Advanced Programming in the UNIX® Environment

ptg10805159

206 Process Environment Chapter 7

stack

heap

uninitialized data

(bss)

initialized data

text

command-line arguments

and environment variables

initialized to

zerobyexec

read from

program file

byexec

high address

low address

Figure 7.6 Typical memory arrangement

Thesize( 1 ) command reports the sizes (in bytes) of the text, data, and bss

segments. For example:

$size /usr/bin/cc /bin/sh

text data bss dec hex filename

346919 3576 6680 357175 57337 /usr/bin/cc

102134 1776 11272 115182 1c1ee /bin/sh

The fourth and fifth columns arethe total of the three sizes, displayed in decimal and

hexadecimal, respectively.

7.7 Shared Librar ies

Most UNIX systems today support shared libraries. Arnold[ 1986 ]describes an early

implementation under System V,and Gingell et al. [ 1987 ] describe a different

implementation under SunOS. Shared libraries remove the common library routines

from the executable file, instead maintaining a single copy of the library routine

somewhere in memory that all processes reference. Thisreduces the size of each

executable file but may add some runtime overhead, either when the program is first

executed or the first time each shared library function is called. Another advantage of

shared libraries is that library functions can be replaced with new versions without

having to relink edit every program that uses the library (assuming that the number and

type of arguments haven’t changed).

Different systems provide different ways for a program to say that it wants to use or

not use the shared libraries. Options for thecc( 1 )andld( 1 )commands aretypical. As