mmap(2) SYSTEM CALLS mmap(2)
NAME
mmap - map pages of memory
SYNOPSIS
#include <sys/types.h>
#include <sys/mman.h>
caddrt mmap(caddrt addr, sizet len, int prot, int flags,
int fd, offt 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 func-
tion 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 map-
pings 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:
PROTREAD Page can be read.
PROTWRITE Page can be written.
PROTEXEC Page can be executed.
PROTNONE Page can not be accessed.
Not all implementations literally provide all possible com-
binations. PROTWRITE is often implemented as
PROTREAD|PROTWRITE and PROTEXEC as PROTREAD|PROTEXEC.
However, no implementation will permit a write to succeed
where PROTWRITE has not been set. The behavior of
PROTWRITE can be influenced by setting MAPPRIVATE in the
flags parameter, described below. The parameter flags pro-
vides other information about the handling of the mapped
pages. The options are defined in <sys/mman.h> as:
MAPSHARED Share changes.
MAPPRIVATE Changes are private.
MAPFIXED Interpret addr exactly.
MAPSHARED and MAPPRIVATE describe the disposition of write
references to the memory object. If MAPSHARED is speci-
fied, write references will change the memory object. If
MAPPRIVATE is specified, the initial write reference will
create a private copy of the memory object page and redirect
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mmap(2) SYSTEM CALLS mmap(2)
the mapping to the copy. Either MAPSHARED or MAPPRIVATE
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 MAPSHARED can change the object.
MAPFIXED informs the system that the value of pa must be
addr, exactly. The use of MAPFIXED is discouraged, as it
may prevent an implementation from making the most effective
use of system resources. When MAPFIXED 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 com-
plete 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 MAPFIXED 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 con-
straint, 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 (caddrt)-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 PROTWRITE
was specified for a MAPSHARED type mapping.
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mmap(2) SYSTEM CALLS mmap(2)
ENXIO Addresses in the range [off, off + len) are invalid
for fd.
EINVAL The arguments addr (if MAPFIXED was specified) or
off are not multiples of the page size as returned by
sysconf.
EINVAL The field in flags is invalid (neither MAPPRIVATE or
MAPSHARED).
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 MAPFIXED was specified and the range [addr, addr +
len) exceeds that allowed for the address space of a
process, or MAPFIXED 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 manipula-
tions 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((caddrt) 0, len, (PROTREAD | PROTWRITE),
MAPPRIVATE, fd, offset)
/* use data at address */
SEE ALSO
fcntl(2), fork(2), lockf(3C), mlockall(3C), mprotect(2),
munmap(2), plock(2), sysconf(2).
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