what is the difference between __attribute__((__packed__)); and #pragma pack(1)

Question

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 error on windows:

'__packed__' : undeclared identifier 

I am wondering if I can fix this error by using

#pragma pack(1)

is there any difference between them? Is there any syntax that can be used in Linux and Windows for this attribute?

Sprout

From my understanding these should be the same. Don't forget to reset the alignment again with #pragma pack() at the end of the struct. Question might be duplicate of #1538464 – Classicist

Possible duplicate of What are the differences between #pragma pack(push, n)/#pragma pack(pop) and __attribute__((__packed__, aligned(n) )) on GCC? – Approbation

Answer 1

__attribute__ is a GCC extension, specific to GCC (and other compilers which attempts to be compatible with GCC).

#pragma pack is originally a Visual C++ compiler specific extension. It has, as noted by commenters, been implemented in GCC as well for VC++ compatibility.

Normally you can't use extensions in one compiler in another compiler. Case in point: __attribute__ doesn't exist as an extension in the Visual C++ compiler.

Rampageous

#pragma pack can be used in gcc too (gcc.gnu.org/onlinedocs/gcc/Structure-Packing-Pragmas.html) – Sprout

#pragma pack is nonstandard extension, but woks with most compilers – Nullipore

Note also that #pragma pack takes effect from the point of definition and until another #pragma pack, while __attribute__((__packed__)) is effective only for the definition it's attached to. This is important. – Equanimity