Non-Fortran Main Program

Even if you are doing mixed-language programming, it is very likely that you do not need to know or use the information in this section. Since it is about the internal structure of GNU Fortran, it may also change in GCC minor releases.

When you compile a PROGRAM with GNU Fortran, a function with the name main (in the symbol table of the object file) is generated, which initializes the libgfortran library and then calls the actual program which uses the name MAIN__, for historic reasons. If you link GNU Fortran compiled procedures to, e.g., a C or C++ program or to a Fortran program compiled by a different compiler, the libgfortran library is not initialized and thus a few intrinsic procedures do not work properly, e.g. those for obtaining the command-line arguments.

Therefore, if your PROGRAM is not compiled with GNU Fortran and the GNU Fortran compiled procedures require intrinsics relying on the library initialization, you need to initialize the library yourself. Using the default options, gfortran calls _gfortran_set_args and _gfortran_set_options. The initialization of the former is needed if the called procedures access the command line (and for backtracing); the latter sets some flags based on the standard chosen or to enable backtracing. In typical programs, it is not necessary to call any initialization function.

If your PROGRAM is compiled with GNU Fortran, you shall not call any of the following functions. The libgfortran initialization functions are shown in C syntax but using C bindings they are also accessible from Fortran.

_gfortran_set_args — Save command-line arguments

void _gfortran_set_args(int argc, char *argv[])

_gfortran_set_args saves the command-line arguments; this initialization is required if any of the command-line intrinsics is called. Additionally, it shall be called if backtracing is enabled (see _gfortran_set_options).

Parameters

• argc – number of command line argument strings

• argv – the command-line argument strings; argv[0] is the pathname of the executable itself.

Example:

int main (int argc, char *argv[])
{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  return 0;
}

_gfortran_set_options — Set library option flags

void _gfortran_set_options(int num, int options[])

_gfortran_set_options sets several flags related to the Fortran standard to be used, whether backtracing should be enabled and whether range checks should be performed. The syntax allows for upward compatibility since the number of passed flags is specified; for non-passed flags, the default value is used. See also see Options for code generation conventions. Please note that not all flags are actually used.

Parameters

• num – number of options passed

• argv – The list of flag values

option flag list:

option [0]	Allowed standard; can give run-time errors if e.g. an input-output edit descriptor is invalid in a given standard. Possible values are (bitwise or-ed) GFC_STD_F77 (1), GFC_STD_F95_OBS (2), GFC_STD_F95_DEL (4), GFC_STD_F95 (8), GFC_STD_F2003 (16), GFC_STD_GNU (32), GFC_STD_LEGACY (64), GFC_STD_F2008 (128), GFC_STD_F2008_OBS (256), GFC_STD_F2008_TS (512), GFC_STD_F2018 (1024), GFC_STD_F2018_OBS (2048), and GFC_STD=F2018_DEL (4096). Default: GFC_STD_F95_OBS | GFC_STD_F95_DEL | GFC_STD_F95 | GFC_STD_F2003 | GFC_STD_F2008 | GFC_STD_F2008_TS | GFC_STD_F2008_OBS | GFC_STD_F77 | GFC_STD_F2018 | GFC_STD_F2018_OBS | GFC_STD_F2018_DEL | GFC_STD_GNU | GFC_STD_LEGACY.
option [1]	Standard-warning flag; prints a warning to standard error. Default: GFC_STD_F95_DEL | GFC_STD_LEGACY.
option [2]	If non zero, enable pedantic checking. Default: off.
option [3]	Unused.
option [4]	If non zero, enable backtracing on run-time errors. Default: off. (Default in the compiler: on.) Note: Installs a signal handler and requires command-line initialization using _gfortran_set_args.
option [5]	If non zero, supports signed zeros. Default: enabled.
option [6]	Enables run-time checking. Possible values are (bitwise or-ed): GFC_RTCHECK_BOUNDS (1), GFC_RTCHECK_ARRAY_TEMPS (2), GFC_RTCHECK_RECURSION (4), GFC_RTCHECK_DO (8), GFC_RTCHECK_POINTER (16), GFC_RTCHECK_MEM (32), GFC_RTCHECK_BITS (64). Default: disabled.
option [7]	Unused.
option [8]	Show a warning when invoking STOP and ERROR STOP if a floating-point exception occurred. Possible values are (bitwise or-ed) GFC_FPE_INVALID (1), GFC_FPE_DENORMAL (2), GFC_FPE_ZERO (4), GFC_FPE_OVERFLOW (8), GFC_FPE_UNDERFLOW (16), GFC_FPE_INEXACT (32). Default: None (0). (Default in the compiler: GFC_FPE_INVALID | GFC_FPE_DENORMAL | GFC_FPE_ZERO | GFC_FPE_OVERFLOW | GFC_FPE_UNDERFLOW.)

Example:

/* Use gfortran 4.9 default options.  */
static int options[] = {68, 511, 0, 0, 1, 1, 0, 0, 31};
_gfortran_set_options (9, &options);

_gfortran_set_convert — Set endian conversion

void _gfortran_set_convert(int conv)

_gfortran_set_convert set the representation of data for unformatted files.

Parameters

conv – Endian conversion, possible values: GFC_CONVERT_NATIVE (0, default), GFC_CONVERT_SWAP (1), GFC_CONVERT_BIG (2), GFC_CONVERT_LITTLE (3).

Example:

int main (int argc, char *argv[])
{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  _gfortran_set_convert (1);
  return 0;
}

_gfortran_set_record_marker — Set length of record markers

void _gfortran_set_record_marker(int val)

_gfortran_set_record_marker sets the length of record markers for unformatted files.

Parameters

val – Length of the record marker; valid values are 4 and 8. Default is 4.

Example:

int main (int argc, char *argv[])
{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  _gfortran_set_record_marker (8);
  return 0;
}

_gfortran_set_fpe — Enable floating point exception traps

void _gfortran_set_fpe(int val)

_gfortran_set_fpe enables floating point exception traps for the specified exceptions. On most systems, this will result in a SIGFPE signal being sent and the program being aborted.

Parameters

option} [0] – IEEE exceptions. Possible values are (bitwise or-ed) zero (0, default) no trapping, GFC_FPE_INVALID (1), GFC_FPE_DENORMAL (2), GFC_FPE_ZERO (4), GFC_FPE_OVERFLOW (8), GFC_FPE_UNDERFLOW (16), and GFC_FPE_INEXACT (32).

Example:

int main (int argc, char *argv[])
{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  /* FPE for invalid operations such as SQRT(-1.0).  */
  _gfortran_set_fpe (1);
  return 0;
}

_gfortran_set_max_subrecord_length — Set subrecord length

void _gfortran_set_max_subrecord_length(int val)

_gfortran_set_max_subrecord_length set the maximum length for a subrecord. This option only makes sense for testing and debugging of unformatted I/O.

Parameters

val – the maximum length for a subrecord; the maximum permitted value is 2147483639, which is also the default.

Example:

int main (int argc, char *argv[])
{
  /* Initialize libgfortran.  */
  _gfortran_set_args (argc, argv);
  _gfortran_set_max_subrecord_length (8);
  return 0;
}