The GNU Fortran compiler’s parser operates by testing each piece of source code against a variety of matchers. In some cases, if these matchers do not match the source code, they will store an error message in a buffer. If the parser later finds a matcher that does correctly match the source code, then the buffered error is discarded. However, if the parser cannot find a match, then the buffered error message is reported to the user. This enables the compiler to provide more meaningful error messages even in the many cases where (erroneous) Fortran syntax is ambiguous due to things like the absence of reserved keywords.
As an example of how this works, consider the following line:
IF = 3
Hypothetically, this may get passed to the matcher for an IF
statement. Since this could plausibly be an erroneous IF
statement, the matcher will buffer an error message reporting the
absence of an expected ‘(’ following an IF. Since no
matchers reported an error-free match, however, the parser will also try
matching this against a variable assignment. When IF is a valid
variable, this will be parsed as an assignment statement, and the error
discarded. However, when IF is not a valid variable, this
buffered error message will be reported to the user.
The error handling code is implemented in error.cc. Errors are
normally entered into the buffer with the gfc_error function.
Warnings go through a similar buffering process, and are entered into
the buffer with gfc_warning. There is also a special-purpose
function, gfc_notify_std, for things which have an error/warning
status that depends on the currently-selected language standard.
The gfc_error_check function checks the buffer for errors,
reports the error message to the user if one exists, clears the buffer,
and returns a flag to the user indicating whether or not an error
existed. To check the state of the buffer without changing its state or
reporting the errors, the gfc_error_flag_test function can be
used. The gfc_clear_error function will clear out any errors in
the buffer, without reporting them. The gfc_warning_check and
gfc_clear_warning functions provide equivalent functionality for
the warning buffer.
Only one error and one warning can be in the buffers at a time, and
buffering another will overwrite the existing one. In cases where one
may wish to work on a smaller piece of source code without disturbing an
existing error state, the gfc_push_error, gfc_pop_error,
and gfc_free_error mechanism exists to implement a stack for the
error buffer.
For cases where an error or warning should be reported immediately
rather than buffered, the gfc_error_now and
gfc_warning_now functions can be used. Normally, the compiler
will continue attempting to parse the program after an error has
occurred, but if this is not appropriate, the gfc_fatal_error
function should be used instead. For errors that are always the result
of a bug somewhere in the compiler, the gfc_internal_error
function should be used.
The syntax for the strings used to produce the error/warning message in
the various error and warning functions is similar to the printf
syntax, with ‘%’-escapes to insert variable values. The details,
and the allowable codes, are documented in the error_print
function in error.cc.