Common Usage C language level for the z/OS platform

The X/Open Portability Guide (XPG) Issue 3 describes a C language definition referred to as Common Usage C. This language definition is roughly equivalent to K&R C, and differs from the ISO C language definition. It is based on various C implementations that predate the ISO standard.

Common Usage C is supported with the LANGLVL(COMMONC) compiler option or the #pragma langlvl(commonc) directive. These cause the compiler to accept C source code containing Common Usage C constructs.

Many of the Common Usage C constructs are already supported by #pragma langlvl(extended). The following language elements are different from those accepted by pragma langlvl(extended).

Standard integral promotions preserve sign. For example, unsigned char or unsigned short are promoted to unsigned int. This is functionally equivalent to specifying the UPCONV compiler option.
Trigraphs are not processed in string or character literals. For example, consider the following source line:
```
??=define STR "??= not processed"
```
The above line gets preprocessed to:
```
  #define STR "??= not processed"
```
The sizeof operator is permitted on bitfields. The result is the size of an unsigned int (4).
Bitfields other than type int are permitted. The compiler issues a warning and changes the type to unsigned int.
Macro parameters found within single or double quotation marks are expanded. For example, consider the following source lines:
```
   #define STR(AAA) "String is: AAA"
   #define ST STR(BBB)
```
The above lines are preprocessed to:
```
   "String is: BBB"
```
Macros can be redefined without first being undefined (that is, without an intervening #undef). An informational message is issued saying that the second definition is used.
The empty comment (/**/) in a function-like macro is equivalent to the ISO token concatenation operator ##.

The LANGLVL compiler option is described in z/OS XL C/C++ User's Guide. The #pragma langlvl is described in #pragma langlvl (C only).