Smart variadic expansion based on format string - c

Question

I have a daemon that reads a configuration file in order to know where to write something. In the configuration file, a line like this exists:
output = /tmp/foo/%d/%s/output
Or, it may look like this:
output = /tmp/foo/%s/output/%d
... or simply like this:
output = /tmp/foo/%s/output
... or finally:
output = /tmp/output
I have that line as cfg->pathfmt within my program. What I am trying to do now is to come up with some clever way of using it.
A little more explanation, the path can contain up to two components to be formatted. %d will be expanded as a job ID (int), %s as a job name (string). The user may want to use one, both or none in the configuration file. I need to know what they want and in what order before I finally pass it to snprintf(). I can kind of narrow it down, but I keep wanting to talk to strtok() and that seems ugly.
I want to give users this kind of flexibility, however I'm getting lost looking for a sensible, portable way to implement it. I'm also at a complete and total loss for how to begin searching for this.
I'd be very happy if:
Someone could help me narrow down the search phrase to find good examples
Someone could post a link to some OSS project implementing this
Someone could post some psuedo code
I don't want the code written for me, I'm just really stuck on what (I think) should be something very simple and need some help taking the first bite. I really feel like I'm over thinking and overlooking the obvious.
The end result should be a boolean function like this:
bool output_sugar(const char *fmt, int jobid, const char *jobname, struct job *j);
It would then call snprintf() (sensibly) on j->outpath, returning false if some kind of garbage (i.e. % followed by something not s, d or %) is in the config line (or its null). The sanity checks are easy, I'm just having a bit of a time getting the number (and order) of arguments to format correct.
Thanks in advance. Also, feel free to edit this title if you have the reputation to do so, as I said, I'm not quite sure how to ask the question in a single line. I think what I need is a parser, but it feels awkward using a full blown lexer / parser to handle one simple string.

Answer 1

Yes, you need a parser of some sort. It need not be complex, though:
void format_filename(const char *fmt, int jobid, const char *jobname,
char *buffer, size_t buflen)
{
char *end = buffer + buflen - 1;
const char *src = fmt;
char *dst = buffer;
char c;
assert(buffer != 0 && fmt != 0 && buflen != 0 && jobname != 0);
while ((c = *src++) != '\0')
{
if (dst >= end)
err_exit("buffer overflow in %s(): format = %s\n",
__func__, fmt);
else if (c != '%')
*dst++ = c;
else if ((c = *src++) == '\0' || c == '%')
{
*dst++ = '%';
if (c == '\0')
break;
}
else if (c == 's')
{
size_t len = strlen(jobname);
if (len > end - dst)
err_exit("buffer overflow on jobname in %s(): format = %s\n",
__func__, fmt);
else
{
strcpy(dst, jobname);
dst += len;
}
}
else if (c == 'd')
{
int nchars = snprintf(dst, end - dst, "%d", jobid);
if (nchars < 0 || nchars >= end - dst)
err_exit("format error on jobid in %s(); format = %s\n",
__func__, fmt);
dst += nchars;
}
else
err_exit("invalid format character %d in %s(): format = %s\n",
c, __func__, fmt);
}
*dst = '\0';
}
Now tested code. Note that it supports the '%%' notation to allow the user to embed a single '%' in the output. Also, it treats a single '%' at the end of the string as valid and equivalent to '%%'. It calls err_exit() on error; you can choose alternative error strategies as suits your system. I simply assume you have included <assert.h>, <stdio.h> and <string.h> and the header for the err_exit() (variadic) function.
Test code...
#include <stdio.h>
#include <string.h>
#include <stdarg.h>
#include <assert.h>
static void err_exit(const char *fmt, ...)
{
va_list args;
va_start(args, fmt);
vfprintf(stderr, fmt, args);
va_end(args);
exit(1);
}
... then format_filename() as above, then ...
#define DIM(x) (sizeof(x)/sizeof(*(x)))
static const char *format[] =
{
"/tmp/%d/name/%s",
"/tmp/%s/number/%d",
"/tmp/%s.%d%%",
"/tmp/%",
};
int main(void)
{
char buffer[64];
size_t i;
for (i = 0; i < DIM(format); i++)
{
format_filename(format[i], 1234, "job-name", buffer, sizeof(buffer));
printf("fmt = %-20s; name = %s\n", format[i], buffer);
}
return(0);
}

Answer 2

Using strtok is a error prone. You can treat your variables as a mini language using (fl)lex and yacc. There is simple tutorial here
%{
#include <stdio.h>
%}
%%
%d printf("%04d",jobid);
%s printf("%s",stripspaces(dirname));
%%
I made an ODBC wrapper that would let you do stuff like dbprintf("insert into blah values %s %D %T %Y", stuff here...); But it was many years ago and I bit it and parsed the format string using strtok.

Answer 3

If the number of options is small and you don't otherwise want/need the extra flexibility and complexity of a parser, you could simply search for each potential replacement substring using strstr().
If you have only the two options, you could tolerably create a four-branched if/else structure (only A, only B, both with A before B, both with B before A) in which to call sprintf() with the correctly ordered arguments. Otherwise, make multiple sprintf() calls, each of which replaces only the first replacement-marker in the format string. (This implies building a list of which replacements are needed and sorting them in appearance-order...)