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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. 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 |
