We mentioned in Section 2.8 that some of the primitive system data types are defined in more than one header. For example, on FreeBSD 5.2.1, size_t is defined in 26 different headers. Because all 26 headers could be included in a program and because ISO C does not allow multiple typedefs for the same name, how must the headers be written?

#ifndef _MACHINE__TYPES_H_
#define _MACHINE__TYPES_H_
typedef int __int32_t;
typedef unsigned int __uint32_t;
...
typedef __uint32_t __size_t;
...
#endif /* _MACHINE__TYPES_H_ */
In each of the headers that can define the size_t primitive system data type, we have the sequence
#ifndef _SIZE_T_DECLARED
typedef __size_t size_t;
#define _SIZE_T_DECLARED
#endif
This way, the typedef for size_t is executed only once.