c

How can I get a string representation for the error return values of wordexp(3) in C?

I'm writing a program where I need to do some shell path expansion, and I've decided to use wordexp(3) for this. Since the argument I'm giving to it is user-provided, I would like to present a relevant error message in the case of a non-zero return value. However, I can't seem to figure out how I would do this. Based on my reading of the manual page for this function, it doesn't seem to make use of errno, so I can't use perror(). I can't print the constant values it provides either since they just are integers at runtime.

How can I get a string representation of the error return values?

Solution

  • Regarding printing some enum error codes that are essentially just integers: just make a string table of error messages and print the one you want based on the error code.

    Solution 1

    It could be semi-automated somewhat by using "X macros":

    #include <wordexp.h>

#define WORDEXP_RESULT(X)                 \
/*  code          custom error message */ \
  X(WRDE_BADCHAR, "Bad char is bad!")     \
  X(WRDE_BADVAL,  "Bad value is bad!")    \
  // and so on...

    Then you'd call the function and check the result, for example with a switch:

    int result = wordexp(...);
switch(result)
{
  case 0: /* ok */ break;
  WORDEXP_RESULT(WORDEXP_ERR_CASE)
};

    Where the macro list boils down to some convenient macro:

    #define WORDEXP_ERR_CASE(val, str)                 \
  case (val):                                      \
  {                                                \
    fprintf(stderr,                                \
            "Error: wordexp failed. Code: %s. %s", \
            #val, str);                            \
    break;                                         \
  }

    Output from this will look like:

    Error: wordexp failed. Code: WRDE_BADCHAR. Bad char is bad!

    The advantage of the X macro here is that you get a direct coupling between the wordexp.h error codes and your error messages, plus macros mean we can "stringificate" the enumeration constants.

    Solution 2

    Although I cheated and took a peek at what these enums expand to. It turns out they are adjacent number from 1 to upward. So we could actually get rid of the switch without getting a huge string table. And at the same time write something more readable than X macros:

    if(result != 0)
{
  const char* wordexp_errmsg[] = 
  {
    [WRDE_BADCHAR] = "Bad char is bad!",
    [WRDE_BADVAL]  = "Bad value is bad!"
  };

  fprintf(stderr, "Error: wordexp failed. %s\n", wordexp_errmsg[result]);
}

    This has a coupling between error codes and strings too, but we lose the handy stringification of the enumeration constants themselves.