ezconf_1.c

typedef enum {
  Ezconf_w_ok = 0,
  Ezconf_w_bad_conf_name,
  Ezconf_w_oom,
  Ezconf_w_no_home,
  Ezconf_w_mkdir_failed,
  Ezconf_w_conf_dir_not_dir,
  Ezconf_w_old_temp_file_stuck,
  Ezconf_w_temp_file_perm_denied,
  Ezconf_w_temp_open_failed,
  Ezconf_w_temp_fsync_failed,
  Ezconf_w_temp_close_failed,
  Ezconf_w_rename_failed,
  Ezconf_w_line_too_long,
  Ezconf_w_existing_read_error,
  Ezconf_w_unknown_error,
} Ezconf_w_error;

char const *ezconf_w_errorstring(Ezconf_w_error error);

typedef struct {
  char const *name;
  intptr_t id;
  uint8_t flags;
} Ezconf_opt;

typedef struct {
  Conf_save save; // 4 ptrs (FILE* and path strings)
  Ezconf_opt *opts;
  size_t optscount, optscap;
  intptr_t optid;
  FILE *file;
  Ezconf_w_error error;
  uint32_t stateflags;
} Ezconf_w;

void ezconf_w_start(Ezconf_w *ezcw, Ezconf_opt *optsbuffer,
                    size_t buffercap, char const *confname);
void ezconf_w_addopt(Ezconf_w *ezcw, char const *key, intptr_t id);
bool ezconf_w_step(Ezconf_w *ezcw);

// example

void write_my_conf(char const *username, int pickles, int expiration_year) {
  enum { Username, Coolpref, Pickles };
  Ezconf_opt optsbuff[32];
  Ezconf_w w;
  ezconf_w_start(&w, optsbuff, sizeof optsbuff / sizeof optsbuff[0],
                 "myprog.conf");
  ezconf_w_addopt(&w, "user_name", Username);
  ezconf_w_addopt(&w, "my_cool_pref", Coolpref);
  if (pickles > 0)
    ezconf_w_addopt(&w, "pickles", Pickles);
  while (ezconf_w_step(&w)) {
    switch (w.optid) {
    case Username:
      fputs(username, w.file);
      break;
    case Coolpref:
      fputs("some value", w.file);
      break;
    case Pickles:
      fprintf(w.file, "%d (expiring %d)", pickles, expiration_year);
      break;
    }
  }
  if (w.error)
    fprintf(stderr, "Conf writing error: %s\n", ezconf_w_errorstring(w.error));
}

#if 0

~/.config/myprog.conf (before)

  user_name = Dudeguy
  #some comment in this file
  random junk line, who knows
  user_name = Dudebro
  pickles = 3 (expiring 2019)
  my_cool_pref = some value


~/.config/myprog.conf (after)

  user_name = Dudeguy
  #some comment in this file
  random junk line, who knows
  #user_name = Dudebro
  pickles = 5 (expiring 2020)
  my_cool_pref = some value

~/.config replaced with $XDG_CONFIG_HOME if present.
Directory automatically created, if necessary.
Configuration file is first written to a temp file, and then moved onto existing.

#endif