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# i f ndef DISKSYS f o r (i = 1; i <= msg->dbgmsgmsg-total; i++) #endi f #i fdef DISKSYS i = dbgmsgno; i f (i <= msg->dbgmsg msg-total) #endi f
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i f (msg->dbgmsgmsg-total == i ) #i f ndef DISKSYS break; /* no more messages t o wait f o r */ about 30 more lines, with further conditional compilation #endi f
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Even when apparently innocuous, conditional compilation can frequently be replaced by cleaner methods For instance, #ifdefs are often used to control debugging code:
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#ifdef DEBUG p r i n t f ( ) ; #endif
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but a regular i f statement with a constant condition may work just as well: enum { DEBUG
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i f (DEBUG) { printf ( ); If DEBUG is zero, most compilers won't generate any code for this, but they will check the syntax of the excluded code An #ifdef, by contrast, can conceal syntax errors that will prevent compilation if the #i fdef is later enabled Sometimes conditional compilation excludes large blocks of code: #ifdef notdef
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but conditional code can often be avoided altogether by using files that are conditionally substituted during compilation We will return to this topic in the next section When you must modify a program to adapt to a new environment, don't begin by making a copy of the entire program Instead, adapt the existing source You will
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probably need to make changes to the main body of the code, and if you edit a copy, before long you will have divergent versions As much as possible there should only be a single source for a program; if you find you need to change something to port to a particular environment, find a way to make the change work everywhere Change internal interfaces if you need to, but keep the code consistent and #ifdef-free This will make your code more portable over time, rather than more specialized Narrow the intersection, don't broaden the union We have spoken out against conditional compilation and shown some of the problems it causes But the nastiest problem is one we haven't mentioned: it is almost impossible to test An #ifdef turns a single program into two separately-compiled programs It is difficult to know whether all the variant programs have been compiled and tested If a change is made in one #ifdef block, we may need to make it in others, but the changes can be verified only within the environment that causes those #i fdefs to be enabled If a similar change needs to be made for other configurations, it cannot be tested Also, when we add a new #ifdef block, it is hard to isolate the change to determine what other conditions need to be satisfied to get here, and where else this problem might need to be fixed Finally, if something is in code that is conditionally omitted, the compiler doesn't see it It could be utter nonsense and we won't know until some unlucky customer tries to compile it in the environment that triggers that condition This program compiles when -MAC is defined and fails when it is not: #ifdef -MAC pri ntf ("Thi s i s Mad ntosh\rU) ; #el se This will give a syntax error on other systems #endi f So our preference is to use only features that are common to all target environments We can compile and test all the code If something is a portability problem, we rewrite to avoid it rather than adding conditional code; this way, portability will steadily increase and the program itself will improve rather than becoming more complicated Some large systems are distributed with a configuration script to tailor code to the local envimnment At compilation time, the script tests the envimnment properties-location of header files and libraries, byte order within words, size of and types, implementations known to be broken (surprisingly common), and so ongenerates configuration parameters or makefiles that will give the right configuration settings for that situation, These scripts can be large and intricate, a significant fraction of a software distribution, and require continual maintenance to keep them working Sometimes such techniques are necessary but the more portable and #i fdef-free the code is, the simpler and more reliable the configuration and installation will be
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