@@ -1574,10 +1574,15 @@ permitted. They include @code{PROT_READ}, @code{PROT_WRITE}, and
of address space for future use. The @code{mprotect} function can be
used to change the protection flags. @xref{Memory Protection}.
-@var{flags} contains flags that control the nature of the map.
-One of @code{MAP_SHARED} or @code{MAP_PRIVATE} must be specified.
+The @var{flags} parameter contains flags that control the nature of
+the map. One of @code{MAP_SHARED}, @code{MAP_SHARED_VALIDATE}, or
+@code{MAP_PRIVATE} must be specified. Additional flags may be bitwise
+OR'd to further define the mapping.
-They include:
+Note that, aside from @code{MAP_PRIVATE} and @code{MAP_SHARED}, not
+all flags are supported on all versions of all operating systems.
+Consult the kernel-specific documentation for details. The flags
+include:
@vtable @code
@item MAP_PRIVATE
@@ -1599,9 +1604,19 @@ Note that actual writing may take place at any time. You need to use
@code{msync}, described below, if it is important that other processes
using conventional I/O get a consistent view of the file.
+@item MAP_SHARED_VALIDATE
+Similar to @code{MAP_SHARED} except that additional flags will be
+validated by the kernel, and the call will fail if an unrecognized
+flag is provided. With @code{MAP_SHARED} using a flag on a kernel
+that doesn't support it causes the flag to be ignored.
+@code{MAP_SHARED_VALIDATE} should be used when the behavior of all
+flags is required.
+
@item MAP_FIXED
This forces the system to use the exact mapping address specified in
-@var{address} and fail if it can't.
+@var{address} and fail if it can't. Note that if the new mapping
+would overlap an existing mapping, the overlapping portion of the
+existing map is unmapped.
@c One of these is official - the other is obviously an obsolete synonym
@c Which is which?
@@ -1642,10 +1657,78 @@ The @code{MAP_HUGETLB} flag is specific to Linux.
@c There is a mechanism to select different hugepage sizes; see
@c include/uapi/asm-generic/hugetlb_encode.h in the kernel sources.
-@c Linux has some other MAP_ options, which I have not discussed here.
-@c MAP_DENYWRITE, MAP_EXECUTABLE and MAP_GROWSDOWN don't seem applicable to
-@c user programs (and I don't understand the last two). MAP_LOCKED does
-@c not appear to be implemented.
+@item MAP_32BIT
+Require addresses that can be accessed with a signed 32 bit pointer,
+i.e., within the first 2 GiB. Ignored if MAP_FIXED is specified.
+
+@item MAP_DENYWRITE
+@itemx MAP_EXECUTABLE
+@itemx MAP_FILE
+
+Provided for compatibility. Ignored by the Linux kernel.
+
+@item MAP_FIXED_NOREPLACE
+Similar to @code{MAP_FIXED} except the call will fail with
+@code{EEXIST} if the new mapping would overwrite an existing mapping.
+To test for this, specify MAP_FIXED_NOREPLACE without MAP_FIXED, and
+check the actual address returned. If it does not match the address
+passed, then this flag is not supported.
+
+@item MAP_GROWSDOWN
+This flag is used to make stacks, and is typically only needed inside
+the program loader to set up the main stack for the running process.
+The mapping is created according to the other flags, except an
+additional page just prior to the mapping is marked as a ``guard
+page''. If a write is attempted inside this guard page, that page is
+mapped, the mapping is extended, and a new guard page is created.
+Thus, the mapping continues to grow towards lower addresses until it
+encounters some other mapping.
+
+Note that accessing memory beyond the guard page will not trigger this
+feature. In gcc, use @code{-fstack-clash-protection} to ensure the
+guard page is always touched.
+
+@item MAP_LOCKED
+A hint that requests that mapped pages are locked in memory (i.e. not
+paged out). Note that this is a request and not a requirement; use
+@code{mlock} if locking is required.
+
+@item MAP_POPULATE
+@itemx MAP_NONBLOCK
+These two are opposites. @code{MAP_POPULATE} is a hint that requests
+that the kernel read-ahead a file-backed mapping, causing more pages
+to be mapped before they're needed. @code{MAP_NONBLOCK} is a hint
+that requests that the kernel @emph{not} attempt such, only mapping
+pages when they're actually needed. Note that neither of these hints
+affects future paging activity, use @code{mlock} if such needs to be
+controlled.
+
+@item MAP_NORESERVE
+Asks the kernel to not reserve physical backing for a mapping. This
+would be useful for, for example, a very large but sparsely used
+mapping which need not be limited in span by available RAM or swap.
+Note that writes to such a mapping may cause a @code{SIGSEGV} if the
+amount of backing required eventualy exceeds system resources.
+
+On Linux, this flag's behavior may be overwridden by
+@file{/proc/sys/vm/overcommit_memory} as documented in the proc(5) man
+page.
+
+@item MAP_STACK
+Ensures that the resulting mapping is suitable for use as a program
+stack. For example, the use of huge pages might be precluded.
+
+@item MAP_SYNC
+This flag is used to map persistent memory devices into the running
+program in such a way that writes to the mapping are immediately
+written to the device as well. Unlike most other flags, this one will
+fail unless @code{MAP_SHARED_VALIDATE} is also given.
+
+@item MAP_UNINITIALIZED
+This flag allows the kernel to map anonymous pages without zeroing
+them out first. This is, of course, a security risk, and will only
+work if the kernel is built to allow it (typically, on single-process
+embedded systems).
@end vtable
@@ -1656,6 +1739,24 @@ Possible errors include:
@table @code
+@item EACCES
+
+@var{filedes} was not open for the type of access specified in @var{protect}.
+
+@item EAGAIN
+
+The system has temporarily run out of resources.
+
+@item EBADF
+
+The @var{fd} passes is invalid, and a valid file descriptor is
+required (i.e. MAP_ANONYMOUS was not specified).
+
+@item EEXIST
+
+@code{MAP_FIXED_NOREPLACE} was specified and an existing mapping was
+found overlapping the requested address range.
+
@item EINVAL
Either @var{address} was unusable (because it is not a multiple of the
@@ -1664,23 +1765,33 @@ applicable page size), or inconsistent @var{flags} were given.
If @code{MAP_HUGETLB} was specified, the file or system does not support
large page sizes.
-@item EACCES
+@item ENODEV
-@var{filedes} was not open for the type of access specified in @var{protect}.
+This file is of a type that doesn't support mapping.
@item ENOMEM
-Either there is not enough memory for the operation, or the process is
-out of address space.
-
-@item ENODEV
-
-This file is of a type that doesn't support mapping.
+There is not enough memory for the operation, the process is out of
+address space, or there are too many mappings. On Linux, the maximum
+number of mappings can be controlled via
+@file{/proc/sys/vm/max_map_count} or, if your OS supports it, via
+the @code{vm.max_map_count} @code{sysctl} setting.
@item ENOEXEC
The file is on a filesystem that doesn't support mapping.
+@item EPERM
+
+@code{PROT_EXEC} was requested but the file is on a filesystem that
+was mounted with execution denied.
+
+@item EOVERFLOW
+
+Either the offset into the file plus the length of the mapping causes
+internal page counts to overflow, or the offset requested exceeds the
+length of the file.
+
@c On Linux, EAGAIN will appear if the file has a conflicting mandatory lock.
@c However mandatory locks are not discussed in this manual.
@c