Advanced Programming in the UNIX® Environment

ptg10805159

Section 20.8 Source Code 753

fgets,for example, to return data that was read into a standardI/O buffer 10 minutes

ago if the data was modified by another process 5 minutes ago.

Our discussion of concurrency is predicated on the simple needs of the database

library.Commercial systems often have additional requirements. See Chapter 16 of

Date[ 2004 ]for additional details on concurrency.

20.7 Building the Librar y

The database library consists of two files: a public C header file and a C source file.We

can build a static library using the commands

gcc -I../include -Wall -c db.c

ar rsv libapue_db.a db.o

Applications that want to link with libapue_db.a will also need to link with

libapue.a,since we use some of our common functions in the database library.

If, on the other hand, we want to build a dynamic shared library version of the

database library, we can use the following commands:

gcc -I../include -Wall -fPIC -c db.c

gcc -shared -Wl,-soname,libapue_db.so.1 -o libapue_db.so.1 \

-L../lib -lapue -lc db.o

The resulting shared library,libapue_db.so.1,needs to be placed in a common

directory wherethe dynamic linker/loader can find it. Alternatively, we can place it in

aprivate directory and modify our LD_LIBRARY_PATH environment variable to

include the private directory in the search path of the dynamic linker/loader.

The steps used to build shared libraries vary among platforms. Here, we have shown how to

do it on a Linux system with the GNU C compiler.

20.8 Source Code

We start by showing theapue_db.hheader.This header is included by the library

source code and all applications that call the library.

For the remainder of this text, we depart from the style of the previous examples in

several ways. First, because the source code example is longer than usual, we number

the lines. This makes it easier to link the discussion with the corresponding source

code. Second, we place the description of the source code immediately below the

source code on the same page.

This style was inspired by John Lions in his book documenting the UNIX Version 6 operating

system source code[Lions 1977, 1996].Itsimplifies the task of studying large amounts of

source code.

Note that we do not bother to number blank lines. Although this departs from the

normal behavior of such tools aspr( 1 ), we have nothing interesting to say about blank

lines.