of these flags must be selected.

If MAP_SHARED is set in the flags argument:

• If the region is a mapped region, modifications to the region are visible to other processes that mapped the same region using MAP_SHARED.

• If the region is a mapped file region, modifications to the region are written to the file. (Note that the modifications are not immediately written to the file because of buffer cache delay; that is, the write to the file does not occur until there is a need to reuse the buffer cache. If the modifications must be written to the file immediately, use the msync function to ensure that this is done.)

If MAP_PRIVATE is set in the flags argument:

• Modifications to the mapped region by the calling process are not visible to other processes that mapped the same region using either MAP_PRIVATE or MAP_SHARED.

• Modifications to the mapped region by the calling process are not written to the file.

It is unspecified whether modifications by processes that mapped the region using MAP_SHARED are visible to other processes that mapped the same region using MAP_PRIVATE.

The prot argument specifies access permissions for the mapped region. Specify one of the following:

PROT_NONE	No access
PROT_READ	Read-only
PROT_WRITE	Read/Write access

After the successful completion of the mmap function, you can close the filedes argument without effect on the mapped region or on the contents of the mapped file. Each mapped region creates a file reference, similar to an open file descriptor, that prevents the file data from being deallocated.

Note

The following rules apply to OpenVMS specific file references:

• Because of the additional file reference, if filedes is not opened for file sharing, mmap reopens it with file sharing enabled.

• The additional file reference that remains for mapped regions implies that a later open, fopen, or create call to the file that is mapped must specify file sharing.

Modifications made to the file using the write function are visible to mapped regions, and modifications to a mapped region are visible with the read function.

See also read, write, open, fopen, creat, and sysconf in this section.

Return Values

x

The address where the mapping is placed.

(caddr_t) -1

Indicates an error; errno is set to one of the following values: EACCES - The file referred to by filedes is not open for read access, or the file is not open for write access and PROT_WRITE was set for a MAP_SHARED mapping operation. EBADF - The filedes argument is not a valid file descriptor. EINVAL -The flags or prot argument is invalid, or the addr argument or off argument is not a multiple of the page size returned by sysconf(_SC_ PAGE_SIZE). Or MAP_ANONYMOUS was specified in flags and filedes is not -1. ENODEV - The file descriptor filedes refers to an object that cannot be mapped, such as a terminal. ENOMEM - There is not enough address space to map len bytes. ENXIO - The addresses specified by the range [off, off + len] are invalid for filedes. EFAULT - The addr argument is an invalid address.

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