|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
|
|
|
REALLOC(3P) POSIX Programmer's Manual REALLOC(3P)
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.
realloc — memory reallocator
#include <stdlib.h>
void *realloc(void *ptr, size_t size);
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.1‐2017 defers to the ISO C standard.
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. If
the size of the space requested is zero, the behavior shall be
implementation-defined: either a null pointer is returned, or the
behavior shall be as if the size were some non-zero value, except
that the behavior is undefined if the returned pointer is used to
access an object. If the space cannot be allocated, the object
shall remain unchanged.
If ptr is a null pointer, realloc() shall be equivalent to
malloc() for the specified size.
If ptr does not match a pointer returned earlier by calloc(),
malloc(), or realloc() or if the space has previously been
deallocated by a call to free() or realloc(), the behavior is
undefined.
The order and contiguity of storage allocated by successive calls
to realloc() 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 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.
Upon successful completion, realloc() shall return a pointer to
the (possibly moved) allocated space. If size is 0, either:
* A null pointer shall be returned and, if ptr is not a null
pointer, errno shall be set to an implementation-defined
value.
* A pointer to the allocated space shall be returned, and the
memory object pointed to by ptr shall be freed. The
application shall ensure that the pointer is not used to
access an object.
If there is not enough available memory, realloc() shall return a
null pointer and set errno to [ENOMEM]. If realloc() returns a
null pointer and errno has been set to [ENOMEM], the memory
referenced by ptr shall not be changed.
The realloc() function shall fail if:
ENOMEM Insufficient memory is available.
The following sections are informative.
None.
The description of realloc() has been modified from previous
versions of this standard to align with the ISO/IEC 9899:1999
standard. Previous versions explicitly permitted a call to
realloc(p, 0) to free the space pointed to by p and return a null
pointer. While this behavior could be interpreted as permitted by
this version of the standard, the C language committee have
indicated that this interpretation is incorrect. Applications
should assume that if realloc() returns a null pointer, the space
pointed to by p has not been freed. Since this could lead to
double-frees, implementations should also set errno if a null
pointer actually indicates a failure, and applications should only
free the space if errno was changed.
None.
This standard defers to the ISO C standard. While that standard
currently has language that might permit realloc(p, 0), where p is
not a null pointer, to free p while still returning a null
pointer, the committee responsible for that standard is
considering clarifying the language to explicitly prohibit that
alternative.
calloc(3p), free(3p), malloc(3p)
The Base Definitions volume of POSIX.1‐2017, stdlib.h(0p)
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 .
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 .
IEEE/The Open Group 2017 REALLOC(3P)
Pages that refer to this page: stdlib.h(0p), calloc(3p), free(3p), getdelim(3p), malloc(3), malloc(3p)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|