Data structure alignment: Difference between revisions

Revision as of 04:00, 18 September 2021

Revision as of 08:56, 18 September 2021

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Although the [[compiler]] (or [[interpreter (computing)|interpreter]]) normally allocates individual data items on aligned boundaries, data structures often have members with different alignment requirements. To maintain proper alignment the translator normally inserts additional unnamed data members so that each member is properly aligned. In addition, the data structure as a whole may be padded with a final unnamed member. This allows each member of an [[array of structures]] to be properly aligned.

Padding is only inserted when a [[datarecord structure(computer science)|structure]] member is followed by a member with a larger alignment requirement or at the end of the structure. By changing the ordering of members in a structure, it is possible to change the amount of padding required to maintain alignment. For example, if members are sorted by descending alignment requirements a minimal amount of padding is required. The minimal amount of padding required is always less than the largest alignment in the structure. Computing the maximum amount of padding required is more complicated, but is always less than the sum of the alignment requirements for all members minus twice the sum of the alignment requirements for the least aligned half of the structure members.

Although C and C++ do not allow the compiler to reorder structure members to save space, other languages might. It is also possible to tell most C and C++ compilers to "pack" the members of a structure to a certain level of alignment, e.g. "pack(2)" means align data members larger than a byte to a two-byte boundary so that any padding members are at most one byte long. Likewise, in [[PL/I]] a structure may be declared <code>UNALIGNED</code> to eliminate all padding except around bit strings.