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Note that the code documentation says

@todo Use INDEF instead of 0.0 for pointing as RA/DEC selection

This seems another interesting solution, but requires first the implementation of the INDEF logic in the GPar class. Needs some further reading on the IRAP parameter file interface.

Here the relevant code in ape_util.c that shows how APE (HEASARC) is handling the thing. Apparently, the string2double() and string2long() functions return a status that can be undefined for INDEF and NONE (case insensitive) or NaN/LONG_MAX for INF, INFINITY or NAN:

/* Strictly convert input string to output bool, ignoring leading/trailing white space, and flagging every possible error. */
static int string2bool(const char * input, char * result) {
  int status = eOK;
  if (0 != result) {
    *result = 0;
    if (0 != input) {
      /* Truncate white space from input string. */
      char * copy = truncate_white_space(input);

      /* Compare input string (case insensitive) to various boolean expressions. */
      if (0 == ci_strcmp(copy, "true") || 0 == ci_strcmp(copy, "t") ||
        0 == ci_strcmp(copy, "yes") || 0 == ci_strcmp(copy, "y")) {
        *result = 1;
      } else if (0 == ci_strcmp(copy, "false") || 0 == ci_strcmp(copy, "f") ||
        0 == ci_strcmp(copy, "no") || 0 == ci_strcmp(copy, "n")) {
        *result = 0;
      } else {
        /* String does not contain a valid bool expression. */
        status = eConversionError;
      }

      /* Clean up. */
      free(copy); copy = 0;
    } else {
      status = eNullPointer;
    }
  } else {
    status = eNullPointer;
  }
  return status;
}

/* Strictly convert input string to output double, ignoring leading/trailing white space, and flagging every possible error. */
static int string2double(const char * input, double * result) {
  int status = eOK;
  if (0 != result) {
    *result = 0.;
    if (0 != input) {
      char * remainder = 0;
      /* Skip leading white space. */
      while (0 != isspace(*input)) ++input;

      if ('\0' == *input || 0 == ci_strcmp(input, "indef") || 0 == ci_strcmp(input, "none") || 0 == ci_strcmp(input, "undef") ||
        0 == ci_strcmp(input, "undefined")) {
        status = eUndefinedValue;
      } else if (0 == ci_strcmp(input, "inf") || 0 == ci_strcmp(input, "infinity") || 0 == ci_strcmp(input, "nan")) {
        status = eNan;
      } else {
        errno = 0;
        *result = strtod(input, &remainder);
        if (ERANGE == errno) {
          if (0. == *result) status = eUnderflow;
          else status = eOverflow;
        } else if (0 != remainder && '\0' != *remainder) {
          /* Ignore trailing whitespace; the C standard is not specific about whether it's returned or eaten by the conversion. */
          while (0 != isspace(*remainder)) ++remainder;
          if ('\0' != *remainder) status = eStringRemainder;
          /* else conversion succeeded. */
        } else {
          /* Conversion succeeded! */
        }
      }
    } else {
      status = eNullPointer;
    }
  } else {
    status = eNullPointer;
  }
  return status;
}

/* Strictly convert input string to output long, ignoring leading/trailing white space, and flagging every possible error. */
static int string2long(const char * input, long * result) {
  int status = eOK;
  if (0 != result) {
    *result = 0;
    if (0 != input) {
      char * remainder = 0;
      errno = 0;
      /* Skip leading white space. */
      while (0 != isspace(*input)) ++input;

      if ('\0' == *input || 0 == ci_strcmp(input, "indef") || 0 == ci_strcmp(input, "none") || 0 == ci_strcmp(input, "undef") ||
        0 == ci_strcmp(input, "undefined")) {
        status = eUndefinedValue;
      } else if (0 == ci_strcmp(input, "inf") || 0 == ci_strcmp(input, "infinity") || 0 == ci_strcmp(input, "nan")) {
        *result = LONG_MAX;
      } else {
        *result = strtol(input, &remainder, 0);
        if (ERANGE == errno) {
          if (LONG_MIN == *result) status = eUnderflow;
          else if (LONG_MAX == *result) status = eOverflow;
          else status = eConversionError;
        } else if (0 != remainder && '\0' != *remainder) {
          /* Ignore trailing whitespace; the C standard is not specific about whether it's returned or eaten by the conversion. */
          while (0 != isspace(*remainder)) ++remainder;
          if ('\0' != *remainder) status = eStringRemainder;
          /* else conversion succeeded. */
        } else {
          /* Conversion succeeded! */
        }
      }
    } else {
      status = eNullPointer;
    }
  } else {
    status = eNullPointer;
  }
  return status;
}

Parameter usepnt has been added.