Advanced Programming in the UNIX® Environment

ptg10805159

Section 2.9 Differences Between Standards 59

Type Description

clock_t counter of clock ticks (process time) (Section 1.10)

comp_t compressed clock ticks (not defined by POSIX.1; see Section 8.14)

dev_t device numbers (major and minor) (Section 4.24)

fd_set file descriptor sets (Section 14.4.1)

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)

pthread_t thread IDs (Section 11.3)

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

Figure 2.21 Some common primitive system data types

convert this value to seconds, we divide it byCLOCKS_PER_SEC,which is defined in

the<time.h>header.POSIX.1 defines the functiontimesthat returns both the CPU

time (for the caller and all its terminated children) and the clock time. All these time

values areclock_tvalues. Thesysconffunction is used to obtain the number of

clock ticks per second for use with the return values from thetimesfunction. What we

have is the same data type (clock_t)used to hold measurements of time defined with

different units by ISO C and POSIX.1. The difference can be seen in Solaris, where

clockreturns microseconds (henceCLOCKS_PER_SECis 1 million), whereassysconf

returns the value 100 for clock ticks per second. Thus we must take carewhen using

variables of typeclock_tso that we don’t mix variables with different units.

Another area of potential conflict is when the ISO C standardspecifies a function,

but doesn’t specify it as strongly as POSIX.1 does. This is the case for functions that

requireadifferent implementation in a POSIX environment (with multiple processes)

than in an ISO C environment (wherevery little can be assumed about the host

operating system). Nevertheless, POSIX-compliant systems implement the ISO C

function for compatibility.Thesignalfunction is an example. If we unknowingly use

thesignalfunction provided by Solaris (hoping to write portable code that can be run

in ISO C environments and under older UNIX systems), it will provide semantics

different from the POSIX.1sigactionfunction. We’ll have more to say about the

signalfunction in Chapter 10.