Gcc struct alignment

Gcc struct alignment. Consider this example on a 64-bit x86 or ARM May 21, 2012 · Checking against existing implementations shows that both Clang 3. Jun 9, 2021 · Well my question is more focused on how to define the default alignment value, in order to be sure what the compiler will do when reaching a struct with no "#pragma pack" or "attribute ( (aligned))" instruction Nov 27, 2023 · Conclusion: Understanding data alignment and padding is crucial for optimizing memory usage in C++. Way of defining compound data types. 6. sea[4][-2] is a valid array reference. type followed. Jun 20, 2012 · For example in the following example, since double is 8-aligned, the actual size of structure will be sizeof (char) + (alignment for double padding) + sizeof (int) = 20 bytes. For example, the declarations: struct S { short f[3]; } __attribute__ ((aligned (8))); typedef int more_aligned_int __attribute__ ((aligned (8))); 5. Mar 8, 2017 · uint8_t gm_mode; uint8_t mc_state; On 32-bit system , the structure's strict alignment is 4bytes so the members are aligned across 4-byte boundary. h>. Your point of structure alignment should be equal to the alignment of its widest data member is wrong. Mar 20, 2018 · This is exactly correct. Aug 25, 2011 at 4:49. } my_type; c++. This is not supported in C, but the alignment of a struct type can be controlled by using _Alignas (until C23) alignas (since C23) in a member declaration. 5 to 7. Since we upgraded to GCC 4. Keywords. It may go either immediately after the struct, union or enum keyword, or after the closing brace. struct A and the type that is aliased by sampleType. Layout the struct, starting with the largest types. Jan 13, 2020 · 3. Apr 28, 2014 · It may provide a #pragma pack (or align or some other thing) that you can add to your structure defininition. gcc provides a language extension, __attribute__((packed)), which tells the compiler not to insert padding, allowing struct members to be misaligned. There is no padding added in this case because all members together are 4-byte aligned. The CPU in modern computer hardware performs reads and writes to memory most efficiently when the data is naturally aligned, which generally means that the This means that you can effectively adjust the alignment of a struct or union type by attaching an aligned attribute to any one of the members of such a type, but the notation illustrated in the example above is a more obvious, intuitive, and readable way to request the compiler to adjust the alignment of an entire struct or union type. It might be worth adding the downsides of packing. Nov 21, 2015 · 63. – Joe. It consists of three separate but related issues: data alignment, data structure padding, and packing . Consider this example on a 64-bit x86 or ARM Nov 26, 2012 · SampleStructPack1 #pragma pack (1): It allocates 1 byte memory block, so our sample struct fits perfectly, in this case it is true that 4 + 1 = 5. Feb 2, 2022 · The compiler pads a structure, when you have a 64bit field that is not at a multiple of 8 address. Feb 2, 2024 · Although, if we move p to the first place, the following members will squeeze into the 8 bytes and the alignment rule is satisfied as well. #pragma pack( n Jan 4, 2016 · 1. What are some general rules of practice that ensure optimal alignment. And struct alginment is done based on word size 4 bytes. BTW, you could compile with gcc -O3 -fverbose-asm -S (and other optimization flags) and, by looking inside the generated assembly code, see what the use of that attribute changes in the emitted assembly code. allocate memory for several of them, determining the size with sizeof). Feb 8, 2021 · From the gcc manual: This attribute specifies a minimum alignment (in bytes) for variables of the specified type. Documentation for alignment in C Structs. gcc targeting Linux uses and $-32, %rsp (or whatever higher alignment) to align the stack in functions that need to spill an __m256, __m512, or any objects you declared with alignas(32) or anything higher than 16. given the following aligned. However, for your case you need the struct to be both packed and aligned as 32-bit unsigned integer. When used on a struct, or struct member, the aligned attribute can only increase the alignment; in order to decrease it, the packed attribute must be specified as well. Feb 22, 2018 · First, a basic fact in C and C++ is that the alignment of a type can not exceed the size of the type. For example, the declaration: int x __attribute__ ((aligned (16))) = 0; causes the compiler to allocate the global variable x on a 16-byte boundary. Aug 29, 2014 · The gcc has introduced the __attribute__((packed)) precisely to avoid the dangerous effects you are seeking: the definition of the structure should binary compatible between all the user applications and libraries which use the same definition. – nos. 54 x86 Options ¶. However, one compiler I used for 680x0 only aligned on 2-byte boundaries - and there was no option to change the alignment! Jul 20, 2011 · 1) if you're to be interfacing with some external compiled library, you should make sure that structure alignment is in agreement with that used by the library compiler. With variables ordered like following: char AA; char CC; int BB; and with #pragma pack(2), the struct would be laid out like this: and sizeOf(Test) would be 3 × 2 = 6. But this is not true on 64-bit system, where time_t is 64bit. Linux doesn't align the stack by 32 either. GCC and VS10 have different ways to specify alignment of a struct. – Kerrek SB. sea[2][1] will always be a higher address than sea[1][2] sea[2] is calculated using only lea. 10 Structure-Layout Pragmas ¶. The maximum allowed n option value is 65536. 这在许多 RISC 机器 Oct 12, 2023 · このようにして、foo2 のサイズは合計 16 バイトとなり、struct を詰めるように呼ばれることになります。 gcc コンパイラは特別な __attribute__ ((packed)) 指定子を持っていて、順序のない struct メンバであっても強制的にパックさせることができることに注意して Mar 20, 2014 · In previous versions of GCC everything worked fine. The Clang source code reproduces the same grammar from N1570, except Parser::ParseSpecifierQualifierList allows alignment specifiers; the function does contain a TODO element, though, that We would like to show you a description here but the site won’t allow us. E. int a; int b; // I want the compiler to create a padding here for cache alignment. I've three options (priority ordered): C++11's alignas(1) struct __attribute__ ((packed)) BMPHeader. By default, when the struct is packed, compilers also change alignment of the struct to 1 byte. gcc-5. For compatibility with Microsoft Windows compilers, GCC supports a set of #pragma directives that change the maximum alignment of members of structures (other than zero-width bit-fields), unions, and classes subsequently defined. Valid non-zero alignments that are weaker than another alignas on the same declaration are ignored. uint32_t lk; }; always got compiled to an address which is aligned with 8 bytes. You have have to use __builtin_assume_aligned if you want to portably convey an alignment promise to GNU C compilers other than GCC itself. A packing a struct). Also, alignment is not guaranteed -- different platforms may have upper limits based on the linker involved. Sep 15, 2017 · 2. Its syntax is just like sizeof . #pragma pack(1) However the last option (with least priority) alone seems to work giving me 54. Enabled at levels -O2, -O3. Figure 1: StructA, Unoptimized Memory Layout. Message. Generally speaking, you can minimize the padding in structure layouts by ordering members by decreasing alignment requirement. Apr 13, 2012 · So for example, for the following struct I want to ask the compiler to create necessary padding so that any object of this structure is always aligned with a cache line. Biggest alignment that any structure or union field can require on this machine, in bits. 19. If the operand of __alignof__ is an lvalue rather than a type, its value is the required alignment for its type, taking into account any minimum alignment specified with GCC's __attribute__ extension (see Variable Attributes). Since you specify that you are using GCC, you should be looking at type attributes. 下面的 n 值始终需要是 2 的小幂，并指定新的字节对齐方式。. For example, gcc allows you to add attributes to a definition, one of which controls alignment: struct mystruct { int val[7]; } __attribute__ ((aligned (16))); 6. C allows programs to rely on and be sensitive to structure member order. Structure alginment is based on the word size of the architecture (not the widest data member). It clearly says that the attribute may only be attached to struct, union and enum types. filler bytes to have the structure aligned). The double data in memory starts at the address immediately after the 13th int member. There is a GCC __aligned__ attribute and its C++11 alignas counterpart. Dec 27, 2023 · No. With arm gcc cross compiler for aarch64, the following structure: struct lock {. 44 确定函数、类型或变量的对齐. For example, if the target machine requires a double value to be aligned on an 8-byte boundary, then __alignof__ (double) is 8. Integer attribute fits because it just needs 2 blocks of 2 Bytes. This attribute, attached to an enum, struct, or union type definition, specified that the minimum required memory be used to represent the type. Oct 2, 2011 at 21:16. If n is not specified or is zero, use a machine-dependent default. Jan 4, 2016 · 1. It consists of two separate but related issues: data alignment and data structure padding. An example struct as: _STRUCT(): a(0), b(0) {}; uint32_t a; uint32_t b; Compiling this code throws the following warning where this strcut is used: Jun 22, 2022 · I'm trying to implement a polymorphic data structure e. If you are not happy with the use of these cookies, please review our Cookie Policy to learn how they can be disabled. } Here we inspect StructA which has a largest alignment which is the D field with 8-bytes alignment, so this struct itself should have 8-byte alignment. I would guess that the problem is that 42 isn't divisible by 4, and so they get out of alignment if you put several of these structs back to back (e. 关键字 __alignof__ 确定函数、对象或类型的对齐要求，或者类型通常所需的最小对齐。. " – Aug 25, 2011 · 2. Macro: ADJUST_FIELD_ALIGN (field, type, computed) ¶ This is documented as not working[1] in all but the most recent GCC versions. sea[1][1] makes two memory accesses. 5, but I expect it is the same in this . My problem is I have a structure that contains 13 int types followed by a double. If you didn't align your a array I guess that the compiler would have to emit additional instructions and they would run slower. This concerns the structures present in library's headers. Taken from the GCC Online docs:. You can check older documentation. The trouble is casting a nested struct to the containing struct leads to gcc issuing a memory-alignment warning. Aug 3, 2021 · All the credit is due to @Clifford. ) Oct 2, 2011 · Yes, gcc aligns the structs to a boundary (in my specific case, a 4-byte boundary), but this does not change the size of the Pe_SymbolHeader struct. Note that the gcc compiler has the special __attribute__ ((packed)) specifier that can force even unordered struct members to be This attribute specifies a minimum alignment for the variable or structure field, measured in bytes. This rule controls what sizeof() will return. A structured group of variables, possibly including other structs. Apparently, although this warning was only recently added, GCC and LLVM/clang do not, and never have, guaranteed that data alignment for data structures that include either base structures or virtual functions will be equivalent with MSVC, even when using the ms_struct pragma. Jan 15, 2013 · To quote the manual, aligned (alignment) This attribute specifies a minimum alignment for the variable or structure field, measured in bytes. K. Consider the structure struct x_ in the following code example: struct x_ { char a; // 1 byte int b; // 4 bytes short c; // 2 bytes char d; // 1 byte } bar[3]; The compiler pads this structure to enforce alignment naturally. In particular, if you want RADIO_PACKET s to be aligned on 4-byte (or wider) boundaries, then you would use __attribute__((aligned (4))) on the type. gcc aligns doubles on linux x86 to 4 bytes. The interviewer said that in general C and C++ are packing structs in different ways and we should never expect the opposite. The n value below always is required to be a small power of two and specifies the 15. Dec 27, 2021 · When used on a struct, or struct member, the aligned attribute can only increase the alignment; in order to decrease it, the packed attribute must be specified as well… That is from the GCC 11. There is none for 7. Here’s an example: Without __attribute__ ( (packed)), this structure occupies 12 bytes (as described in the previous section), assuming 4-byte alignment for int. For this reason is done alignment. This will force gcc to not do any padding around members. Compilers are not free to reorder structure members. Use -malign-double if you'd want 8 byte alignment. Feb 15, 2013 · 3. Its syntax is just like sizeof and C11 _Alignof . One way to "hack" it is to pad your struct with unused variables until it matches the alignment you want. e. Jul 26, 2012 · Is it possible to align a struct member to 4-byte if that struct is single-byte aligned using #pragma??? Mar 30, 2014 · GCC Manual: Note that the alignment of any given struct or union type is required by the ISO C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the struct or union in question. Specifying no alignment argument implies the maximum alignment for the target, which is often, but by no means always, 8 or 16 bytes. 0 and GCC 4. I. The n value below always is required to be a small power of two and specifies Jan 17, 2024 · struct alignas (8) S {}; struct alignas (1) U {S s;}; // error: alignment of U would have been 8 without alignas(1) Invalid non-zero alignments, such as alignas ( 3 ) are ill-formed. {. -march=cpu-type ¶ Generate instructions for the machine type cpu-type. You want the struct to be packed, not the single fields (which are already packed as they are scalar types). There's a similar problem with alignment within structs. this structs largest member is a pointer (8 bytes on Aug 25, 2023 · D int64 // 8-byte alignment. For instance, Sparc, PowerPC, and 680x0 CPUs are all big-endian, and the common default for Sparc and PowerPC compilers is to align struct members on 4-byte boundaries. But the gcc also provides a way to do the packing the old fashioned, dangerous way - #pragma pack Jun 22, 2012 · Could you union your vect2 with __m128? That should instruct gcc to align your struct to 16 bytes on the stack. alignas, _Alignas Example -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned. Also, this typically introduces an element of padding (i. In contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. These ‘-m’ options are defined for the x86 family of computers. The general rule of trailing structure padding is this: the compiler will behave as though the structure has trailing padding out to its stride address. The easiest way to pack a struct is to order them from largest to smallest, as a struct is always aligned to their largest data type. The former syntax is preferred. For example, the declaration: int x __attribute__ ( (aligned (16))) = 0; causes the compiler to allocate the global variable x on a 16-byte boundary. " In addition the document shows the following example: struct S { short f[3 Apr 30, 2019 · The gcc bug report discusses the i386 System V ABI, which has different struct-packing rules than MSVC: based on minimum alignment, not preferred. Boolean attribute needs just 1 Byte, but minimum block size is 2 Sep 26, 2018 · I correctly got the alignment warning. and, packed. 6), we get a bunch of warnings regarding these structs. I've been asked in a recent interview about C++ struct fields alignment and theoretized that C and C++ follows the same strategy in struct packing. 52. When specified, alignment must be an integer constant power of 2. 0 or ICC19, see the x86-64 non-AVX asm they emit on Godbolt. 2 (previously we used 4. The packed attribute specifies that a On an Intel processor with the GCC compiler, the fields were laid out in memory as they are shown. By continuing to use our site, you consent to our cookies. Alternatively, you can leave out the alignment factor and just ask the compiler to align a type to the maximum useful alignment for the target machine you are compiling for. It is the first address following the structure data that has the same alignment as the structure. Nov 22, 2013 · As per the requirement of the BMP file format, the structure should've no padding. This means that a field less than the size of a word will be padded to take up an entire word. However, we can remove unnecessary padding by simply ordering our variables within a struct so they align to the largest member (A. Having the size as 44 forces the alignment in these cases as you requested. 例如，如果目标机器要求 double 值在 8 字节边界上对齐，则 __alignof__ (double) 为 8。. 46 Determining the Alignment of Functions, Types or Variables. Data structure alignment is the way data is arranged and accessed in computer memory. gcc will insert padding after a given element to ensure that the next one (or the struct if we are talking about the last element) is correctly aligned. E byte // 1-byte alignment. As Micheal mentions in his answer, there are two types involved. If I find equivalent struct packing options for various compilers, will this be cross-platform? Jan 13, 2020 · Are the members of a structure packed in C/C++? By packed I mean that they are compact and among the fields there aren't memory spaces. aligned (alignment) The aligned attribute specifies a minimum alignment (in bytes) for variables of the specified type. ¶. For compatibility with Microsoft Windows compilers, GCC supports a set of #pragma directives which change the maximum alignment of members of structures (other than zero-width bitfields), unions, and classes subsequently defined. Note that __ attribute __ is a GCC-specific thing. As in the preceding example, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given struct or union type. Quoting the gcc Feb 28, 2020 · For compatibility with Microsoft Windows compilers, GCC supports a set of #pragma directives which change the maximum alignment of members of structures (other than zero-width bitfields), unions, and classes subsequently defined. In GNU C you can force a structure to be laid out with no gaps by adding __attribute__ ( (packed)) after struct (or at the end of the structure type declaration). 7 Structure-Packing Pragmas. aligned (alignment) This attribute specifies a minimum alignment for the variable or structure field, measured in bytes. 为了与 Microsoft Windows 编译器兼容， GCC 支持一组 #pragma 指令，这些指令可更改随后定义的结构（零宽度位字段除外）、联合和类的成员的最大对齐方式。. The question you link to states false facts, as alignment is a platform dependent matter, and even windows and linux platforms aligns a double differently on x86. For example, if the system normally requires all int objects to have 4-byte alignment, __attribute__((packed)) can cause int struct members to be allocated at odd offsets. Is this a bug in the compiler or I've completely mistook something I've run into a data alignment problem in my code running on the AM389x Sitara (ARM Cortex A8). Hovewer, it was the wrong assumption. When I try to put the structure onto a 4-byte aligned address, gcc will fill 4 empty bytes before the structure to make it on 8-bytes aligned address. If defined, this overrides BIGGEST_ALIGNMENT for structure and union fields only, unless the field alignment has been set by the __attribute__ ((aligned (n))) construct. 6 Packed Structures. Jun 29, 2023 · In C++, the alignas specifier may also be applied to the declarations of class/struct/union types and enumerations. This is important when doing things like accessing a Pe_SymbolHeader in the PE file (sizeof (Pe_SymbolHeader * symbol_index)) – endeavormac. It seems like a weird bug that MinGW gcc doesn't use the same sequence to save the We would like to show you a description here but the site won’t allow us. 7 support _Alignas on structure members without complaining (with -pedantic ). In short, they allow to specify the alignment (in bytes) for a variable or struct. SampleStructPack2 #pragma pack (2): Minimum block size is 2 bytes. Even if you use the same compiler, the packing alignment for structs can be different based on what pragma pack you're using. The compiler’s role in managing memory, aligning data, and introducing padding underscores the intricate balance between performance and memory efficiency in game development and other resource-intensive applications. 38 Inquiring on Alignment of Types or Variables. IMHO it's strange statement. On a 68040, this could be used in conjunction with an asm expression to This helps gcc but not clang7. The specifics are provided below: Feb 22, 2018 · Struct padding is done differently by different compilers. Jan 4, 2016 at 9:37. struct { uint64_t alpha; uint32_t beta; uint32_t gamma; uint8_t delta; Oct 2, 2011 · Yes, gcc aligns the structs to a boundary (in my specific case, a 4-byte boundary), but this does not change the size of the Pe_SymbolHeader struct. 8. This can be done by changing the packing attribute as following: #include <stdint. c: char not_aligned_char ; int not_aligned_int ; 3. When specified, alignment must be a power of 2. This site uses cookies to store information on your computer. #pragma pack( n Oct 3, 2011 · According to GNU "You may specify aligned and transparent_union attributes either in a typedef declaration or just past the closing curly brace of a complete enum, struct or union type definition and the packed attribute only past the closing brace of a definition. 它的语法就像 sizeof 和 C11 _Alignof 。. The keyword __alignof__ allows you to inquire about how an object is aligned, or the minimum alignment usually required by a type. an intrusive linked list (I already know the kernel has one - this is more of learning experience). Sep 9, 2016 · The __attribute__((aligned(byte-alignment))) affect the minimum alignment of the variable (or struct field if specified within the struct) I believe the following are equivalent If you want to avoid this penalty on non-x86 hardware, you should avoid packing the struct whenever possible, and specify the overall struct alignment otherwise (so the compiler doesn't need to generate code to handle every possible offset from correct alignment). If it would, then it would not be possible to allocate memory using malloc(N*sizeof(the_type)). In 32 bit system also size of uint64_t is 8 bytes. However, when the next member of the struct can also fit inside the same word, then the compiler will put both members into the same word. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the maximum useful alignment for the target machine you are compiling for. Oct 31, 2023 · The compiler also pads structures in a way that naturally aligns each element of the structure. It will never rearrange the order of the elements in the struct, even if that will save memory. 62. (Only GCC folds a load into a memory operand for mulps, instead of using a separate movups). g. For example, after this declaration: struct foo { int x; char y; } foo1; As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. But modern i386 System V maintains 16-byte stack alignment, so it's only inside structs (because of struct-packing rules that are part of the ABI) that the compiler ever creates int64_t and double The aligned attribute specifies a minimum alignment for the variable or structure field, measured in bytes. The keyword __alignof__ determines the alignment requirement of a function, object, or a type, or the minimum alignment usually required by a type. In case you have a structure that has only 32 bit Jul 26, 2013 · Afaik there is no generic option in gcc to change the default alignment for variables (some architectures have special -malign switches, but I thing for arm are none available), and a quick glance into the IAR doc say, that the IAR compiler behaves the same way. The struct A will consume 8*4 = 32 bytes. (unaligned object accesses are slow in the best case, but will cause errors on some platforms. Thus, foo2 ’s size totals 16 bytes and it’s called to be packed struct. Good libraries try to minimize the effort by providing #pragma 's for some compilers to ensure the proper alignment. When applied to a struct, it describes the alignment of instances of the overall struct, not (directly) the What you are looking for is the packed attribute. It may also provide this as a language extension. The strict alignment in that case is 8-byte. 4. 1 at that site; it jumps from 6. c. cast increases required alignment of target type [-Wcast-align] from the GCC compiler due to the following code: uint8_t array[100]; uint32_t foo; foo = * ( (uint32_t *) &array[10]); Then I used the aligned attribute to figure out the issue: 6. 66. packed. typedef struct. The code is compiled using the CodeSourcery gcc compiler. In case you have to access it, the cpu should have to make two 64bit accesses to load/store the data if the data is not aligned to a multiple of 8 address. This helps gcc but not clang7. typedef struct structc_tag { char c; double d; int s; } structc_t; Thank you. When used as part of a typedef, the aligned attribute can both increase and decrease alignment, and specifying the packed attribute generates a warning. Jun 3, 2014 · @unwind Actually, according to the GCC manual's page on attribute syntax both placements of attribute are valid: "An attribute specifier list may appear as part of a struct, union or enum specifier. -fmin-function-alignment ¶ Specify minimal alignment of functions to the next power-of-two greater than or equal to n. 1. cast increases required alignment of target type [-Wcast-align] from the GCC compiler due to the following code: uint8_t array[100]; uint32_t foo; foo = * ( (uint32_t *) &array[10]); Then I used the aligned attribute to figure out the issue: Sep 26, 2018 · I correctly got the alignment warning. 10 结构布局编译指令. I cannot seem to find the right document for avr specific information. That means that he would align a variable of your struct type at 4 byte boundaries Aug 31, 2015 · When used on a struct, or struct member, the aligned attribute can only increase the alignment; in order to decrease it, the packed attribute must be specified as well. Typically (but under no guarantees), members of a struct are word -aligned. Please always read the documentation before using a vendor extension. 2 documentation. So, in that respect, it is "wrong" to pack structs when you don't need to. 4 documents: > "The aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well. version was the first 3 bits in the buffer, and Message. There seem to me to be a lot of forum posts on alignment of members in a c struct, but I think I may be having difficulty finding the right gcc-avr documentation to get a 'definitive' understanding of alignment of members in a c struct. This thing is basically ignored in our port of GCC now. Structs in C. xk ul yq hm re aq mo ny bk si