These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of -ffoo would be -fno-foo. In the table below, only one of the forms is listed—the one which is not the default. You can figure out the other form by either removing no- or adding it.
Treat each program unit (except those marked as RECURSIVE) as if the
SAVE statement were specified for every local variable and array
referenced in it. Does not affect common blocks. (Some Fortran compilers
provide this option under the name -static or -save.)
The default, which is -fautomatic, uses the stack for local
variables smaller than the value given by -fmax-stack-var-size.
Use the option -frecursive to use no static memory.
Generate code designed to be compatible with code generated
The calling conventions used by
g77 (originally implemented
f2c) require functions that return type
REAL to actually return the C type
functions that return type
COMPLEX to return the values via an
extra argument in the calling sequence that points to where to
store the return value. Under the default GNU calling conventions, such
functions simply return their results as they would in GNU
REAL functions return the C type
COMPLEX functions return the GNU C type
Additionally, this option implies the -fsecond-underscore
option, unless -fno-second-underscore is explicitly requested.
This does not affect the generation of code that interfaces with
Caution: It is not a good idea to mix Fortran code compiled with
-ff2c with code compiled with the default -fno-f2c
calling conventions as, calling
COMPLEX or default
functions between program parts which were compiled with different
calling conventions will break at execution time.
Caution: This will break code which passes intrinsic functions
of type default
COMPLEX as actual arguments, as
the library implementations use the -fno-f2c calling conventions.
Do not transform names of entities specified in the Fortran source file by appending underscores to them.
With -funderscoring in effect, GNU Fortran appends one underscore to external names with no underscores. This is done to ensure compatibility with code produced by many UNIX Fortran compilers.
Caution: The default behavior of GNU Fortran is
g77, please use the
-ff2c option if you want object files compiled with
GNU Fortran to be compatible with object code created with these
Use of -fno-underscoring is not recommended unless you are experimenting with issues such as integration of GNU Fortran into existing system environments (vis-à-vis existing libraries, tools, and so on).
For example, with -funderscoring, and assuming that
max_count() are external functions while
lvar are local variables, a statement like
I = J() + MAX_COUNT (MY_VAR, LVAR)
is implemented as something akin to:
i = j_() + max_count__(&my_var__, &lvar);
With -fno-underscoring, the same statement is implemented as:
i = j() + max_count(&my_var, &lvar);
Use of -fno-underscoring allows direct specification of user-defined names while debugging and when interfacing GNU Fortran code with other languages.
Note that just because the names match does not mean that the interface implemented by GNU Fortran for an external name matches the interface implemented by some other language for that same name. That is, getting code produced by GNU Fortran to link to code produced by some other compiler using this or any other method can be only a small part of the overall solution—getting the code generated by both compilers to agree on issues other than naming can require significant effort, and, unlike naming disagreements, linkers normally cannot detect disagreements in these other areas.
Also, note that with -fno-underscoring, the lack of appended underscores introduces the very real possibility that a user-defined external name will conflict with a name in a system library, which could make finding unresolved-reference bugs quite difficult in some cases—they might occur at program run time, and show up only as buggy behavior at run time.
In future versions of GNU Fortran we hope to improve naming and linking issues so that debugging always involves using the names as they appear in the source, even if the names as seen by the linker are mangled to prevent accidental linking between procedures with incompatible interfaces.
By default, GNU Fortran appends an underscore to external names. If this option is used GNU Fortran appends two underscores to names with underscores and one underscore to external names with no underscores. GNU Fortran also appends two underscores to internal names with underscores to avoid naming collisions with external names.
This option has no effect if -fno-underscoring is in effect. It is implied by the -ff2c option.
Otherwise, with this option, an external name such as
is implemented as a reference to the link-time external symbol
max_count__, instead of
max_count_. This is required
for compatibility with
f2c, and is implied
by use of the -ff2c option.
Disable coarray support; using coarray declarations and image-control statements will produce a compile-time error. (Default)
Single-image mode, i.e.
num_images() is always one.
Library-based coarray parallelization; a suitable GNU Fortran coarray library needs to be linked.
Enable the generation of run-time checks; the argument shall be a comma-delimited list of the following keywords. Prefixing a check with no- disables it if it was activated by a previous specification.
Enable all run-time test of -fcheck.
Warns at run time when for passing an actual argument a temporary array had to be generated. The information generated by this warning is sometimes useful in optimization, in order to avoid such temporaries.
Note: The warning is only printed once per location.
Enable generation of run-time checks for array subscripts and against the declared minimum and maximum values. It also checks array indices for assumed and deferred shape arrays against the actual allocated bounds and ensures that all string lengths are equal for character array constructors without an explicit typespec.
Some checks require that -fcheck=bounds is set for the compilation of the main program.
Note: In the future this may also include other forms of checking, e.g., checking substring references.
Enable generation of run-time checks for invalid modification of loop iteration variables.
Enable generation of run-time checks for memory allocation.
Note: This option does not affect explicit allocations using the
ALLOCATE statement, which will be always checked.
Enable generation of run-time checks for pointers and allocatables.
Enable generation of run-time checks for recursively called subroutines and functions which are not marked as recursive. See also -frecursive. Note: This check does not work for OpenMP programs and is disabled if used together with -frecursive and -fopenmp.
Example: Assuming you have a file foo.f90, the command
gfortran -fcheck=all,no-array-temps foo.f90
will compile the file with all checks enabled as specified above except warnings for generated array temporaries.
Deprecated alias for -fcheck=bounds.
Deprecated alias for -fcheck=array-temps.
This option can be used to increase the upper limit permitted in array constructors. The code below requires this option to expand the array at compile time.
program test implicit none integer j integer, parameter :: n = 100000 integer, parameter :: i(n) = (/ (2*j, j = 1, n) /) print '(10(I0,1X))', i end program test
Caution: This option can lead to long compile times and excessively large object files.
The default value for n is 65535.
This option specifies the size in bytes of the largest array that will be put on the stack; if the size is exceeded static memory is used (except in procedures marked as RECURSIVE). Use the option -frecursive to allow for recursive procedures which do not have a RECURSIVE attribute or for parallel programs. Use -fno-automatic to never use the stack.
This option currently only affects local arrays declared with constant bounds, and may not apply to all character variables. Future versions of GNU Fortran may improve this behavior.
The default value for n is 32768.
Adding this option will make the Fortran compiler put all arrays of unknown size and array temporaries onto stack memory. If your program uses very large local arrays it is possible that you will have to extend your runtime limits for stack memory on some operating systems. This flag is enabled by default at optimization level -Ofast unless -fmax-stack-var-size is specified.
This option tells GNU Fortran to pack derived type members as closely as possible. Code compiled with this option is likely to be incompatible with code compiled without this option, and may execute slower.
In some circumstances GNU Fortran may pass assumed shape array sections via a descriptor describing a noncontiguous area of memory. This option adds code to the function prologue to repack the data into a contiguous block at runtime.
This should result in faster accesses to the array. However it can introduce significant overhead to the function call, especially when the passed data is noncontiguous.
This option is provided for interoperability with C code that was
compiled with the -fshort-enums option. It will make
GNU Fortran choose the smallest
INTEGER kind a given
enumerator set will fit in, and give all its enumerators this kind.
This option will make
gfortran generate calls to BLAS functions
for some matrix operations like
MATMUL, instead of using our own
algorithms, if the size of the matrices involved is larger than a given
limit (see -fblas-matmul-limit). This may be profitable if an
optimized vendor BLAS library is available. The BLAS library will have
to be specified at link time.
Only significant when -fexternal-blas is in effect.
Matrix multiplication of matrices with size larger than (or equal to) n
will be performed by calls to BLAS functions, while others will be
gfortran internal algorithms. If the matrices
involved are not square, the size comparison is performed using the
geometric mean of the dimensions of the argument and result matrices.
The default value for n is 30.
When front-end optimiztion is active, some calls to the
intrinsic function will be inlined. This may result in code size
increase if the size of the matrix cannot be determined at compile
time, as code for both cases is generated. Setting
-finline-matmul-limit=0 will disable inlining in all cases.
Setting this option with a value of n will produce inline code
for matrices with size up to n. If the matrices involved are not
square, the size comparison is performed using the geometric mean of
the dimensions of the argument and result matrices.
The default value for n is 30. The
can be used to change this value.
Allow indirect recursion by forcing all local arrays to be allocated on the stack. This flag cannot be used together with -fmax-stack-var-size= or -fno-automatic.
The -finit-local-zero option instructs the compiler to
variables to zero,
LOGICAL variables to false, and
CHARACTER variables to a string of null bytes. Finer-grained
initialization options are provided by the
-finit-real=<zero|inf|-inf|nan|snan> (which also initializes
the real and imaginary parts of local
-finit-character=n (where n is an ASCII character
With -finit-derived, components of derived type variables will be initialized according to these flags. Components whose type is not covered by an explicit -finit-* flag will be treated as described above with -finit-local-zero.
These options do not initialize
(These limitations may be removed in future releases).
Note that the -finit-real=nan option initializes
COMPLEX variables with a quiet NaN. For a signalling NaN
use -finit-real=snan; note, however, that compile-time
optimizations may convert them into quiet NaN and that trapping
needs to be enabled (e.g. via -ffpe-trap).
Finally, note that enabling any of the -finit-* options will silence warnings that would have been emitted by -Wuninitialized for the affected local variables.
gfortran enforces proper alignment of all variables in a
COMMON block by padding them as needed. On certain platforms this is mandatory,
on others it increases performance. If a
COMMON block is not declared with
consistent data types everywhere, this padding can cause trouble, and
-fno-align-commons can be used to disable automatic alignment. The
same form of this option should be used for all files that share a
To avoid potential alignment issues in
COMMON blocks, it is recommended to order
objects from largest to smallest.
By default the parentheses in expression are honored for all optimization
levels such that the compiler does not do any re-association. Using
-fno-protect-parens allows the compiler to reorder
COMPLEX expressions to produce faster code. Note that for the re-association
optimization -fno-signed-zeros and -fno-trapping-math
need to be in effect. The parentheses protection is enabled by default, unless
-Ofast is given.
An allocatable left-hand side of an intrinsic assignment is automatically (re)allocated if it is either unallocated or has a different shape. The option is enabled by default except when -std=f95 is given. See also -Wrealloc-lhs.
Functions with identical argument lists are eliminated within
statements, regardless of whether these functions are marked
PURE or not. For example, in
a = f(b,c) + f(b,c)
there will only be a single call to
f. This option only works
if -ffrontend-optimize is in effect.
This option performs front-end optimization, based on manipulating
parts the Fortran parse tree. Enabled by default by any -O
option. Optimizations enabled by this option include inlining calls
MATMUL, elimination of identical function calls within
expressions, removing unnecessary calls to
TRIM in comparisons
and assignments and replacing
a(1:LEN_TRIM(a)). It can be deselected by specifying
Attempt to interchange loops in the Fortran front end where
profitable. Enabled by default by any -O option.
At the moment, this option only affects
DO CONCURRENT statements with several forall triplets.
See Options for Code Generation Conventions in Using the GNU Compiler Collection (GCC), for information on more options
offered by the GBE
gcc, and other GNU compilers.