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Questions tagged [pragma-pack]

Packing class or struct to place its members directly after each other in memory, which can mean that some or all members can be aligned on a boundary smaller than the default alignment the target architecture

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#pragma pack effect

I was wondering if someone could explain to me what the #pragma pack preprocessor statement does, and more importantly, why one would want to use it. I checked out the MSDN page, which offered some insight, but I was hoping to hear more from people…
Cenoc
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Is gcc's __attribute__((packed)) / #pragma pack unsafe?

In C, the compiler will lay out members of a struct in the order in which they're declared, with possible padding bytes inserted between members, or after the last member, to ensure that each member is aligned properly. gcc provides a language…
Keith Thompson
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Scope of the pragma pack directive in Visual Studio

What is the scope of the #pragma pack alignment in Visual C++? The API reference https://msdn.microsoft.com/en-us/library/vstudio/2e70t5y1%28v=vs.120%29.aspx says: pack takes effect at the first struct, union, or class declaration after the…
rgb
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C++ #pragma pack

What does the following statement actually do and what are it's effects? #pragma pack(push,8)
Tony The Lion
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what is the difference between __attribute__((__packed__)); and #pragma pack(1)

I am porting a code which runs perfectly on Linux to windows visual c++. I have this code in Linux: struct exif_desc { uint16_t tag; uint16_t type; uint32_t length; uint32_t value; } __attribute__((__packed__)); I am getting…
mans
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How should #pragma pack(8) work?

I'm new to structure alignment and packing. I thought I understood it, but I'm finding some results that I did not expect (see below). My understanding of structure alignment is: Types are generally aligned on memory addresses that are multiples of…
jne
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Why #pragma pack also affects structs' own alignment?

I noticed that when #pragma pack is used around a struct, the alignment inside of it is not the only one that is affected, but also the alignment of the struct itself changes. Consider the following: #include #include #pragma…
ZenJ
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pragma pack(push) without corresponding pop leads to stack smashing

I used #pragma pack(push, 2) at the beginning of a struct in a header file but forgot the corresponding #pragma pack(pop). After including this header file, I included fstream. On creating an ofstream object, I am seeing stack smashing. Details of…
vkj
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pragma pack(push) on iphone

I have a structure within #pragma pack(push,1) and #pragma pack(pop). I would like to know if this will properly work on iphone compiler, and if there might be other side effects outside of those macros. Thanks, Raxvan.
Raxvan
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Struct alignment is always 4

Why pragma pack does not work in here? sizeof() returning the size aligned to 4. I use compiler which comes with Microsoft Visual Studio 2015 #pragma pack(push,1) struct Header { uint16_t zero = 0; uint32_t id{}; }; struct ImageBlock :…
Valerii Chistyakov
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I don't understand this mistake about #pragma

This is the first part of my code and I can't understand the mistake written at the end of the message. Maybe I don't know enough to understand it. I would be very grateful if someone can explain me how #pragma works and what means the mistake.…
Ernestina Lede
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What is the alternative to using #pragma pack (push n) and #pragma pack (pop)

I've been reading a lot of comments on the use of #pragma pack, nearly all negative. I understand what it does and I can see the negative effects it may have but is there an alternative? Most of the code I write is to read/write/manipulate GIS data.…
Colins2
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Nim: How to wrap a c struct that contains bitfields?

The question is if I see a bitfield in a c struct as such: struct Example { int A; int B; int C : 3; int D : 3; int E : 3; int F : 8; bool G : 1; bool H : 1; }; Would the correct binding in Nim be as such? type…
Joshua Fenner
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Problem in converting Packed C structs to go structure by cgo

When we use #pragma pack (push, 1) to pack the C structure and convert to go structure by cgo, some of the fields are missing in the converted Go structure. I am using Go version: 1.16.6 Windows/386 For example, my C struct is like this: #pragma…
Priya Purushoth
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Influence of #pragma pack on outer struct?

Problem I stumbled over alignment issues when accessing a member of a struct (on ARMv7). The accessed member itself is also a struct but packed with the infamous #pragma pack(1) preprocessor directive. The outer struct is not packed so a naive…
MarcusS
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