What is the size of an empty struct in C?

by

A struct cannot be empty in C because the syntax forbids it. Furthermore, there is a semantic constraint that makes behavior undefined if a struct has no named member:

struct-or-union-specifier:
  struct-or-union identifieropt { struct-declaration-list }
  struct-or-union identifier

struct-or-union:
  struct
  union

struct-declaration-list:
  struct-declaration
  struct-declaration-list struct-declaration

struct-declaration:
  specifier-qualifier-list struct-declarator-list ;

/* type-specifier or qualifier required here! */
specifier-qualifier-list:
  type-specifier specifier-qualifier-listopt
  type-qualifier specifier-qualifier-listopt

struct-declarator-list:
  struct-declarator
  struct-declarator-list , struct-declarator

struct-declarator:
  declarator
  declaratoropt : constant-expression

If you write

struct identifier { };

It will give you a diagnostic message, because you violate syntactic rules. If you write

struct identifier { int : 0; };

Then you have a non-empty struct with no named members, thus making behavior undefined, and not requiring a diagnostic:

If the struct-declaration-list contains no named members, the behavior is undefined.

Notice that the following is disallowed because a flexible array member cannot be the first member:

struct identifier { type ident[]; };

More Related Contents:

  1. Are compilers allowed to eliminate infinite loops?
  2. Compilation error: stray ‘\302’ in program, etc
  3. initial value of int array in C
  4. Why is the .bss segment required?
  5. How does an uninitiliazed variable get a garbage value?
  6. What do R_X86_64_32S and R_X86_64_64 relocation mean?
  7. Why are global variables in x86-64 accessed relative to the instruction pointer?
  8. C compiler asserts – how to implement?
  9. When / How does Linux load shared libraries into address space?
  10. How to compile .c file with OpenSSL includes?
  11. How many GCC optimization levels are there?
  12. Are GCC and Clang parsers really handwritten?
  13. Compiler warning – suggest parentheses around assignment used as truth value
  14. What happens with an extern inline function?
  15. C compiler for Windows? [closed]
  16. Strange array initialize expression?
  17. Which C99 features are available in the MS Visual Studio compiler?
  18. Are constant C expressions evaluated at compile time or at runtime?
  19. Does GCC’s __attribute__((__packed__)) retain the original ordering?
  20. Is there a C compiler that targets the 8086? [closed]
  21. Avoiding the main (entry point) in a C program
  22. Statically link ncurses to program
  23. What decides the sizeof an integer?
  24. Why GDB jumps unpredictably between lines and prints variables as “”?
  25. how do compilers assign memory addresses to variables?
  26. library not found for -lrt with QtCreator [mac os]
  27. How do I compile the asm generated by GCC?
  28. Compiler support of GNU Statement Expression
  29. Are compound statements (blocks) surrounded by parens expressions in ANSI C?
  30. C reverse bits in unsigned integer

Leave a Comment