mickjc750 / prnf Goto Github PK

When attempting to print strings generated by application specific functions, there are some difficulties...

For example, consider a function which takes an integer 'seconds' and describes the duration as xxDxxHxxMxxS.

char* period2txt(char* buf, int seconds);

If this function is used for multiple arguments to a single prnf(), a separate buffer must be provided for each call, eg.

char buf1[20];
char buf2[20];
prnf("Fred is %s, Bob is %s\n", period2txt(buf1, freds_age), period2txt(buf2, bobs_age));

The above is ugly and error prone. Accidentally put buf1 in both, and it breaks. Undersize the buffer and it breaks.

Proposed solution

The standard printf() provides %n, which is normally used to write the number of characters printed so far, to an integer pointed to by the argument. I cannot think of a practical application of this, and I don't know why it exists.
As %n expects a pointer (int*) as an argument, this could actually point to a string in a heap allocation. prnf() could then free the allocation after printing it.
If our function period2txt() makes a heap allocation when buf is passed as NULL, then we could do this:

prnf("Fred is %n, Bob is %n\n", (int*)period2txt(NULL, freds_age), (int*)period2txt(NULL, bobs_age));

Or, if period2txt is only to be used as a prnf argument, it could be of the form:

int* prnf_n_period(int seconds);

This would allow usage as simple as:

prnf("Fred is %n, Bob is %n\n", prnf_n_period(freds_age), prnf_n_period(bobs_age));

There are some benefits to this.

1. Mixing up %s and %n will give you a compilation warning, as %s expects (char*) and %n expects(int*)
2. Misuse of prnf_n_period() for other purposes, such as using it in an assignment to a char* will also cause a compilation warning due to pointer types.

Unfortunately the compilers format checking will complain if we try to specify width or precision for %n. So no alignment by prnf for %n strings would be possible.

If anyone is out there.. please share your thoughts on this.

If the buffer output, and function pointer output, were to be handled separately, this could improve the performance of both.

Buffer output, would no longer need to call a function pointer per character.
Function pointer output, would no longer need to test for a size limitation.