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HP-UX Reference Volume 3 of 5 > m
mmap(2) |
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NAMEmmap — map pages of memory SYNOPSIS#include <sys/mman.h> void *mmap(void *addr, size_t len, int prot, int flags, int fildes, off_t off); DESCRIPTIONNote: This manpage contains HP-UX extensions. The mmap() function establishes a mapping between a process' address space and a file. The format of the call is as follows: pa=mmap(addr, len, prot, flags, fildes, off); The mmap() function establishes a mapping between the process' address space at an address pa for len bytes and the file associated with the file descriptor fildes at offset off for len bytes. The value of pa is an unspecified function of the argument 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 file, respectively. If the size of the mapped file changes after the call to mmap(), the effect of references to portions of the mapped region that correspond to added or removed portions of the file is unspecified. The mmap() function is supported for regular files. Support for any other type of file is unspecified. The prot argument 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>:
Implementations need not enforce all combinations of access permissions. However, writes shall only be permitted when PROT_WRITE has been set. The flags argument provides other information about the handling of the mapped pages. The options are defined in <sys/mman.h>:
The MAP_PRIVATE and MAP_SHARED flags control the visibility of write references to the memory region. Exactly one of these flags must be specified. The mapping type is retained across a fork(). If MAP_SHARED is set in flags, write references to the memory region by the calling process may change the file and are visible in all MAP_SHARED mappings of the same portion of the file by any process of the same executable type. That is, an application compiled as a 32-bit process will be able to share the same mappings with other 32-bit processes, and an application compiled as a 64-bit process will be able to share the same mappings with other 64-bit processes. If a 64-bit and a 32-bit application want to share the same mapping, both MAP_ADDR32 and MAP_SHARED must be set in flags by the 64-bit application. The 32-bit application does not need to set MAP_ADDR32 in flags. When MAP_SHARED is set in flags, write references to the memory region by the calling process may change the file. Changes are visible in all 32-bit processes which specify MAP_SHARED and by all 64-bit processes which specify both MAP_SHARED and MAP_ADDR32 for the same portion of the file. If MAP_PRIVATE is set in flags, write references to the memory region by the calling process do not change the file and are not visible to any process in other mappings of the same portion of the file. It is unspecified whether write references by processes that have mapped the memory region using MAP_SHARED are visible to processes that have mapped the same portion of the file using MAP_PRIVATE. It is also unspecified whether write references to a memory region mapped with MAP_SHARED are visible to processes reading the file and whether writes to a file are visible to processes that have mapped the modified portion of that file, except for the effect of msync(). When MAP_FIXED is set in the flags argument, the implementation is informed that the value of pa must be addr, exactly. If MAP_FIXED is set, mmap() may return MAP_FAILED and set errno to EINVAL. If a MAP_FIXED request is successful, the mapping established by mmap() replaces any previous mappings for the process' pages in the range [pa, pa+len]. When MAP_FIXED is not set, the implementation uses addr in an unspecified manner to arrive at pa. The pa so chosen will be an area of the address space which the implementation deems suitable for a mapping of len bytes to the file. All implementations interpret an addr value of 0 as granting the implementation 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 implementation selects a value for pa, it never places a mapping at address 0, nor does it replace any extant mapping, nor map into dynamic memory allocation areas. The off argument is constrained to be aligned and sized according to the value returned by sysconf() when passed _SC_PAGESIZE or _SC_PAGE_SIZE. When MAP_FIXED is specified, the argument addr must also meet these constraints. The implementation performs mapping operations over whole pages. Thus, while the argument len need not meet a size or alignment constraint, the implementation will include, in any unmapping operation, any partial page specified by the range [pa, pa+len]. The implementation always zero-fills any partial page at the end of a memory region. Further, the implementation never writes out any modified portions of the last page of a file that are beyond the end of the mapped portion of the file. If the mapping established by mmap() extends into pages beyond the page containing the last byte of the file, an application reference to any of the pages in the mapping that are beyond the last page results in the delivery of a SIGBUS or SIGSEGV signal. The mmap() function adds an extra reference to the file associated with the file descriptor fildes which is not removed by a subsequent close() on that file descriptor. This reference is removed when there are no more mappings to the file. The st_atime field of the mapped file may be marked for update at any time between the mmap() call and the corresponding munmap() call. The initial read or write reference to a mapped region will cause the file's st_atime field to be marked for update if it has not already been marked for update. The st_ctime and st_mtime fields of a file that is mapped with MAP_SHARED and PROT_WRITE, will be marked for update at some point in the interval between a write reference to the mapped region and the next call to msync() with MS_ASYNC or MS_SYNC for that portion of the file by any process. If there is no such call, these fields may be marked for update at any time after a write reference if the underlying file is modified as a result. There may be implementation-dependent limits on the number of memory regions that can be mapped (per process or per system). If such a limit is imposed, whether the number of memory regions that can be mapped by a process is decreased by the use of shmat() is implementation-dependent. RETURN VALUEUpon successful completion, mmap() returns the address, (pa), at which the mapping was placed. Otherwise, it returns MAP_FAILED (defined in <sys/mman.h>) and sets errno to indicate the error. ERRORSThe mmap() function will fail if:
APPLICATION USAGEUse of mmap() may reduce the amount of memory available to other memory allocation functions. Use of MAP_FIXED may result in unspecified behavior in further use of brk(), sbrk(), malloc(), and shmat(). The use of MAP_FIXED is discouraged, as it may prevent an implementation from making the most effective use of resources. The application must ensure correct synchronization when using mmap() in conjunction with any other file access method, such as read() and write(), standard input/output, and shmat(). The mmap() function allows access to resources via address space manipulations, instead of read()/write(). Once a file is mapped, all a process has to do to access it is use the data at the address to which the file was mapped. So, using pseudo-code to illustrate the way in which an existing program might be changed to use mmap(), the following: fildes = open(...) lseek(fildes, some_offset) read(fildes, buf, len) /* use data in buf */ becomes: fildes = open(...) address = mmap(0, len, PROT_READ, MAP_PRIVATE, fildes, some_offset) /* use data at address */ SEE ALSOexec(2), fcntl(2), fork(2), lockf(2), msync(2), munmap(2), mprotect(2), shmop(2), sysconf(2). SYNOPSIS (HP-UX)#include <sys/mman.h> caddr_t mmap( caddr_t addr, size_t len, int prot, int flags, int fildes, off_t off); DESCRIPTION
The MAP_FILE and MAP_ANONYMOUS flags control whether the region to be mapped is a mapped file region or an anonymous shared memory region. Exactly one of these flags must be selected. If MAP_FILE is set in flags:
If MAP_ANONYMOUS is set in flags:
The MAP_VARIABLE and MAP_FIXED flags control the placement of the region as described below. Exactly one of these flags must be selected. If MAP_VARIABLE is set in flags:
If MAP_FIXED is set in flags:
If MAP_NORESERVE is set in flags:
If MAP_LOCAL is set in flags:
If MAP_IO is set in flags :
The prot argument can be PROT_NONE, or any combination of PROT_READ, PROT_WRITE, and PROT_EXEC OR'ed together. If PROT_NONE is not specified, the system may grant other access permissions to the region in addition to those explicitly requested, except that write access will not be granted unless PROT_WRITE is specified. mmap() cannot create a mapped file region unless the file descriptor used to map the file is open for reading. For a mapped file region that is mapped with MAP_SHARED, mmap() grants write access permission only if the file descriptor is open for writing. If a region was mapped with either MAP_PRIVATE or MAP_ANONYMOUS, mmap() grants all requested access permissions. After the successful completion of mmap(), fildes can be closed without effect on the mapped region or on the contents of the mapped file. Each mapped region creates a file reference, similar to an open file descriptor, that prevents the file data from being deallocated. Whether modifications made to the file using write() are visible to mapped regions, and whether modifications to a mapped region are visible with read(), is undefined except for the effect of msync(). If an enforcement-mode file lock is in effect for any range of a file, a call to mmap() to map any range of the file with access rights that would violate the lock fails. The msem_lock() and msem_unlock() semaphore interfaces can be used to coordinate shared access to a region created with the MAP_SHARED flag. The advisory locks of the lockf() or fcntl() interfaces have no effect on memory mapped access, but they can be used to coordinate shared access to a MAP_SHARED mapped file region. For a memory mapped file, the st_atime and st_mtime values returned by stat() are updated when a page in the memory mapped region is read from or written to the file system. After a call to fork(), the child process inherits all mapped regions with the same data and the same sharing and protection attributes as in the parent process. Each mapped file and anonymous memory region created with mmap() is unmapped upon process exit, and by a successful call to any of the exec functions. MAP_NORESERVE attribute is inherited across a fork() call; at the time of the fork(), swap space for a mapping is reserved in the child only for dirtied private pages that currently exist in the parent; thereafter the child's mapping reservation policy is as described above. A SIGBUS signal is delivered to a process when a write reference to a mapped file region would cause a file system error condition such as exceeding quota or file system space limits. A SIGBUS signal is delivered to a process upon a write reference to a region without PROT_WRITE protection, or any reference to a region with PROT_NONE protection. A call to mmap() with PROT_EXECUTE specified, but without PROT_WRITE specified for a MAP_SHARED|MAP_FILE mapping is treated by the system as the execution of the underlying file. This implies that such a call fails if the file is currently open for writing or mapped with MAP_SHARED|PROT_WRITE options by any process, and that if the call succeeds, the file cannot be opened for writing or subsequently mapped with MAP_SHARED|PROT_WRITE options as long as such mappings are present. A file's status as an active executable file is determined only at the time of an exec(), exit(), or mmap() operation. mprotect() operations on a MAP_FILE|MAP_SHARED mapping have no effect on the underlying file's status as an active executable file. ERRORS
DEPENDENCIESSeries 700/800Because the PA-RISC memory architecture utilizes a globally shared virtual address space between processes and discourages multiple virtual address translations to the same physical address, all concurrently existing MAP_SHARED mappings of a file range must share the same virtual address offsets and hardware translations. PA-RISC-based HP-UX systems allocate virtual address ranges for shared memory and shared mapped files in the range 0x80000000 through 0xefffffff for those applications compiled as 32-bit executables or for those 64-bit applications which specify MAP_SHARED and MAP_ADDR32 in the flags argument of the mmap() function. For applications compiled as 64-bit executables which specify MAP_SHARED and do not specify MAP_ADDR32, the shared mapped files are in the range 0x00000011 00000000 through 0x000003ff ffffffff and 0xc0000000 00000000 through 0xc00003ff ffffffff. These address ranges are used globally for all memory objects shared between processes. This implies the following:
MAP_FIXED is discouraged, but there are some applications which by design must fix pointer offsets into file data. The application must map the file at a specific address in order for the file offsets embedded in the file to make sense. Processes cannot control the usage of global virtual address space, but they can control what happens within their private data area. The Series 700/800 allows a single process to exclusively map a file MAP_SHARED into its private data space. When a process specifies MAP_SHARED and MAP_FIXED with a private data address (i.e. second quadrant for 32-bit executable, third quadrant for 64-bit executable), the kernel interprets this as an exclusive mapping of the file. The request will only succeed if no other processes in the system currently have that file mapped through MAP_SHARED. If the file is already mapped the caller receives an EBUSY error. If the call is successful, the calling process is the only process allowed to map that file using MAP_SHARED until it unmaps the file using munmap(). Because it is exclusive, the mmap() is not inherited across fork(). When a file is exclusively mapped only MAP_PRIVATE mappings are allowed by other processes. The following combinations of protection modes are supported:
If a MAP_PRIVATE mapping is created of a file for which a MAP_SHARED mapping exists, a separate copy of a page for the MAP_PRIVATE mapping is created at the time of the first access to the page through the private mapping. SEE ALSOfcntl(2), fork(2), truncate(2), lockf(2), madvise(2), creat64(2), mprotect(2), msem_init(2), msem_lock(2), msem_unlock(2), msync(2), munmap(2), rtctl(2), sysconf(2), mman(5), stat(5). |
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