As I understand the gfortran docs, the -ff2c/-fno-f2c switches change how functions returning complex numbers are implemented:
-ff2c: the return value is returned in an additional argument as g77 and other fortran compilers do it (can be seen e. g. in http://www.pgroup.com/doc/pvfug.pdf, chapter 11 and http://www.intel.com/support/performancetools/libraries/mkl/sb/cs-017175.htm) -fno-f2c (which is the default): the return value is returned in a C compatible manner without an additional argument. The latter does not work for me. To reproduce, compile the snippet complex function fcomplex_func(i) implicit none integer i fcomplex_func = cmplx(i, 2*i) end double complex function dcomplex_func(i) implicit none integer i dcomplex_func = dcmplx(i, 2*i) end with gfortran and the snippet #include <stdio.h> typedef struct { float r; float i; } fcomplex; typedef struct { double r; double i; } dcomplex; #ifdef FF2C extern fcomplex fcomplex_func__(int const *); extern dcomplex dcomplex_func__(int const *); #else extern void fcomplex_func_(fcomplex *, int const *); extern void dcomplex_func_(dcomplex *, int const *); #endif int main(){ int const i = 12; int const j = 13; fcomplex fc = {7,8}; dcomplex dc = {9,10}; #ifdef FF2C printf("c return %d %d\n", i, j); fc = fcomplex_func__(&i); dc = dcomplex_func__(&j); #else printf("nonc return %d %d\n", i, j); fcomplex_func_(&fc, &i); dcomplex_func_(&dc, &j); #endif printf("fcomplex_func: %d %g %g\n", i, fc.r, fc.i); printf("dcomplex_func: %d %lg %lg\n", j, dc.r, dc.i); return 0; } with gcc and link both together. The expected output of the program is nonc return 12 13 fcomplex_func: 12 12 24 dcomplex_func: 13 13 26 but here it goes into an endless loop and prints nonc return 12 13 fcomplex_func: 12 7 8 dcomplex_func: 13 13 26 repeatedly until manual abort. Note that the double complex result is correct, but nevertheless the call of dcomplex_func does something bad (stack corruption?). The endless loop persists if you comment out the call of fcomplex_func. If you use the -ff2c switch of gfortran, and define the FF2C preprocessor variable for gcc everything works well, and the output is as expected nonc return 12 13 fcomplex_func: 12 12 24 dcomplex_func: 13 13 26 . I tried 4.2.4 svn from today, and the debian packages 4.2.3-2 and 4.3.0-1, and all versions show this problem. -- Summary: return type of complex functions not C compatible Product: gcc Version: 4.2.4 Status: UNCONFIRMED Severity: normal Priority: P3 Component: fortran AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: Georg dot Baum at post dot rwth-aachen dot de GCC target triplet: i686-pc-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35765
