#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);
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 (see ftruncate(S)).
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:
The parameter flags provides other information about the handling of the mapped pages. The options are defined in <sys/mman.h> as:
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 filesystem conditions, including quota exceeded errors.
mmap adds an extra reference to the object associated
with the file descriptor fd which is not removed
by a subsequent close on that file descriptor.
This reference is removed when the entire range is unmapped
(explicitly or implicitly).
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 */
AT&T SVID Issue 3;
IEEE POSIX Std 1003.4 1992 System Application Program Interface (API) Realtime Extension [C Language] (ISO/IEC 9945-1).