2.8. Primitive System Data Types

Historically, certain C data types have been associated with certain UNIX system variables. For example, the major and minor device numbers have historically been stored in a 16-bit short integer, with 8 bits for the major device number and 8 bits for the minor device number. But many larger systems need more than 256 values for these device numbers, so a different technique is needed. (Indeed, Solaris uses 32 bits for the device number: 14 bits for the major and 18 bits for the minor.)

The header <sys/types.h> defines some implementation-dependent data types, called the primitive system data types. More of these data types are defined in other headers also. These data types are defined in the headers with the C typedef facility. Most end in _t. Figure 2.20 lists many of the primitive system data types that we'll encounter in this text.

Figure 2.20. Some common primitive system data types

Type	Description
caddr_t	core address (Section 14.9)
clock_t	counter of clock ticks (process time) (Section 1.10)
comp_t	compressed clock ticks (Section 8.14)
dev_t	device numbers (major and minor) (Section 4.23)
fd_set	file descriptor sets (Section 14.5.1)
fpos_t	file position (Section 5.10)
gid_t	numeric group IDs
ino_t	i-node numbers (Section 4.14)
mode_t	file type, file creation mode (Section 4.5)
nlink_t	link counts for directory entries (Section 4.14)
off_t	file sizes and offsets (signed) (lseek, Section 3.6)
pid_t	process IDs and process group IDs (signed) (Sections 8.2 and 9.4)
ptrdiff_t	result of subtracting two pointers (signed)
rlim_t	resource limits (Section 7.11)
sig_atomic_t	data type that can be accessed atomically (Section 10.15)
sigset_t	signal set (Section 10.11)
size_t	sizes of objects (such as strings) (unsigned) (Section 3.7)
ssize_t	functions that return a count of bytes (signed) (read, write, Section 3.7)
time_t	counter of seconds of calendar time (Section 1.10)
uid_t	numeric user IDs
wchar_t	can represent all distinct character codes

By defining these data types this way, we do not build into our programs implementation details that can change from one system to another. We describe what each of these data types is used for when we encounter them later in the text.