Struct memory layout in C

Question

I have a C  background  I am very much a newbie to a low-level language like C   In C   struct s memory is laid out by the compiler by default  The compiler can re-order data fields or pad additional bits between fields implicitly  So  I had to specify some special attribute to override this behavior for exact layout   AFAIK  C does not reorder or align memory layout of a struct by default  However  I heard there s a little exception that is very hard to find   What is C s memory layout behavior  What should be re-ordered aligned and not

User · Answer

You can start by reading the data structure alignment wikipedia article to get a better understanding of data alignment.

From the wikipedia article:

Data alignment means putting the data at a memory offset equal to some multiple of the word size, which increases the system's performance due to the way the CPU handles memory. To align the data, it may be necessary to insert some meaningless bytes between the end of the last data structure and the start of the next, which is data structure padding.

From 6.54.8 Structure-Packing Pragmas of the GCC documentation:

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. The n value below always is required to be a small power of two and specifies the new alignment in bytes.

#pragma pack(n) simply sets the new alignment.

#pragma pack() sets the alignment to the one that was in effect when compilation started (see also command line option -fpack-struct[=] see Code Gen Options).

#pragma pack(push[,n]) pushes the current alignment setting on an internal stack and then optionally sets the new alignment.

#pragma pack(pop) restores the alignment setting to the one saved at the top of the internal stack (and removes that stack entry). Note that #pragma pack([n]) does not influence this internal stack; thus it is possible to have #pragma pack(push) followed by multiple #pragma pack(n) instances and finalized by a single #pragma pack(pop).

Some targets, e.g. i386 and powerpc, support the ms_struct #pragma which lays out a structure as the documented __attribute__ ((ms_struct)).

#pragma ms_struct on turns on the layout for structures declared.

#pragma ms_struct off turns off the layout for structures declared.

#pragma ms_struct reset goes back to the default layout.

User · Answer

It s implementation-specific  but in practice the rule  in the absence of  pragma pack or the like  is    Struct members are stored in the order they are declared    This is required by the C99 standard  as mentioned here earlier   If necessary  padding is added before each struct member  to ensure correct alignment  Each primitive type T requires an alignment of sizeof T  bytes    So  given the following struct   struct ST      char ch1     short s     char ch2     long long ll     int i        ch1 is at offset 0 a padding byte is inserted to align    s at offset 2 ch2 is at offset 4  immediately after s 3 padding bytes are inserted to align    ll at offset 8 i is at offset 16  right after ll 4 padding bytes are added at the end so that the overall struct is a multiple of 8 bytes   I checked this on a 64-bit system  32-bit systems may allow structs to have 4-byte alignment    So sizeof ST  is 24   It can be reduced to 16 bytes by rearranging the members to avoid padding   struct ST      long long ll       0    int i              8    short s            12    char ch1           14    char ch2           15   ST

User · Answer

In C  the compiler is allowed to dictate some alignment for every primitive type  Typically the alignment is the size of the type  But it s entirely implementation-specific  Padding bytes are introduced so every object is properly aligned  Reordering is not allowed  Possibly every remotely modern compiler implements  pragma pack which allows control over padding and leaves it to the programmer to comply with the ABI   It is strictly nonstandard  though   From C99   6 7 2 1   12    Each non-bit-field member of a structure or union object is aligned in an implementation- defined manner appropriate to its type  13    Within a structure object  the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared  A pointer to a structure object  suitably converted  points to its initial member  or if that member is a bit-field  then to the unit in which it resides   and vice versa  There may be unnamed padding within a structure object  but not at its beginning

[c] Struct memory layout in C

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