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PROLOG |NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |EXAMPLES |APPLICATION USAGE |RATIONALE |FUTURE DIRECTIONS |SEE ALSO |COPYRIGHT | |
REALLOC(3P) POSIX Programmer's ManualREALLOC(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_tsize);
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. Therealloc() function shall deallocate the old object pointed to byptr and return a pointer to a new object that has the size specified bysize. 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. Ifptr is a null pointer,realloc() shall be equivalent tomalloc() for the specified size. Ifptr does not match a pointer returned earlier bycalloc(),malloc(), orrealloc() or if the space has previously been deallocated by a call tofree() orrealloc(), the behavior is undefined. The order and contiguity of storage allocated by successive calls torealloc() 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. Ifsize is 0, either: * A null pointer shall be returned and, ifptr 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 byptr 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 seterrno to[ENOMEM]. Ifrealloc() returns a null pointer anderrno has been set to[ENOMEM], the memory referenced byptr shall not be changed.
Therealloc() function shall fail if:ENOMEMInsufficient memory is available.The following sections are informative.
None.
The description ofrealloc() has been modified from previous versions of this standard to align with the ISO/IEC 9899:1999 standard. Previous versions explicitly permitted a call torealloc(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 ifrealloc() returns a null pointer, the space pointed to byp has not been freed. Since this could lead to double-frees, implementations should also seterrno if a null pointer actually indicates a failure, and applications should only free the space iferrno was changed.
None.
This standard defers to the ISO C standard. While that standard currently has language that might permitrealloc(p, 0), where p is not a null pointer, to freep 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 athttp://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, seehttps://www.kernel.org/doc/man-pages/reporting_bugs.html .IEEE/The Open Group 2017REALLOC(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 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |