C Language Conditional inclusion and conditional function signature modification


To conditionally include a block of code, the preprocessor has several directives (e.g #if, #ifdef, #else, #endif, etc).

/* Defines a conditional `printf` macro, which only prints if `DEBUG`
 * has been defined
#ifdef DEBUG
#define DLOG(x) (printf(x))
#define DLOG(x)

Normal C relational operators may be used for the #if condition

#if __STDC_VERSION__ >= 201112L
/* Do stuff for C11 or higher */
#elif __STDC_VERSION__ >= 199901L
/* Do stuff for C99 */
/* Do stuff for pre C99 */

The #if directives behaves similar to the C if statement, it shall only contain integral constant expressions, and no casts. It supports one additional unary operator, defined( identifier ), which returns 1 if the identifier is defined, and 0 otherwise.

#if defined(DEBUG) && !defined(QUIET)
#define DLOG(x) (printf(x))
#define DLOG(x)

Conditional Function Signature Modification

In most cases a release build of an application is expected to have as little overhead as possible. However during testing of an interim build, additional logs and information about problems found can be helpful.

For example assume there is some function SHORT SerOpPluAllRead(PLUIF *pPif, USHORT usLockHnd) which when doing a test build it is desired will generate a log about its use. However this function is used in multiple places and it is desired that when generating the log, part of the information is to know where is the function being called from.

So using conditional compilation you can have something like the following in the include file declaring the function. This replaces the standard version of the function with a debug version of the function. The preprocessor is used to replace calls to the function SerOpPluAllRead() with calls to the function SerOpPluAllRead_Debug() with two additional arguments, the name of the file and the line number of where the function is used.

Conditional compilation is used to choose whether to override the standard function with a debug version or not.

#if 0
// function declaration and prototype for our debug version of the function.
SHORT   SerOpPluAllRead_Debug(PLUIF *pPif, USHORT usLockHnd, char *aszFilePath, int nLineNo);

// macro definition to replace function call using old name with debug function with additional arguments.
#define SerOpPluAllRead(pPif,usLock) SerOpPluAllRead_Debug(pPif,usLock,__FILE__,__LINE__)
// standard function declaration that is normally used with builds.
SHORT   SerOpPluAllRead(PLUIF *pPif, USHORT usLockHnd);

This allows you to override the standard version of the function SerOpPluAllRead() with a version that will provide the name of the file and line number in the file of where the function is called.

There is one important consideration: any file using this function must include the header file where this approach is used in order for the preprocessor to modify the function. Otherwise you will see a linker error.

The definition of the function would look something like the following. What this source does is to request that the preprocessor rename the function SerOpPluAllRead() to be SerOpPluAllRead_Debug() and to modify the argument list to include two additional arguments, a pointer to the name of the file where the function was called and the line number in the file at which the function is used.

#if defined(SerOpPluAllRead)
// forward declare the replacement function which we will call once we create our log.
SHORT    SerOpPluAllRead_Special(PLUIF *pPif, USHORT usLockHnd);

SHORT    SerOpPluAllRead_Debug(PLUIF *pPif, USHORT usLockHnd, char *aszFilePath, int nLineNo)
    int iLen = 0;
    char  xBuffer[256];

    // only print the last 30 characters of the file name to shorten the logs.
    iLen = strlen (aszFilePath);
    if (iLen > 30) {
        iLen = iLen - 30;
    else {
        iLen = 0;

    sprintf (xBuffer, "SerOpPluAllRead_Debug(): husHandle = %d, File %s, lineno = %d", pPif->husHandle, aszFilePath + iLen, nLineNo);

    // now that we have issued the log, continue with standard processing.
    return SerOpPluAllRead_Special(pPif, usLockHnd);

// our special replacement function name for when we are generating logs.
SHORT    SerOpPluAllRead_Special(PLUIF *pPif, USHORT usLockHnd)
// standard, normal function name (signature) that is replaced with our debug version.
SHORT   SerOpPluAllRead(PLUIF *pPif, USHORT usLockHnd)
    if (STUB_SELF == SstReadAsMaster()) {
        return OpPluAllRead(pPif, usLockHnd);
    return OP_NOT_MASTER;