c++ - Repeated use of a variadic function argument doesn't work

Question

I have a function that tries to log stuff to the console and also to a log file, but it doesn't work. The second use of the variable length argument gives garbage written to the console. Any ideas?

    void logPrintf(const char *fmt, ...) {
        va_list ap;    // log to logfile
        va_start(ap, fmt);
        logOpen;
        vfprintf(flog, fmt, ap);
        logClose;
        va_end(ap);
        va_list ap2;   // log to console
        va_start(ap2, fmt);
        printf(fmt, ap2);
        va_end(ap2);
    }

Answer 1

The original code fails because it tries to use printf() where it needs to use vprintf(). Taking dubious points like the logOpen and logClose statements at face value (given the notation, presumably they're macros which open and close the flog file stream), the code should be:

void logPrintf(const char *fmt, ...) {
    va_list ap;
    va_start(ap, fmt);
    logOpen;
    vfprintf(flog, fmt, ap);
    logClose;
    va_end(ap);
    va_list ap2;
    va_start(ap2, fmt);
    vprintf(fmt, ap2);
    va_end(ap2);
}

There's no particular requirement to use two separate va_list variables; it is perfectly OK to use the same one twice as long as you use va_end() before you use va_start() again.

void logPrintf(const char *fmt, ...) {
    va_list ap;
    va_start(ap, fmt);
    logOpen;
    vfprintf(flog, fmt, ap);
    logClose;
    va_end(ap);
    va_start(ap, fmt);
    vprintf(fmt, ap);
    va_end(ap);
}

When a va_list value is passed to another function (vfprintf() and vprintf() in this code), you should assume that it is no longer usable in the current function. It is only safe to call va_end() on it.

There is no need for va_copy() in this code. It works, but it isn't needed. You need va_copy() in other circumstances, such as when your function is passed a va_list and you need to process the list twice:

void logVprintf(const char *fmt, va_list args1)
{
    va_list args2;
    va_copy(args2, args1);
    logOpen;
    vfprintf(flog, fmt, args1);
    logClose;
    vprintf(fmt, args2);
    va_end(args2);
}

Note that in this code, it is the calling code's responsibility to call va_end() on args1. Indeed, the standard says:

Each invocation of the va_start and va_copy macros shall be matched by a corresponding invocation of the va_end macro in the same function.

Since the logVprintf() function doesn't call either va_start or va_copy to initialize args1, it cannot legitimately call va_end on args1. On the other hand, the standard requires it to call va_end for args2.

The logPrintf() function can be implemented in terms of logVprintf() now:

void logPrintf(const char *fmt, ...)
{
    va_list args;
    va_start(args, fmt);
    logVprintf(fmt, args);
    va_end(args);
}

This structure — an operational function that takes a va_list and a cover function that takes ellipsis (variable arguments) and passes them to the operational function after conversion to a va_list — is often a good way to work. Sooner or later, you usually find a need for the version with a va_list argument.