Compiler Warning (level 4) C4121

'symbol' : alignment of a member was sensitive to packing

The compiler added padding to align a structure member on the packing boundary but the packing value is less than the member's size. For example, the following code snippet produces C4121:

// C4121.cpp
// compile with: /W4 /c
#pragma pack(2)
struct s
   char a;
   int b; // C4121
   long long c;

To fix this issue, make one of the following changes:

  • Change the packing size to the size of the member that caused the warning or larger. For example, in this snippet, change pack(2) to pack(4) or pack(8).

  • Reorder member declarations by size, from largest to smallest. In the snippet, reverse the order of the structure members such that the long long member precedes the int, and the int precedes the char.

This warning only happens when the compiler adds padding before data members. It does not happen when packing has placed data at a memory location that is not aligned for the data type, but no padding was placed before the data member. When data is not aligned on boundaries that are multiples of the data's size, performance can degrade. Reads and writes of unaligned data cause processor faults on some architectures and the faults may take two or three orders of magnitude more time to resolve. Unaligned data accesses cannot be ported to some RISC architectures.

You can use #pragma pack or /Zp to specify the structure alignment. (The compiler does not generate this warning when /Zp1 is specified.)