mmap(2) — SYSTEM CALLS

NAME

mmap − map pages of memory

SYNOPSIS

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

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

DESCRIPTION

The function mmap establishes a mapping between a process’s address space and a virtual memory object. The format of the call is as follows:

pa = mmap(addr, len, prot, flags, fd, off);

mmap establishes a mapping between the process’s address space at an address pa for len bytes to the memory object represented by the file descriptor fd at offset off for len bytes. The value of pa is an implementation-dependent function of the parameter addr and values of flags, further described below. A successful mmap call returns pa as its result. The address ranges covered by [pa, pa + len) and [off, off + len) must be legitimate for the possible (not necessarily current) address space of a process and the object in question, respectively. mmap cannot grow a file.

The mapping established by mmap replaces any previous mappings for the process’s pages in the range [pa, pa + len).

The parameter prot determines whether read, write, execute, or some combination of accesses are permitted to the pages being mapped. The protection options are defined in sys/mman.h as:

PROT_READPage can be read.
PROT_WRITEPage can be written.
PROT_EXECPage can be executed.
PROT_NONEPage can not be accessed.

Not all implementations literally provide all possible combinations. PROT_WRITE is often implemented as PROT_READ|PROT_WRITE and PROT_EXEC as PROT_READ|PROT_EXEC. However, no implementation will permit a write to succeed where PROT_WRITE has not been set. The behavior of PROT_WRITE can be influenced by setting MAP_PRIVATE in the flags parameter, described below. See the System V Application Binary Interface for further information concerning combinations of the PROT_READ, PROT_WRITE, and PROT_EXEC flags.

The parameter flags provides other information about the handling of the mapped pages. The options are defined in sys/mman.h as:

MAP_SHAREDShare changes.
MAP_PRIVATEChanges are private.
MAP_FIXEDInterpret addr exactly.

MAP_SHARED and MAP_PRIVATE describe the disposition of write references to the memory object. If MAP_SHARED is specified, write references will change the memory object. If MAP_PRIVATE is specified, the initial write reference will create a private copy of the memory object page and redirect the mapping to the copy. Either MAP_SHARED or MAP_PRIVATE must be specified, but not both. The mapping type is retained across a fork(2).

Note that the private copy is not created until the first write; until then, other users who have the object mapped MAP_SHARED can change the object.

MAP_FIXED informs the system that the value of pa must be addr, exactly. The use of MAP_FIXED is discouraged, as it may prevent an implementation from making the most effective use of system resources.

When MAP_FIXED is not set, the system uses addr in an implementation-defined manner to arrive at pa. The pa so chosen will be an area of the address space which the system deems suitable for a mapping of len bytes to the specified object. All implementations interpret an addr value of zero as granting the system complete freedom in selecting pa, subject to constraints described below. A non-zero value of addr is taken to be a suggestion of a process address near which the mapping should be placed. When the system selects a value for pa, it will never place a mapping at address 0, nor will it replace any extant mapping, nor map into areas considered part of the potential data or stack “segments”.

The parameter off is constrained to be aligned and sized according to the value returned by sysconf. When MAP_FIXED is specified, the parameter addr must also meet these constraints. The system performs mapping operations over whole pages. Thus, while the parameter len need not meet a size or alignment constraint, the system will include, in any mapping operation, any partial page specified by the range [pa, pa + len).

The system will always zero-fill any partial page at the end of an object. Further, the system will never write out any modified portions of the last page of an object which are beyond its end. References to whole pages following the end of an object will result in the delivery of a SIGBUS signal. SIGBUS signals may also be delivered on various file system conditions, including quota exceeded errors.

RETURN VALUE

On success, mmap returns the address at which the mapping was placed (pa). On failure it returns (caddr_t)−1 and sets errno to indicate an error.

ERRORS

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

EAGAIN The mapping could not be locked in memory.

EBADF fd is not open.

EACCES
fd is not open for read, regardless of the protection specified, or fd is not open for write and PROT_WRITE was specified for a MAP_SHARED type mapping.

ENXIO Addresses in the range [off, off + len) are invalid for fd.

EINVAL The arguments addr (if MAP_FIXED was specified) or off are not multiples of the page size as returned by sysconf.

EINVAL The field in flags is invalid (neither MAP_PRIVATE or MAP_SHARED).

EINVAL The argument len has a value less than or equal to 0.

ENODEV
fd refers to an object for which mmap is meaningless, such as a terminal.

ENOMEM
MAP_FIXED was specified and the range [addr, addr + len) exceeds that allowed for the address space of a process, or MAP_FIXED was not specified and there is insufficient room in the address space to effect the mapping.

NOTES

mmap allows access to resources via address space manipulations instead of the read/write interface. Once a file is mapped, all a process has to do to access it is use the data at the address to which the object was mapped. Consider the following pseudo-code:

fd = open(...)
lseek(fd, offset)
read(fd, buf, len)
/∗ use data in buf ∗/

Here is a rewrite using mmap:

fd = open(...)
address = mmap((caddr_t) 0, len, (PROT_READ | PROT_WRITE),
MAP_PRIVATE, fd, offset)
/∗ use data at address ∗/

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