1POSIX_MEMALIGN(3) Linux Programmer's Manual POSIX_MEMALIGN(3)
2
6 posix_memalign, aligned_alloc, memalign, valloc, pvalloc - allocate
7 aligned memory
8
10 #include <stdlib.h>
11 int posix_memalign(void **memptr, size_t alignment, size_t size);
13 void *aligned_alloc(size_t alignment, size_t size);
14 void *valloc(size_t size);
15 #include <malloc.h>
17 void *memalign(size_t alignment, size_t size);
19 void *pvalloc(size_t size);
20 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
22 posix_memalign(): _POSIX_C_SOURCE >= 200112L
24 aligned_alloc(): _ISOC11_SOURCE
26 valloc():
28 Since glibc 2.12:
29 (_XOPEN_SOURCE >= 500) && !(_POSIX_C_SOURCE >= 200112L)
30 || /* Glibc since 2.19: */ _DEFAULT_SOURCE
31 || /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
32 Before glibc 2.12:
33 _BSD_SOURCE || _XOPEN_SOURCE >= 500
34 (The (nonstandard) header file <malloc.h> also exposes the dec‐
35 laration of valloc(); no feature test macros are required.)
36
38 The function posix_memalign() allocates size bytes and places the ad‐
39 dress of the allocated memory in *memptr. The address of the allocated
40 memory will be a multiple of alignment, which must be a power of two
41 and a multiple of sizeof(void *). This address can later be success‐
42 fully passed to free(3). If size is 0, then the value placed in
43 *memptr is either NULL or a unique pointer value.
44 The obsolete function memalign() allocates size bytes and returns a
46 pointer to the allocated memory. The memory address will be a multiple
47 of alignment, which must be a power of two.
48 The function aligned_alloc() is the same as memalign(), except for the
50 added restriction that size should be a multiple of alignment.
51 The obsolete function valloc() allocates size bytes and returns a
53 pointer to the allocated memory. The memory address will be a multiple
54 of the page size. It is equivalent to memalign(sysconf(_SC_PAGE‐
55 SIZE),size).
56 The obsolete function pvalloc() is similar to valloc(), but rounds the
58 size of the allocation up to the next multiple of the system page size.
59 For all of these functions, the memory is not zeroed.
61
63 aligned_alloc(), memalign(), valloc(), and pvalloc() return a pointer
64 to the allocated memory on success. On error, NULL is returned, and
65 errno is set to indicate the cause of the error.
66 posix_memalign() returns zero on success, or one of the error values
68 listed in the next section on failure. The value of errno is not set.
69 On Linux (and other systems), posix_memalign() does not modify memptr
70 on failure. A requirement standardizing this behavior was added in
71 POSIX.1-2008 TC2.
72
74 EINVAL The alignment argument was not a power of two, or was not a mul‐
75 tiple of sizeof(void *).
76 ENOMEM There was insufficient memory to fulfill the allocation request.
78
80 The functions memalign(), valloc(), and pvalloc() have been available
81 since at least glibc 2.0.
82 The function aligned_alloc() was added to glibc in version 2.16.
84 The function posix_memalign() is available since glibc 2.1.91.
86
88 For an explanation of the terms used in this section, see at‐
89 tributes(7).
90 ┌─────────────────┬───────────────┬────────────────┐
92 │Interface │ Attribute │ Value │
93 ├─────────────────┼───────────────┼────────────────┤
94 │aligned_alloc(), │ Thread safety │ MT-Safe │
95 │memalign(), │ │ │
96 │posix_memalign() │ │ │
97 ├─────────────────┼───────────────┼────────────────┤
98 │valloc(), │ Thread safety │ MT-Unsafe init │
99 │pvalloc() │ │ │
100 └─────────────────┴───────────────┴────────────────┘
101
103 The function valloc() appeared in 3.0BSD. It is documented as being
104 obsolete in 4.3BSD, and as legacy in SUSv2. It does not appear in
105 POSIX.1.
106 The function pvalloc() is a GNU extension.
108 The function memalign() appears in SunOS 4.1.3 but not in 4.4BSD.
110 The function posix_memalign() comes from POSIX.1d and is specified in
112 POSIX.1-2001 and POSIX.1-2008.
113 The function aligned_alloc() is specified in the C11 standard.
115 Headers
117 Everybody agrees that posix_memalign() is declared in <stdlib.h>.
118 On some systems memalign() is declared in <stdlib.h> instead of <mal‐
120 loc.h>.
121 According to SUSv2, valloc() is declared in <stdlib.h>. Glibc declares
123 it in <malloc.h>, and also in <stdlib.h> if suitable feature test
124 macros are defined (see above).
125
127 On many systems there are alignment restrictions, for example, on buf‐
128 fers used for direct block device I/O. POSIX specifies the path‐
129 conf(path,_PC_REC_XFER_ALIGN) call that tells what alignment is needed.
130 Now one can use posix_memalign() to satisfy this requirement.
131 posix_memalign() verifies that alignment matches the requirements de‐
133 tailed above. memalign() may not check that the alignment argument is
134 correct.
135 POSIX requires that memory obtained from posix_memalign() can be freed
137 using free(3). Some systems provide no way to reclaim memory allocated
138 with memalign() or valloc() (because one can pass to free(3) only a
139 pointer obtained from malloc(3), while, for example, memalign() would
140 call malloc(3) and then align the obtained value). The glibc implemen‐
141 tation allows memory obtained from any of these functions to be re‐
142 claimed with free(3).
143 The glibc malloc(3) always returns 8-byte aligned memory addresses, so
145 these functions are needed only if you require larger alignment values.
146
148 brk(2), getpagesize(2), free(3), malloc(3)
149
151 This page is part of release 5.10 of the Linux man-pages project. A
152 description of the project, information about reporting bugs, and the
153 latest version of this page, can be found at
154 https://www.kernel.org/doc/man-pages/.
155GNU 2020-12-21 POSIX_MEMALIGN(3)