mmap(2) [opendarwin man page]
MMAP(2) BSD System Calls Manual MMAP(2) NAME
mmap -- map files or devices into memory SYNOPSIS
#include <sys/types.h> #include <sys/mman.h> void * mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset); DESCRIPTION
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, start- ing at byte offset offset. If offset or len is not a multiple of the pagesize, the mapped region may extend past the specified range. 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. 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. Sharing, mapping type and options are specified in the flags argument by or'ing the following values: MAP_ANON Map anonymous memory not associated with any specific file. The file descriptor used for creating MAP_ANON regions is used only for naming, and may be specified as -1 if no name is associated with the region. MAP_FILE Mapped from a regular file or character-special device memory. (This is the default mapping type, and need not be specified.) 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_PRIVATE Modifications are private. 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. RETURN VALUES
Upon successful completion, mmap returns a pointer to the mapped region. Otherwise, a value of -1 is returned and errno is set to indicate the error. ERRORS
Mmap() will fail if: [EACCES] The flag PROT_READ was specified as part of the prot parameter and fd was not open for reading. The flags PROT_WRITE and MAP_SHARED were specified as part of the flags and prot parameters and fd was not open for writing. [EBADF] fd is not a valid open file descriptor. [EINVAL] MAP_FIXED was specified and the parameter was not page aligned. fd did not reference a regular or character special file. [ENOMEM] MAP_FIXED was specified and the addr parameter wasn't available. MAP_ANON was specified and insufficient memory was available. SEE ALSO
getpagesize(2), msync(2), munmap(2), mprotect(2), madvise(2), mincore(2), mlock(2) 4th Berkeley Distribution June 4, 1993 4th Berkeley Distribution
Check Out this Related Man Page
MMAP(2) Linux Programmer's Manual MMAP(2) NAME
mmap, munmap - map or unmap files or devices into memory SYNOPSIS
#include <sys/mman.h> #ifdef _POSIX_MAPPED_FILES void * mmap(void *start, size_t length, int prot , int flags, int fd, off_t offset); int munmap(void *start, size_t length); #endif DESCRIPTION
The mmap function asks to map length bytes starting at offset offset from the file (or other object) specified by the file descriptor fd into memory, preferably at address start. This latter address is a hint only, and is usually specified as 0. The actual place where the object is mapped is returned by mmap, and is never 0. The prot argument describes the desired memory protection (and must not conflict with the open mode of the file). It is either PROT_NONE or is the bitwise OR of one or more of the other PROT_* flags. PROT_EXEC Pages may be executed. PROT_READ Pages may be read. PROT_WRITE Pages may be written. PROT_NONE Pages may not be accessed. 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. It has bits MAP_FIXED Do not select a different address than the one specified. If the specified address cannot be used, mmap will fail. If MAP_FIXED is specified, start must be a multiple of the pagesize. Use of this option is discouraged. MAP_SHARED Share this mapping with all other processes that map this object. Storing to the region is equivalent to writing to the file. The file may not actually be updated until msync(2) or munmap(2) are called. MAP_PRIVATE Create a private copy-on-write mapping. Stores to the region do not affect the original file. It is unspecified whether changes made to the file after the mmap call are visible in the mapped region. You must specify exactly one of MAP_SHARED and MAP_PRIVATE. The above three flags are described in POSIX.1b (formerly POSIX.4) and SUSv2. Linux also knows about the following non-standard flags: MAP_DENYWRITE This flag is ignored. (Long ago, it signalled that attempts to write to the underlying file should fail with ETXTBUSY. But this was a source of denial-of-service attacks.) MAP_EXECUTABLE This flag is ignored. MAP_NORESERVE (Used together with MAP_PRIVATE.) Do not reserve swap space pages for this mapping. When swap space is reserved, one has the guaran- tee that it is possible to modify this private copy-on-write region. When it is not reserved one might get SIGSEGV upon a write when no memory is available. MAP_LOCKED This flag is ignored. MAP_GROWSDOWN Used for stacks. Indicates to the kernel VM system that the mapping should extend downwards in memory. MAP_ANONYMOUS The mapping is not backed by any file; the fd and offset arguments are ignored. This flag in conjunction with MAP_SHARED is imple- mented since Linux 2.4. MAP_ANON Alias for MAP_ANONYMOUS. Deprecated. MAP_FILE Compatibility flag. Ignored. Some systems document the additional flags MAP_AUTOGROW, MAP_AUTORESRV, MAP_COPY, and MAP_LOCAL. fd should be a valid file descriptor, unless MAP_ANONYMOUS is set, in which case the argument is ignored. offset should be a multiple of the page size as returned by getpagesize(2). Memory mapped by mmap is preserved across fork(2), with the same attributes. A file is mapped in multiples of the page size. For a file that is not a multiple of the page size, the remaining memory is zeroed when mapped, and writes to that region are not written out to the file. The effect of changing the size of the underlying file of a mapping on the pages that correspond to added or removed regions of the file is unspecified. The munmap system call deletes the mappings for the specified address range, and causes further references to addresses within the range to generate invalid memory references. The region is also automatically unmapped when the process is terminated. On the other hand, closing the file descriptor does not unmap the region. The address start must be a multiple of the page size. All pages containing a part of the indicated range are unmapped, and subsequent ref- erences to these pages will generate SIGSEGV. It is not an error if the indicated range does not contain any mapped pages. For file-backed mappings, the st_atime field for the mapped file may be updated at any time between the mmap() and the corresponding unmap- ping; the first reference to a mapped page will update the field if it has not been already. The st_ctime and st_mtime field for a file mapped with PROT_WRITE and MAP_SHARED will be updated after a write to the mapped region, and before a subsequent msync() with the MS_SYNC or MS_ASYNC flag, if one occurs. RETURN VALUE
On success, mmap returns a pointer to the mapped area. On error, MAP_FAILED (-1) is returned, and errno is set appropriately. On success, munmap returns 0, on failure -1, and errno is set (probably to EINVAL). ERRORS
EBADF fd is not a valid file descriptor (and MAP_ANONYMOUS was not set). EACCES A file descriptor refers to a non-regular file. Or MAP_PRIVATE was requested, but fd is not open for reading. Or MAP_SHARED was requested and PROT_WRITE is set, but fd is not open in read/write (O_RDWR) mode. Or PROT_WRITE is set, but the file is append-only. EINVAL We don't like start or length or offset. (E.g., they are too large, or not aligned on a PAGESIZE boundary.) ETXTBSY MAP_DENYWRITE was set but the object specified by fd is open for writing. EAGAIN The file has been locked, or too much memory has been locked. ENOMEM No memory is available, or the process's maximum number of mappings would have been exceeded. ENODEV The underlying filesystem of the specified file does not support memory mapping. Use of a mapped region can result in these signals: SIGSEGV Attempted write into a region specified to mmap as read-only. SIGBUS Attempted access to a portion of the buffer that does not correspond to the file (for example, beyond the end of the file, including the case where another process has truncated the file). CONFORMING TO
SVr4, POSIX.1b (formerly POSIX.4), 4.4BSD, SUSv2. SVr4 documents additional error codes ENXIO and ENODEV. SUSv2 documents additional error codes EMFILE and EOVERFLOW. SEE ALSO
getpagesize(2), mmap2(2), mremap(2), msync(2), shm_open(2), B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391. Linux 2.3.51 2000-03-25 MMAP(2)