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.

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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); }