REALLOC(3P)

💖 REALLOC(3P) — the POSIX Programmer's Manual, bestie edition 💖

okay so before we even get into it — this page is literally part of the POSIX Programmer's Manual. That's the vibe. But heads up, the Linux implementation of this interface may differ (so go consult the corresponding Linux manual page for the deets on Linux behavior), or honestly the interface may not even be implemented on Linux. It's giving "portable, not guaranteed". 🫶

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🌟 PROLOG 🌟

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.

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✨ NAME ✨

realloc — memory reallocator. yes that's the whole name. memory. reallocator. iconic brevity.

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🎀 SYNOPSIS 🎀

#include <stdlib.h>

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

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💅 DESCRIPTION 💅

So the functionality described on this reference page is aligned with the ISO C standard. If anything here conflicts with the ISO C standard, that conflict is unintentional — no drama intended. This volume of POSIX.1‐2017 defers to the ISO C standard. periodt.

Now, the actual tea:

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 — so don't come for them, they're just ✨mystery bytes✨.

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. Like, do not touch it. fr fr.

If the space cannot be allocated, the object shall remain unchanged. phew.

If ptr is a null pointer, realloc() shall be equivalent to malloc() for the specified size. cute little shortcut, slay.

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. bestie. do not do this. this is the forbidden move. 💀

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. And if the space cannot be allocated, a null pointer shall be returned. okay that's a lot but you got this 🦋

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🔥 RETURN VALUE 🔥

Upon successful completion, realloc() shall return a pointer to the (possibly moved) allocated space. yes, possibly moved — she may have relocated, keep up.

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]. And — important, bestie — if realloc() returns a null pointer and errno has been set to [ENOMEM], the memory referenced by ptr shall not be changed. 🫶

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🚨 ERRORS 🚨

The realloc() function shall fail if:

• ENOMEM — Insufficient memory is available.

The following sections are informative. (i.e. the vibes section, not the rules section)

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🌈 EXAMPLES 🌈

None.

...yeah. none. we're all just out here raw-dogging the standard. 😭

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🧚 APPLICATION USAGE 🧚

okay so this part is actually important lore. 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. (the committee said no 💅)

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.

Double frees are the villain origin story of C bugs. check your errno, bestie. always. 🔍

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💜 RATIONALE 💜

None.

(the standard said "no thoughts, head empty" on this one 😌)

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🦋 FUTURE DIRECTIONS 🦋

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. so watch this space, the plot is still developing 👀

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👑 SEE ALSO 👑

• calloc()
• free()
• malloc()

The Base Definitions volume of POSIX.1‐2017, <stdlib.h>

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📜 COPYRIGHT 📜

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 .

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and that's the whole page, bestie 💖 this lil masterpiece was brought to you by IEEE/The Open Group, dated 2017, and the manual page is REALLOC(3P). go forth and reallocate responsibly. mwah. 💅✨🎀