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getpagesize(2) [linux man page]

GETPAGESIZE(2)						     Linux Programmer's Manual						    GETPAGESIZE(2)

NAME
getpagesize - get memory page size SYNOPSIS
#include <unistd.h> int getpagesize(void); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): getpagesize(): Since glibc 2.12: _BSD_SOURCE || !(_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) Before glibc 2.12: _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED DESCRIPTION
The function getpagesize() returns the number of bytes in a page, where a "page" is the thing used where it says in the description of mmap(2) that files are mapped in page-sized units. The size of the kind of pages that mmap(2) uses, is found using #include <unistd.h> long sz = sysconf(_SC_PAGESIZE); (most systems allow the synonym _SC_PAGE_SIZE for _SC_PAGESIZE), or #include <unistd.h> int sz = getpagesize(); CONFORMING TO
SVr4, 4.4BSD, SUSv2. In SUSv2 the getpagesize() call is labeled LEGACY, and in POSIX.1-2001 it has been dropped; HP-UX does not have this call. Portable applications should employ sysconf(_SC_PAGESIZE) instead of this call. NOTES
Whether getpagesize() is present as a Linux system call depends on the architecture. If it is, it returns the kernel symbol PAGE_SIZE, whose value depends on the architecture and machine model. Generally, one uses binaries that are dependent on the architecture but not on the machine model, in order to have a single binary distribution per architecture. This means that a user program should not find PAGE_SIZE at compile time from a header file, but use an actual system call, at least for those architectures (like sun4) where this depen- dency exists. Here libc4, libc5, glibc 2.0 fail because their getpagesize() returns a statically derived value, and does not use a system call. Things are OK in glibc 2.1. SEE ALSO
mmap(2), sysconf(3) COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2010-09-20 GETPAGESIZE(2)

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POSIX_MEMALIGN(3)					     Linux Programmer's Manual						 POSIX_MEMALIGN(3)

NAME
posix_memalign, memalign, valloc - Allocate aligned memory SYNOPSIS
#include <stdlib.h> int posix_memalign(void **memptr, size_t alignment, size_t size); #include <malloc.h> void *valloc(size_t size); void *memalign(size_t boundary, size_t size); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): posix_memalign(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600 valloc(): Since glibc 2.12: _BSD_SOURCE || (_XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) && !(_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) Before glibc 2.12: _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED DESCRIPTION
The function posix_memalign() allocates size bytes and places the address of the allocated memory in *memptr. The address of the allocated memory will be a multiple of alignment, which must be a power of two and a multiple of sizeof(void *). If size is 0, then posix_memalign() returns either NULL, or a unique pointer value that can later be successfully passed to free(). The obsolete function memalign() allocates size bytes and returns a pointer to the allocated memory. The memory address will be a multiple of boundary, which must be a power of two. The obsolete function valloc() allocates size bytes and returns a pointer to the allocated memory. The memory address will be a multiple of the page size. It is equivalent to memalign(sysconf(_SC_PAGESIZE),size). For all three routines, the memory is not zeroed. RETURN VALUE
memalign() and valloc() return the pointer to the allocated memory, or NULL if the request fails. posix_memalign() returns zero on success, or one of the error values listed in the next section on failure. Note that errno is not set. ERRORS
EINVAL The alignment argument was not a power of two, or was not a multiple of sizeof(void *). ENOMEM There was insufficient memory to fulfill the allocation request. VERSIONS
The functions memalign() and valloc() have been available in all Linux libc libraries. The function posix_memalign() is available since glibc 2.1.91. CONFORMING TO
The function valloc() appeared in 3.0BSD. It is documented as being obsolete in 4.3BSD, and as legacy in SUSv2. It does not appear in POSIX.1-2001. The function memalign() appears in SunOS 4.1.3 but not in 4.4BSD. The function posix_memalign() comes from POSIX.1d. Headers Everybody agrees that posix_memalign() is declared in <stdlib.h>. On some systems memalign() is declared in <stdlib.h> instead of <malloc.h>. According to SUSv2, valloc() is declared in <stdlib.h>. Libc4,5 and glibc declare it in <malloc.h> and perhaps also in <stdlib.h> (namely, if _GNU_SOURCE is defined, or _BSD_SOURCE is defined, or, for glibc, if _XOPEN_SOURCE_EXTENDED is defined, or, equivalently, _XOPEN_SOURCE is defined to a value not less than 500). NOTES
On many systems there are alignment restrictions, for example, on buffers used for direct block device I/O. POSIX specifies the path- conf(path,_PC_REC_XFER_ALIGN) call that tells what alignment is needed. Now one can use posix_memalign() to satisfy this requirement. posix_memalign() verifies that alignment matches the requirements detailed above. memalign() may not check that the boundary argument is correct. POSIX requires that memory obtained from posix_memalign() can be freed using free(3). Some systems provide no way to reclaim memory allo- cated with memalign() or valloc() (because one can only pass to free(3) a pointer gotten from malloc(3), while, for example, memalign() would call malloc(3) and then align the obtained value). The glibc implementation allows memory obtained from any of these three routines to be reclaimed with free(3). The glibc malloc(3) always returns 8-byte aligned memory addresses, so these routines are only needed if you require larger alignment val- ues. SEE ALSO
brk(2), getpagesize(2), free(3), malloc(3) COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. GNU
2010-09-20 POSIX_MEMALIGN(3)

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