Content-type: text/html Man page of mmap

mmap

Section: System Calls (2)
Index Return to Main Contents
 

NAME

mmap - Maps file system object into virtual memory  

SYNOPSIS

#include <sys/mman.h>

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

[Digital]  The following definition of the mmap() function does not conform to current standards and is supported only for backward compatibility (see standards(5)):

caddr_t mmap (        caddr_t addr,
       size_t len,
       int prot,
       int flags,
       int filedes,
       off_t off );
 

STANDARDS

Interfaces documented on this reference page conform to industry standards as follows:

mmap(): XPG4-UNIX

Refer to the standards(5) reference page for more information about industry standards and associated tags.

Standards: standards(5)  

PARAMETERS

Specifies the starting address of the new region (truncated to a page boundary). Specifies the length in bytes of the new region (rounded up to a page boundary). Specifies access permissions as either PROT_NONE or the result of a logical OR operation on any combination of PROT_READ, PROT_WRITE, and PROT_EXEC. Specifies attributes of the mapped region as the results of a bitwise-inclusive OR operation on any combination of MAP_FILE, MAP_ANONYMOUS, MAP_VARIABLE, MAP_FIXED, MAP_SHARED, MAP_PRIVATE, MAP_INHERIT, or MAP_UNALIGNED . Specifies the file to be mapped to the new mapped file region returned by open(). Specifies the offset into the file that gets mapped at address addr.  

DESCRIPTION

The mmap() function creates a new mapped file region, a new private region, or a new shared memory region.

The addr and len parameters 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 parameter 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 parameter specifies attributes of the mapped region. Values of the flags parameter are constructed by a bitwise-inclusive OR operation on the flags from the following list of symbolic names defined in the sys/mman.h file: Create a mapped file region. Create an unnamed memory region. Place region at the computed address. Place region at fixed address. Share changes. Changes are private. Region not unmapped by exec(2). Do not verify that the file offset is page aligned.

The MAP_FILE and MAP_ANONYMOUS flags control whether the region to be mapped 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 parameter: A new mapped file region is created, mapping the file associated with the filedes parameter. The off parameter specifies the file byte offset at which 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 parameter: A new memory region is created and initialized to all zeros. This memory region can be shared only with descendents of the current process. If the filedes parameter 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 parameter: 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 parameter: 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 will be unmapped.) If the requested address is null and MAP_FIXED is specified, the region is placed at the default exact mapping address for the region. It places the region at this value exactly, replacing previous mappings if necessary. The exact mapping address is determined from a combination of the flag and protection parameters passed to the mmap() function.

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 parameter: If the region is a mapped region, modifications to the region are visible to other processes that have 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, the msync() function can be used to ensure that this is done.)

If MAP_PRIVATE is set in the flags parameter: Modifications to the mapped region by the calling process are not visible to other processes that have 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 have mapped the region using MAP_SHARED are visible to other processes that have mapped the same region using MAP_PRIVATE.

If MAP_INHERIT is set in the flags parameter: The mapped region cannot be removed from the address space of the calling process by the exec(2) system call.

If MAP_UNALIGNED is set in the flags parameter: The mmap system call is prevented from verifying that the file offset is page aligned.

The prot parameter specifies the mapped region's access permissions. The sys/mman.h header file defines the following access options: The mapped region can be read. The mapped region can be written. The mapped region can be executed. The mapped region cannot be accessed.

The prot parameter can be either PROT_NONE or the results of a logical OR operation on any combination of PROT_READ, PROT_WRITE, and PROT_EXEC. If PROT_NONE is not specified, access permissions may be granted to the region in addition to those explicitly requested, except that write access is not granted unless PROT_WRITE is specified.

If the region is a mapped file that was mapped with MAP_SHARED, the mmap() function grants read or execute access permission only if the file descriptor used to map the file is open for reading, and grants write access permission only if the file descriptor used to map the file is open for writing. If the region is a mapped file that was mapped with MAP_PRIVATE, the mmap() function grants read, write, or execute access permission only if the file descriptor used to map the file is open for reading. If the region is a shared memory region that was mapped with MAP_ANONYMOUS, the mmap() function grants all requested access permissions.

After the successful completion of the mmap() function, the filedes parameter can be closed 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, which prevents the file data from being deallocated.

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.

After a call to the fork() function, the child process inherits all mapped regions with the same sharing and protection attributes as in the parent process. Each mapped file and shared memory region created with the mmap() function is unmapped by a successful call to any of the exec functions, unless that region is made inheritable across exec.  

NOTES

Note that memory acquired with the mmap() function is not locked, regardless of the previous use of the plock() function.  

RETURN VALUES

Upon successful completion, the mmap() function returns the address at which the mapping was placed. Otherwise, mmap() returns (caddr_t)-1 and sets errno to indicate the error.  

ERRORS

If the mmap() function fails, errno may be set to one of the following values:

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. The filedes parameter is not a valid file descriptor. The addr parameter is an invalid address. The flags or prot parameter is invalid, or the addr parameter or off parameter is not a multiple of the page size returned by sysconf(_SC_PAGE_SIZE). The file descriptor filedes refers to an object that cannot be mapped, such as a terminal. There is not enough address space to map len bytes. The addresses specified by the range [off, off + len) are invalid for filedes.  

RELATED INFORMATION

Functions: exec(2), fcntl(2), fork(2), madvise(2), mprotect(2), msync(2), munmap(2), plock(2), sysconf(3)

Standards: standards(5) delim off


 

Index

NAME
SYNOPSIS
STANDARDS
PARAMETERS
DESCRIPTION
NOTES
RETURN VALUES
ERRORS
RELATED INFORMATION

This document was created by man2html, using the manual pages.
Time: 02:40:14 GMT, October 02, 2010