Technical Reference: Base Operating System and Extensions, Volume 1

mmap or mmap64 Subroutine

Purpose

Maps a file-system object into virtual memory.

Library

Standard C library (libc.a)

Syntax

#include <sys/types.h>
#include <sys/mman.h>

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

void *mmap64 (addr, len, prot, flags, fildes, off) void * addr; size_t len; int prot, flags, fildes; off64_t off;

Description

Attention: A file-system object should not be simultaneously mapped using both the mmap and shmat subroutines. Unexpected results may occur when references are made beyond the end of the object.

The mmap subroutine creates a new mapped file or anonymous memory region by establishing a mapping between a process-address space and a file-system object. Care needs to be taken when using the mmap subroutine if the program attempts to map itself. If the page containing executing instructions is currently referenced as data through an mmap mapping, the program will hang. Use the -H4096 binder option, and that will put the executable text on page boundaries. Then reset the file that contains the executable material, and view via an mmap mapping.

A region created by the mmap subroutine cannot be used as the buffer for read or write operations that involve a device. Similarly, an mmap region cannot be used as the buffer for operations that require either a pin or xmattach operation on the buffer.

Modifications to a file-system object are seen consistently, whether accessed from a mapped file region or from the read or write subroutine.

Child processes inherit all mapped regions from the parent process when the fork subroutine is called. The child process also inherits the same sharing and protection attributes for these mapped regions. A successful call to any exec subroutine will unmap all mapped regions created with the mmap subroutine.

The mmap64 subroutine is identical to the mmap subroutine except that the starting offset for the file mapping is specified as a 64-bit value. This permits file mappings which start beyond OFF_MAX.

In the large file enabled programming environment, mmap is redefined to be mmap64.

If the application has requested SPEC1170 compliant behavior then the st_atime field of the mapped file is marked for update upon successful completion of the mmap call.

If the application has requested SPEC1170 compliant behavior then the st_ctime and st_mtime fields of a file that is mapped with MAP_SHARED and PROT_WRITE are marked for update at the next call to msync subroutine or munmap subroutine if the file has been modified.

Parameters

addr	Specifies the starting address of the memory region to be mapped. When the MAP_FIXED flag is specified, this address must be a multiple of the page size returned by the sysconf subroutine using the _SC_PAGE_SIZE value for the Name parameter. A region is never placed at address zero, or at an address where it would overlap an existing region.
len	Specifies the length, in bytes, of the memory region to be mapped. The system performs mapping operations over whole pages only. If the len parameter is not a multiple of the page size, the system will include in any mapping operation the address range between the end of the region and the end of the page containing the end of the region.
prot	Specifies the access permissions for the mapped region. The sys/mman.h file defines the following access options: PROT_READ Region can be read. PROT_WRITE Region can be written. PROT_EXEC Region can be executed. PROT_NONE Region cannot be accessed. The prot parameter can be the PROT_NONE flag, or any combination of the PROT_READ flag, PROT_WRITE flag, and PROT_EXEC flag logically ORed together. If the PROT_NONE flag is not specified, access permissions may be granted to the region in addition to those explicitly requested. However, write access will not be granted unless the PROT_WRITE flag is specified. Note The operating system generates a SIGSEGV signal if a program attempts an access that exceeds the access permission given to a memory region. For example, if the PROT_WRITE flag is not specified and a program attempts a write access, a SIGSEGV signal results. If the region is a mapped file that was mapped with the MAP_SHARED flag, the mmap subroutine grants read or execute access permission only if the file descriptor used to map the file was opened for reading. It grants write access permission only if the file descriptor was opened for writing. If the region is a mapped file that was mapped with the MAP_PRIVATE flag, the mmap subroutine grants read, write, or execute access permission only if the file descriptor used to map the file was opened for reading. If the region is an anonymous memory region, the mmap subroutine grants all requested access permissions.
fildes	Specifies the file descriptor of the file-system object to be mapped. If the MAP_ANONYMOUS flag is set, the fildes parameter must be -1. After the successful completion of the mmap subroutine, the file specified by the fildes parameter may be closed without effecting the mapped region or 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. Note The mmap subroutine supports the mapping of regular files only. An mmap call that specifies a file descriptor for a special file fails, returning the ENODEV error. An example of a file descriptor for a special file is one that might be used for mapping either I/O or device memory.
off	Specifies the file byte offset at which the mapping starts. This offset must be a multiple of the page size returned by the sysconf subroutine using the _SC_PAGE_SIZE value for the Name parameter.
flags	Specifies attributes of the mapped region. Values for the flags parameter are constructed by a bitwise-inclusive ORing of values from the following list of symbolic names defined in the sys/mman.h file: MAP_FILE Specifies the creation of a new mapped file region by mapping the file associated with the fildes file descriptor. The mapped region can extend beyond the end of the file, both at the time when the mmap subroutine is called and while the mapping persists. This situation could occur if a file with no contents was created just before the call to the mmap subroutine, or if a file was later truncated. However, references to whole pages following the end of the file result in the delivery of a SIGBUS signal. Only one of the MAP_FILE and MAP_ANONYMOUS flags must be specified with the mmap subroutine. MAP_ANONYMOUS Specifies the creation of a new, anonymous memory region that is initialized to all zeros. This memory region can be shared only with the descendants of the current process. When using this flag, the fildes parameter must be -1. Only one of the MAP_FILE and MAP_ANONYMOUS flags must be specified with the mmap subroutine. MAP_ VARIABLE Specifies that the system select an address for the new memory region if the new memory region cannot be mapped at the address specified by the addr parameter, or if the addr parameter is null. Only one of the MAP_VARIABLE and MAP_FIXED flags must be specified with the mmap subroutine. MAP_FIXED Specifies that the mapped region be placed exactly at the address specified by the addr parameter. If the application has requested SPEC1170 complaint behavior and the mmap request is successful, the mapping replaces any previous mappings for the process' pages in the specified range. If the application has not requested SPEC1170 compliant behavior and a previous mapping exists in the range then the request fails. Only one of the MAP_VARIABLE and MAP_FIXED flags must be specified with the mmap subroutine. MAP_SHARED When the MAP_SHARED flag is set, modifications to the mapped memory region will be visible to other processes that have mapped the same region using this flag. If the region is a mapped file region, modifications to the region will be written to the file. You can specify only one of the MAP_SHARED or MAP_PRIVATE flags with the mmap subroutine. MAP_PRIVATE is the default setting when neither flag is specified unless you request SPEC1170 compliant behavior. In this case, you must choose either MAP_SHARED or MAP_PRIVATE.

MAP_PRIVATE

When the MAP_PRIVATE flag is specified, modifications to the mapped region by the calling process are not visible to other processes that have mapped the same region. If the region is a mapped file region, modifications to the region are not written to the file.

If this flag is specified, the initial write reference to an object page creates a private copy of that page and redirects the mapping to the copy. Until then, modifications to the page by processes that have mapped the same region with the MAP_SHARED flag are visible.

You can specify only one of the MAP_SHARED or MAP_PRIVATE flags with the mmap subroutine. MAP_PRIVATE is the default setting when neither flag is specified unless you request SPEC1170 compliant behavior. In this case, you must choose either MAP_SHARED or MAP_PRIVATE.

Return Values

If successful, the mmap subroutine returns the address at which the mapping was placed. Otherwise, it returns -1 and sets the errno global variable to indicate the error.

Error Codes

Under the following conditions, the mmap subroutine fails and sets the errno global variable to:

EACCES	The file referred to by the fildes parameter is not open for read access, or the file is not open for write access and the PROT_WRITE flag was specified for a MAP_SHARED mapping operation. Or, the file to be mapped has enforced locking enabled and the file is currently locked.
EBADF	The fildes parameter is not a valid file descriptor, or the MAP_ANONYMOUS flag was set and the fildes parameter is not -1.
EFBIG	The mapping requested extends beyond the maximum file size associated with fildes.
EINVAL	The flags or prot parameter is invalid, or the addr parameter or off parameter is not a multiple of the page size returned by the sysconf subroutine using the _SC_PAGE_SIZE value for the Name parameter.
EINVAL	The application has requested SPEC1170 compliant behavior and the value of flags is invalid (neither MAP_PRIVATE nor MAP_SHARED is set).
EMFILE	The application has requested SPEC1170 compliant behavior and the number of mapped regions would excedd and implementation-dependent limit (per process or per system).
ENODEV	The fildes parameter refers to an object that cannot be mapped, such as a terminal.
ENOMEM	There is not enough address space to map len bytes, or the application has not requested Single UNIX Specification, Version 2 compliant behavior and the MAP_FIXED flag was set and part of the address-space range (addr, addr+len) is already allocated.
ENXIO	The addresses specified by the range (off, off+len) are invalid for the fildes parameter.
EOVERFLOW	The mapping requested extends beyond the offset maximum for the file description associated with fildes.

Related Information

The exec (exec: execl, execle, execlp, execv, execve, execvp, or exect Subroutine) subroutine, fork (fork, f_fork, or vfork Subroutine) subroutine, munmap (munmap Subroutine) subroutine, read subroutine, shmat subroutine, sysconf subroutine, write subroutine.

The pin kernel service, xmattach kernel service.

List of Memory Manipulation Services, List of Memory Mapping Services, Understanding Memory Mapping in AIX 5L Version 5.2 General Programming Concepts: Writing and Debugging Programs.