The Open Group Base Specifications Issue 8
IEEE Std 1003.1-2024
Copyright © 2001-2024 The IEEE and The Open Group

NAME

realloc, reallocarray — memory reallocators

SYNOPSIS

#include <stdlib.h>

void *realloc(void *
ptr, size_t size);

[CX] [Option Start] void *reallocarray(void *ptr, size_t nelem, size_t elsize); [Option End]

DESCRIPTION

For realloc(): [CX] [Option Start] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard. [Option End]

The realloc() function shall deallocate the old object pointed to by ptr and return a pointer to a new object that has the size specified by size. The contents of the new object shall be the same as that of the old object prior to deallocation, up to the lesser of the new and old sizes. Any bytes in the new object beyond the size of the old object have indeterminate values.

[CX] [Option Start] The reallocarray() function shall be equivalent to the call realloc(ptr, nelem * elsize) except that overflow in the multiplication shall be an error. [Option End]

If ptr is a null pointer, realloc() [CX] [Option Start]  or reallocarray() [Option End]  shall be equivalent to malloc() for the specified size. Otherwise, if ptr does not match a pointer earlier returned by aligned_alloc(), calloc(), malloc(), [ADV] [Option Start]  posix_memalign(), [Option End]  realloc(),
[CX] [Option Start] reallocarray(), or a function in POSIX.1-2024 that allocates memory as if by malloc(), [Option End]  or if the space has been deallocated by a call to free(), [CX] [Option Start]  reallocarray(), [Option End]  or realloc(),  the behavior is undefined.

If size is non-zero and memory for the new object is not allocated, the old object shall not be deallocated.

The order and contiguity of storage allocated by successive calls to realloc() [CX] [Option Start]  or reallocarray() [Option End]  is unspecified. The pointer returned if the allocation succeeds shall be suitably aligned so that it may be assigned to a pointer to any type of object with a fundamental alignment requirement and then used to access such an object in the space allocated (until the space is explicitly freed or reallocated). Each such allocation shall yield a pointer to an object disjoint from any other object. The pointer returned shall point to the start (lowest byte address) of the allocated space. If the space cannot be allocated, a null pointer shall be returned.

For purposes of determining the existence of a data race, realloc() [CX] [Option Start]  and reallocarray() [Option End]  shall each behave as though it accessed only memory locations accessible through its argument and not other static duration storage. The function may, however, visibly modify the storage that it allocates. Calls to aligned_alloc(), calloc(), free(), malloc(), [ADV] [Option Start]  posix_memalign(), [Option End]
[CX] [Option Start] reallocarray(), [Option End]  and realloc() that allocate or deallocate a particular region of memory shall occur in a single total order (see 4.15.1 Memory Ordering ), and each such deallocation call shall synchronize with the next allocation (if any) in this order.

RETURN VALUE

Upon successful completion, realloc() [CX] [Option Start]  and reallocarray() [Option End]  shall return a pointer to the new object (which can have the same value as a pointer to the old object), or a null pointer if the new object has not been allocated.

[OB] [Option Start] If size is 0,
[Option End] [OB CX] [Option Start] or either nelem or elsize is 0,
[Option End] [OB] [Option Start] either: [Option End]

If there is not enough available memory, realloc() [CX] [Option Start]  and reallocarray() [Option End]  shall return a null pointer
[CX] [Option Start] and set errno to [ENOMEM]. [Option End]

ERRORS

The realloc() [CX] [Option Start]  and reallocarray() [Option End]  functions shall fail if:

[ENOMEM]
[CX] [Option Start] Insufficient memory is available. [Option End]

[CX] [Option Start] The reallocarray() function shall fail if:

[ENOMEM]
The calculation nelem * elsize would overflow.
[Option End]

The realloc() [CX] [Option Start]  and reallocarray() [Option End]  functions may fail if:

[EINVAL]
[CX] [Option Start] The requested allocation size is 0 and the implementation does not support 0 sized allocations. [Option End]

The following sections are informative.

EXAMPLES

None.

APPLICATION USAGE

The ISO C standard makes it implementation-defined whether a call to realloc(p, 0) frees the space pointed to by p if it returns a null pointer because memory for the new object was not allocated. POSIX.1 instead requires that implementations set errno if a null pointer is returned and the space has not been freed, and POSIX applications should only free the space if errno was changed.

RATIONALE

See the RATIONALE for malloc .

FUTURE DIRECTIONS

The ISO C standard states that invoking realloc() with a size argument equal to zero is an obsolescent feature. This feature may be removed in a future version of this standard.

SEE ALSO

aligned_alloc , calloc , free , malloc

XBD <stdlib.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 6

Extensions beyond the ISO C standard are marked.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0495 [400], XSH/TC1-2008/0496 [400], XSH/TC1-2008/0497 [400], and XSH/TC1-2008/0498 [400] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0309 [526] and XSH/TC2-2008/0310 [526,688] are applied.

Issue 8

Austin Group Defect 374 is applied, adding the [EINVAL] error.

Austin Group Defect 1218 is applied, adding reallocarray().

Austin Group Defect 1302 is applied, aligning the realloc() function with the ISO/IEC 9899:2018 standard.

Austin Group Defect 1387 is applied, changing the RATIONALE section.

End of informative text.

 

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