Microchip Technology XC8 Standard Compiler (Workstation) SW006021-1 SW006021-1 ユーザーズマニュアル

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DS52053A-page 14

 2012 Microchip Technology Inc.

The ANSI C Standard has to reconcile two opposing goals: freedom for compilers ven-
dors to target new devices and improve code generation, with the known functional 
operation of source code for programmers. If both goals can be met, source code can 
be made portable.

The standard is implemented as a set of rules which detail not only the syntax that a 
conforming C program must follow, but the semantic rules by which that program will 
be interpreted. Thus, for a compiler to conform to the standard, it must ensure that a 
conforming C program functions as described by the standard.

The standard describes implementation, the set of tools and the runtime environment 
on which the code will run. If any of these change, e.g., you build for, and run on, a dif-
ferent target device, or if you update the version of the compiler you use to build, then 
you are using a different implementation.

The standard uses the term behavior to mean the external appearance or action of the 
program. It has nothing to do with how a program is encoded.

Since the standard is trying to achieve goals that could be construed as conflicting, 
some specifications appear somewhat vague. For example, the standard states that an 
int

 type must be able to hold at least a 16-bit value, but it does not go as far as saying 

what the size of an int actually is; and the action of right-shifting a signed integer can 
produce different results on different implementations; yet, these different results are 
still ANSI C compliant.

If the standard is too strict, device architectures may not allow the compiler to conform.

1

 

But, if it is too weak, programmers would see wildly differing results within different 
compilers and architectures, and the standard would loose its effectiveness.

The standard organizes source code whose behavior is not fully defined into groups 
that include the following behaviors:

This is unspecified behavior where each implementation documents how the choice 

is made.

Unspecified behavior 

The standard provides two or more possibilities and imposes no further requirements 

on which possibility is chosen in any particular instance.

Undefined behavior 

This is behavior for which the standard imposes no requirements.

Code that strictly conforms to the standard does not produce output that is dependent 
on any unspecified, undefined, or implementation-defined behavior. The size of an 
int

, which we used as an example earlier, falls into the category of behavior that is 

defined by implementation. That is to say, the size of an int is defined by which com-
piler is being used, how that compiler is being used, and the device that is being tar-
geted.

All the MPLAB XC compilers conform to the ANS X3.159-1989 Standard for program-
ming languages (with the exception of the XC8 compiler’s inability to allow recursion, 
as mentioned in the footnote). This is commonly called the C89 Standard. Some fea-
tures from the later standard, C99, are also supported. 

1. Case in point: The mid-range PIC

®

 microcontrollers do not have a data stack. Because a compiler 

targeting this device cannot implement recursion, it (strictly speaking) cannot conform to the ANSI 
C Standard. This example illustrate a situation in which the standard is too strict for mid-range 
devices and tools.