Unaligned pointer. Normally data, pointers and structure members are aligned a...

Unaligned pointer. Normally data, pointers and structure members are aligned according to the table in Data Types. This differentiation still exists in current CPUs, and still some have only instructions that perform aligned accesses. Oct 16, 2018 · As we will see in this article, we need to be careful while casting pointers around to be sure not to break any of these rules. Unaligned Load Register (LDR) instructions generated by the compiler Comparisons of an unpacked struct, a __packed struct, and a struct with individually __packed fields, and of a __packed struct and a #pragma packed struct Compiler support for floating-point arithmetic Default selection of hardware or software floating-point support Jun 5, 2025 · This is one of the ground rules for programming: Pointers must be properly aligned unless explicitly permitted otherwise. In the C and C++ languages, forming an unaligned pointer is explicitly specified to return no useful value. Jun 17, 2013 · Why do I get a warning for an unaligned pointer but not a reference Ask Question Asked 12 years, 8 months ago Modified 5 years, 7 months ago Feb 3, 2025 · It seems strange that debug builds trap for unaligned pointer access, when unaligned binding would be a much less frequent but equally effective check closer to the root cause. So, platform-appropriate code is generated to handle unaligned reads and writes through the pointer. Jun 22, 2018 · I'm writing a const_iterator that takes an aligned pointer and can iterate over the unaligned elements. The goal of this article is to be educative by showcasing the problem and by giving some solutions to easily get over it. Oct 3, 2025 · When you declare a pointer with the __unaligned modifier, the compiler assumes that the pointer addresses data that isn't aligned. Jun 12, 2025 · 文章浏览阅读1. 6k次，点赞26次，收藏14次。 本文分析了C语言编程中出现的"unaligned pointer"警告，主要涉及packed结构体内存对齐问题。 在使用__attribute__ ( (packed))时，结构体成员可能位于非对齐地址，直接取地址会导致警告或运行时错误。 Mar 2, 2026 · But in C, one can cast pointers. C23 6. For some processors, this can be quite expensive. Mar 30, 2014 · When I try to load an exe into IDApro it shows up an error: The input file has unaligned section pointers What does this mean ? Oct 16, 2018 · CPUs used to perform better when memory accesses are aligned, that is when the pointer value is a multiple of the alignment value. On most architectures, reading or writing unaligned pointers suffers some sort of penalty. Pointers manipulations are intentionally using the safe wrapping variants instead of the unsafe variants, as the latter requires pointers to be inside an allocation which is not necessarily true for I/O pointers. With the type qualifier __unaligned you can specify to suppress the alignment of objects or structure members. To take into account this issue, the C standard has alignment rules in place, and so the compilers exploit them to generate efficient code Jun 5, 2025 · The Microsoft C++ compiler has a special nonstandard keyword __unaligned for declaring that a pointer may be unaligned, and this tells the compiler that any accesses to the data behind that pointer must use instructions that are alignment-forgiving. If the resulting pointer is not correctly aligned for the referenced type, the behavior is undefined. A memory pointer that refers to primitive data that is n bytes long is said to be aligned if it is only allowed to contain addresses that are n -byte aligned, otherwise it is said to be unaligned. This can be useful to create compact data structures. 2. After zeroing the memory, we access mem and mem + 1 by casting to different pointer types, knowing that the second address is odd, and therefore unaligned except for char * access. This How to fix unaligned pointer (char**) Asked 12 years, 5 months ago Modified 8 years, 9 months ago Viewed 6k times Jun 17, 2013 · Why do I get a warning for an unaligned pointer but not a reference Ask Question Asked 12 years, 8 months ago Modified 5 years, 7 months ago Nov 30, 2023 · Simplifying, I think this macro should work the same in gcc and similar: apply to any pointer to get something that behaves as *p but will read or write as unaligned. This will form the basis of performing I/O projections. . Jul 28, 2022 · Pointer to int, however, is assumed to point to a valid int, which may mean it has to be aligned (or there will be "bus error" or some such). Compiler generating valid unaligned access machine code for every int pointer dereference would be quite inefficient on CPUs where it matters, so compiler basically has to make this assumption. Nov 25, 2013 · An aligned pointer is one that points to an address that's a multiple of the word size, and an unaligned pointer is one that's not pointing to an address that's a multiple of the word size. net> Add a generic infrastructure for performing field and index projections on raw pointers. Thus, one may create unaligned pointers by conversion from another pointer type and this is an undefined 1 day ago · From: Gary Guo <gary@garyguo. 3. 3 § 7 A pointer to an object type may be converted to a pointer to a different object type. The API for these arrays specifies '__unaligned' for pointers to elements, so I'm mostly trying to confirm or deny my understanding of applying the __unaligned specifier.