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diff --git a/man-pages-posix-2017/man3p/mmap.3p b/man-pages-posix-2017/man3p/mmap.3p new file mode 100644 index 0000000..4ba4a50 --- /dev/null +++ b/man-pages-posix-2017/man3p/mmap.3p @@ -0,0 +1,736 @@ +'\" et +.TH MMAP "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual" +.\" +.SH PROLOG +This manual page is part of the POSIX Programmer's Manual. +The Linux implementation of this interface may differ (consult +the corresponding Linux manual page for details of Linux behavior), +or the interface may not be implemented on Linux. +.\" +.SH NAME +mmap +\(em map pages of memory +.SH SYNOPSIS +.LP +.nf +#include <sys/mman.h> +.P +void *mmap(void *\fIaddr\fP, size_t \fIlen\fP, int \fIprot\fP, int \fIflags\fP, + int \fIfildes\fP, off_t \fIoff\fP); +.fi +.SH DESCRIPTION +The +\fImmap\fR() +function shall establish a mapping between an address space +of a process and a memory object. +.P +The +\fImmap\fR() +function shall be supported for the following memory objects: +.IP " *" 4 +Regular files +.IP " *" 4 +Shared memory objects +.IP " *" 4 +Typed memory objects +.P +Support for any other type of file is unspecified. +.P +The format of the call is as follows: +.sp +.RS 4 +.nf + +\fIpa\fR=\fImmap\fR(\fIaddr\fP, \fIlen\fP, \fIprot\fP, \fIflags\fP, \fIfildes\fP, \fIoff\fP); +.fi +.P +.RE +.P +The +\fImmap\fR() +function shall establish a mapping between the address space of the +process at an address +.IR pa +for +.IR len +bytes to the memory object represented by the file descriptor +.IR fildes +at offset +.IR off +for +.IR len +bytes. The value of +.IR pa +is an implementation-defined function of the parameter +.IR addr +and the values of +.IR flags , +further described below. A successful +\fImmap\fR() +call shall return +.IR pa +as its result. The address range starting at +.IR pa +and continuing for +.IR len +bytes shall be legitimate for the possible (not necessarily current) +address space of the process. The range of bytes starting at +.IR off +and continuing for +.IR len +bytes shall be legitimate for the possible (not necessarily current) +offsets in the memory object represented by +.IR fildes . +.P +If +.IR fildes +represents a typed memory object opened with either the +POSIX_TYPED_MEM_ALLOCATE flag or the POSIX_TYPED_MEM_ALLOCATE_CONTIG +flag, the memory object to be mapped +shall be that portion of the typed memory object allocated by the +implementation as specified below. In this case, if +.IR off +is non-zero, the behavior of +\fImmap\fR() +is undefined. If +.IR fildes +refers to a valid typed memory object that is not accessible from the +calling process, +\fImmap\fR() +shall fail. +.P +The mapping established by +\fImmap\fR() +shall replace any previous mappings for those whole pages containing +any part of the address space of the process starting at +.IR pa +and continuing for +.IR len +bytes. +.P +If the size of the mapped file changes after the call to +\fImmap\fR() +as a result of some other operation on the mapped file, the effect of +references to portions of the mapped region that correspond to added or +removed portions of the file is unspecified. +.P +If +.IR len +is zero, +\fImmap\fR() +shall fail and no mapping shall be established. +.P +The parameter +.IR prot +determines whether read, write, execute, or some combination of +accesses are permitted to the data being mapped. The +.IR prot +shall be either PROT_NONE +or the bitwise-inclusive OR of one or more of the other flags in +the following table, defined in the +.IR <sys/mman.h> +header. +.TS +center box tab(!); +cB | cB +lw(1.5i) | lw(2i). +Symbolic Constant!Description +_ +PROT_READ!Data can be read. +PROT_WRITE!Data can be written. +PROT_EXEC!Data can be executed. +PROT_NONE!Data cannot be accessed. +.TE +.P +If an implementation cannot support the combination of access types +specified by +.IR prot , +the call to +\fImmap\fR() +shall fail. +.P +An implementation may permit accesses other than those specified by +.IR prot ; +however, the implementation shall not permit a write to succeed +where PROT_WRITE has not been set and shall not permit any access where +PROT_NONE alone has been set. The implementation shall support at least +the following values of +.IR prot : +PROT_NONE, PROT_READ, PROT_WRITE, and the bitwise-inclusive OR of +PROT_READ and PROT_WRITE. The file descriptor +.IR fildes +shall have been opened with read permission, regardless of the +protection options specified. If PROT_WRITE is specified, the +application shall ensure that it has opened the file descriptor +.IR fildes +with write permission unless MAP_PRIVATE is specified in the +.IR flags +parameter as described below. +.P +The parameter +.IR flags +provides other information about the handling of the mapped data. +The value of +.IR flags +is the bitwise-inclusive OR of these options, defined in +.IR <sys/mman.h> : +.TS +center box tab(!); +cB | cB +lw(1.5i) | lw(2i). +Symbolic Constant!Description +_ +MAP_SHARED!Changes are shared. +MAP_PRIVATE!Changes are private. +MAP_FIXED!Interpret \fIaddr\fP exactly. +.TE +.P +It is implementation-defined whether MAP_FIXED shall be supported. +MAP_FIXED shall be supported on XSI-conformant systems. +.P +MAP_SHARED and MAP_PRIVATE describe the disposition of write references +to the memory object. If MAP_SHARED is specified, write references +shall change the underlying object. If MAP_PRIVATE is specified, +modifications to the mapped data by the calling process shall be visible +only to the calling process and shall not change the underlying object. +It is unspecified whether modifications to the underlying object done +after the MAP_PRIVATE mapping is established are visible through the +MAP_PRIVATE mapping. Either MAP_SHARED or MAP_PRIVATE can be +specified, but not both. The mapping type is retained across +\fIfork\fR(). +.P +The state of synchronization objects such as mutexes, semaphores, +barriers, and conditional variables placed in shared memory mapped with +MAP_SHARED becomes undefined when the last region in any process +containing the synchronization object is unmapped. +.P +When +.IR fildes +represents a typed memory object opened with either the +POSIX_TYPED_MEM_ALLOCATE flag or the +POSIX_TYPED_MEM_ALLOCATE_CONTIG flag, +\fImmap\fR() +shall, if there are enough resources available, map +.IR len +bytes allocated from the corresponding typed memory object which were +not previously allocated to any process in any processor that may +access that typed memory object. If there are not enough resources +available, the function shall fail. If +.IR fildes +represents a typed memory object opened with the +POSIX_TYPED_MEM_ALLOCATE_CONTIG flag, these allocated bytes shall be +contiguous within the typed memory object. If +.IR fildes +represents a typed memory object opened with the +POSIX_TYPED_MEM_ALLOCATE flag, these allocated bytes may be composed of +non-contiguous fragments within the typed memory object. If +.IR fildes +represents a typed memory object opened with neither the +POSIX_TYPED_MEM_ALLOCATE_CONTIG flag nor the POSIX_TYPED_MEM_ALLOCATE +flag, +.IR len +bytes starting at offset +.IR off +within the typed memory object are mapped, exactly as when mapping a +file or shared memory object. In this case, if two processes map an +area of typed memory using the same +.IR off +and +.IR len +values and using file descriptors that refer to the same memory pool +(either from the same port or from a different port), both processes +shall map the same region of storage. +.P +When MAP_FIXED is set in the +.IR flags +argument, the implementation is informed that the value of +.IR pa +shall be +.IR addr , +exactly. If MAP_FIXED is set, +\fImmap\fR() +may return MAP_FAILED and set +.IR errno +to +.BR [EINVAL] . +If a MAP_FIXED request is successful, then any previous mappings +or memory locks +for those whole pages containing any part of the address range +[\fIpa\fP,\fIpa\fP+\fIlen\fR) shall be removed, as if by an +appropriate call to +\fImunmap\fR(), +before the new mapping is established. +.P +When MAP_FIXED is not set, the implementation uses +.IR addr +in an implementation-defined manner to arrive at +.IR pa . +The +.IR pa +so chosen shall be an area of the address space that the implementation +deems suitable for a mapping of +.IR len +bytes to the file. All implementations interpret an +.IR addr +value of 0 as granting the implementation complete freedom in selecting +.IR pa , +subject to constraints described below. A non-zero value of +.IR 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 +.IR pa , +it never places a mapping at address 0, nor does it replace any extant +mapping. +.P +If MAP_FIXED is specified and +.IR addr +is non-zero, it shall have the same remainder as the +.IR off +parameter, modulo the page size as returned by +\fIsysconf\fR() +when passed _SC_PAGESIZE or _SC_PAGE_SIZE. The implementation may +require that off is a multiple of the page size. If MAP_FIXED is +specified, the implementation may require that +.IR addr +is a multiple of the page size. The system performs mapping operations +over whole pages. Thus, while the parameter +.IR len +need not meet a size or alignment constraint, the system shall include, +in any mapping operation, any partial page specified by the address +range starting at +.IR pa +and continuing for +.IR len +bytes. +.P +The system shall always zero-fill any partial page at the end of an +object. Further, the system shall never write out any modified +portions of the last page of an object which are beyond its end. +References within the address range starting at +.IR pa +and continuing for +.IR len +bytes to whole pages following the end of an object shall result in +delivery of a SIGBUS signal. +.P +An implementation may generate SIGBUS signals when a reference would +cause an error in the mapped object, such as out-of-space condition. +.P +The +\fImmap\fR() +function shall add an extra reference to the file associated with the +file descriptor +.IR fildes +which is not removed by a subsequent +\fIclose\fR() +on that file descriptor. This reference shall be removed when there are +no more mappings to the file. +.P +The last data access timestamp of the mapped file may be marked for +update at any time between the +\fImmap\fR() +call and the corresponding +\fImunmap\fR() +call. The initial read or write reference to a mapped region shall cause +the file's last data access timestamp to be marked for update if it has +not already been marked for update. +.P +The last data modification and last file status change timestamps +of a file that is mapped with MAP_SHARED and PROT_WRITE shall be +marked +for update at some point in the interval between a write reference to +the mapped region and the next call to +\fImsync\fR() +with MS_ASYNC or MS_SYNC for that portion of the file by any process. +If there is no such call and if the underlying file is modified +as a result of a write reference, then these timestamps shall be marked +for update at some time after the write reference. +.P +There may be implementation-defined limits on the number of memory +regions that can be mapped (per process or per system). +.P +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 +\fIshmat\fR() +is implementation-defined. +.P +If +\fImmap\fR() +fails for reasons other than +.BR [EBADF] , +.BR [EINVAL] , +or +.BR [ENOTSUP] , +some of the mappings in the address range starting at +.IR addr +and continuing for +.IR len +bytes may have been unmapped. +.SH "RETURN VALUE" +Upon successful completion, the +\fImmap\fR() +function shall return the address at which the mapping was placed (\c +.IR pa ); +otherwise, it shall return a value of MAP_FAILED and set +.IR errno +to indicate the error. The symbol MAP_FAILED is defined in the +.IR <sys/mman.h> +header. No successful return from +\fImmap\fR() +shall return the value MAP_FAILED. +.SH ERRORS +The +\fImmap\fR() +function shall fail if: +.TP +.BR EACCES +The +.IR fildes +argument is not open for read, regardless of the protection specified, +or +.IR fildes +is not open for write and PROT_WRITE was specified for a MAP_SHARED +type mapping. +.TP +.BR EAGAIN +The mapping could not be locked in memory, if required by +\fImlockall\fR(), +due to a lack of resources. +.TP +.BR EBADF +The +.IR fildes +argument is not a valid open file descriptor. +.TP +.BR EINVAL +The value of +.IR len +is zero. +.TP +.BR EINVAL +The value of +.IR flags +is invalid (neither MAP_PRIVATE nor MAP_SHARED is set). +.TP +.BR EMFILE +The number of mapped regions would exceed an implementation-defined +limit (per process or per system). +.TP +.BR ENODEV +The +.IR fildes +argument refers to a file whose type is not supported by +\fImmap\fR(). +.TP +.BR ENOMEM +MAP_FIXED was specified, and the range +[\fIaddr\fP,\fIaddr\fP+\fIlen\fR) exceeds that allowed for the +address space of a process; or, if MAP_FIXED was not specified and +there is insufficient room in the address space to effect the mapping. +.TP +.BR ENOMEM +The mapping could not be locked in memory, if required by +\fImlockall\fR(), +because it would require more space than the system is able to supply. +.TP +.BR ENOMEM +Not enough unallocated memory resources remain in the typed memory +object designated by +.IR fildes +to allocate +.IR len +bytes. +.TP +.BR ENOTSUP +MAP_FIXED or MAP_PRIVATE was specified in the +.IR flags +argument and the implementation does not support this functionality. +.RS 12 +.P +The implementation does not support the combination of accesses +requested in the +.IR prot +argument. +.RE +.TP +.BR ENXIO +Addresses in the range [\fIoff\fP,\fIoff\fP+\fIlen\fR) are invalid +for the object specified by +.IR fildes . +.TP +.BR ENXIO +MAP_FIXED was specified in +.IR flags +and the combination of +.IR addr , +.IR len , +and +.IR off +is invalid for the object specified by +.IR fildes . +.TP +.BR ENXIO +The +.IR fildes +argument refers to a typed memory object that is not accessible from +the calling process. +.TP +.BR EOVERFLOW +The file is a regular file and the value of +.IR off +plus +.IR len +exceeds the offset maximum established in the open file description +associated with +.IR fildes . +.br +.P +The +\fImmap\fR() +function may fail if: +.TP +.BR EINVAL +The +.IR addr +argument (if MAP_FIXED was specified) or +.IR off +is not a multiple of the page size as returned by +\fIsysconf\fR(), +or is considered invalid by the implementation. +.LP +.IR "The following sections are informative." +.SH EXAMPLES +None. +.SH "APPLICATION USAGE" +Use of +\fImmap\fR() +may reduce the amount of memory available to other memory allocation +functions. +.P +Use of MAP_FIXED may result in unspecified behavior in further use of +\fImalloc\fR() +and +\fIshmat\fR(). +The use of MAP_FIXED is discouraged, as it may prevent an +implementation from making the most effective use of resources. Most +implementations require that +.IR off +and +.IR addr +are multiples of the page size as returned by +\fIsysconf\fR(). +.P +The application must ensure correct synchronization when using +\fImmap\fR() +in conjunction with any other file access method, such as +\fIread\fR() +and +\fIwrite\fR(), +standard input/output, and +\fIshmat\fR(). +.P +The +\fImmap\fR() +function allows access to resources via address space manipulations, +instead of +\fIread\fR()/\c +\fIwrite\fR(). +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 +\fImmap\fR(), +the following: +.sp +.RS 4 +.nf + +fildes = open(...) +lseek(fildes, some_offset) +read(fildes, buf, len) +/* Use data in buf. */ +.fi +.P +.RE +.P +becomes: +.sp +.RS 4 +.nf + +fildes = open(...) +address = mmap(0, len, PROT_READ, MAP_PRIVATE, fildes, some_offset) +/* Use data at address. */ +.fi +.P +.RE +.SH RATIONALE +After considering several other alternatives, it was decided to adopt +the +\fImmap\fR() +definition found in SVR4 for mapping memory objects into process +address spaces. The SVR4 definition is minimal, in that it describes +only what has been built, and what appears to be necessary for a +general and portable mapping facility. +.P +Note that while +\fImmap\fR() +was first designed for mapping files, it is actually a general-purpose +mapping facility. It can be used to map any appropriate object, such +as memory, files, devices, and so on, into the address space of a +process. +.P +When a mapping is established, it is possible that the implementation +may need to map more than is requested into the address space of the +process because of hardware requirements. An application, however, +cannot count on this behavior. Implementations that do not use a paged +architecture may simply allocate a common memory region and return the +address of it; such implementations probably do not allocate any more +than is necessary. References past the end of the requested area are +unspecified. +.P +If an application requests a mapping that overlaps existing mappings +in the process, it might be desirable that an implementation detect +this and inform the application. However, if the program specifies a +fixed address mapping (which requires some implementation knowledge to +determine a suitable address, if the function is supported at all), +then the program is presumed to be successfully managing its own +address space and should be trusted when it asks to map over existing +data structures. Furthermore, it is also desirable to make as few +system calls as possible, and it might be considered onerous to +require an +\fImunmap\fR() +before an +\fImmap\fR() +to the same address range. This volume of POSIX.1\(hy2017 specifies that the new mapping +replaces any existing mappings (implying an automatic +\fImunmap\fR() +on the address range), following existing practice in this regard. +The standard developers also considered whether there should be a way +for new mappings to overlay existing mappings, but found no existing +practice for this. +.P +It is not expected that all hardware implementations are able to +support all combinations of permissions at all addresses. +Implementations are required to disallow write +access to mappings without write permission and to disallow access to +mappings without any access permission. Other than these restrictions, +implementations may allow access types other than those requested by +the application. For example, if the application requests only +PROT_WRITE, the implementation may also allow read access. A call to +\fImmap\fR() +fails if the implementation cannot support allowing all the access +requested by the application. For example, some implementations +cannot support a request for both write access and execute access +simultaneously. All implementations must support requests for no access, +read access, write access, and both read and write access. Strictly +conforming code must only rely on the required checks. These restrictions +allow for portability across a wide range of hardware. +.P +The MAP_FIXED address treatment is likely to fail for non-page-aligned +values and for certain architecture-dependent address ranges. +Conforming implementations cannot count on being able to choose address +values for MAP_FIXED without utilizing non-portable, +implementation-defined knowledge. Nonetheless, MAP_FIXED is provided +as a standard interface conforming to existing practice for utilizing +such knowledge when it is available. +.P +Similarly, in order to allow implementations that do not support +virtual addresses, support for directly specifying any mapping +addresses via MAP_FIXED is not required and thus a conforming +application may not count on it. +.P +The MAP_PRIVATE +function can be implemented efficiently when memory protection hardware +is available. When such hardware is not available, implementations can +implement such ``mappings'' +by simply making a real copy of the relevant data into process private +memory, though this tends to behave similarly to +\fIread\fR(). +.P +The function has been defined to allow for many different models of +using shared memory. However, all uses are not equally portable across +all machine architectures. In particular, the +\fImmap\fR() +function allows the system as well as the application to specify the +address at which to map a specific region of a memory object. The most +portable way to use the function is always to let the system choose +the address, specifying NULL as the value for the argument +.IR addr +and not to specify MAP_FIXED. +.P +If it is intended that a particular region of a memory object be mapped +at the same address in a group of processes (on machines where this is +even possible), then MAP_FIXED can be used to pass in the desired +mapping address. The system can still be used to choose the desired +address if the first such mapping is made without specifying MAP_FIXED, +and then the resulting mapping address can be passed to subsequent +processes for them to pass in via MAP_FIXED. The availability of a +specific address range cannot be guaranteed, in general. +.P +The +\fImmap\fR() +function can be used to map a region of memory that is larger than the +current size of the object. Memory access within the mapping but +beyond the current end of the underlying objects may result in SIGBUS +signals being sent to the process. The reason for this is that the +size of the object can be manipulated by other processes and can change +at any moment. The implementation should tell the application that a +memory reference is outside the object where this can be detected; +otherwise, written data may be lost and read data may not reflect +actual data in the object. +.P +Note that references beyond the end of the object do not extend the +object as the new end cannot be determined precisely by most virtual +memory hardware. Instead, the size can be directly manipulated by +\fIftruncate\fR(). +.P +Process memory locking does apply to shared memory regions, and the +MCL_FUTURE argument to +\fImlockall\fR() +can be relied upon to cause new shared memory regions to be +automatically locked. +.P +Existing implementations of +\fImmap\fR() +return the value \-1 when unsuccessful. Since the casting of this +value to type +.BR "void *" +cannot be guaranteed by the ISO\ C standard to be distinct from a successful +value, this volume of POSIX.1\(hy2017 defines the symbol MAP_FAILED, +which a conforming implementation does not return as the result of a +successful call. +.SH "FUTURE DIRECTIONS" +None. +.SH "SEE ALSO" +.IR "\fIexec\fR\^", +.IR "\fIfcntl\fR\^(\|)", +.IR "\fIfork\fR\^(\|)", +.IR "\fIlockf\fR\^(\|)", +.IR "\fImsync\fR\^(\|)", +.IR "\fImunmap\fR\^(\|)", +.IR "\fImprotect\fR\^(\|)", +.IR "\fIposix_typed_mem_open\fR\^(\|)", +.IR "\fIshmat\fR\^(\|)", +.IR "\fIsysconf\fR\^(\|)" +.P +The Base Definitions volume of POSIX.1\(hy2017, +.IR "\fB<sys_mman.h>\fP" +.\" +.SH COPYRIGHT +Portions of this text are reprinted and reproduced in electronic form +from IEEE Std 1003.1-2017, Standard for Information Technology +-- Portable Operating System Interface (POSIX), The Open Group Base +Specifications Issue 7, 2018 Edition, +Copyright (C) 2018 by the Institute of +Electrical and Electronics Engineers, Inc and The Open Group. +In the event of any discrepancy between this version and the original IEEE and +The Open Group Standard, the original IEEE and The Open Group Standard +is the referee document. The original Standard can be obtained online at +http://www.opengroup.org/unix/online.html . +.PP +Any typographical or formatting errors that appear +in this page are most likely +to have been introduced during the conversion of the source files to +man page format. To report such errors, see +https://www.kernel.org/doc/man-pages/reporting_bugs.html . |