C与Fortran都是应用广泛的语言，由于二者语言特性的不同，应用的场合很有差异。C语言通常用于系统软件和一些应用软件的开发，而Fortran在高性能应用领域有很好的应用。对于通常的用户，接促C语言的较多，但是，C语言的开发者，有时还想利用高效的Fortran 数据包 或者是Fortran语言的开发者想借用C提供的强大辅助功能，为此，为了方便大家对二语言相互调用的学习。这里给出了一些简单的入门性技术介绍。
1. C 调用Fortran
编辑Fortran 源文件 add.f95
subroutine add(a, b, c)
implicit none
real:: a, b, c
!
! the add routine
! c = a+b
!
c = a + b
write(*,*) a, ’+’, b, ’=’, c
return
end subroutine add
这里部介绍Fortran的语法格式，add.f95的功能是建立一个add routine 也就是函数。
编译 add.f95
f95 -o add.o -c add.f95
编辑C 主程序main.c
1 #include <stdio.h>
2
3 extern void add_(float *a, float *b, float *c);
4
5 int main(int argc, char *argvs[])
6 {
7 float a, b, c;
8 a = 5.0;
9 b = 7.0;
10 add_(&a, &b, &c);
11
12 return 0;
13 }
3: 声明你要调用的函数，这里注意，是add_，调用的时候用指针，即变量地址，你应该明白了Fortran的调用是改变参数值的。
10：调用fortran函数
编译与执行
#gfortran -o main main.c add.o
#./main
5.000000 + 7.000000 = 12.00000
NOTES; if you use the gcc tool, it will generate error
#gcc -o main main.c add.o
add.o: In function `add_’:
add.f95:(.text+0x4c): undefined reference to `_gfortran_st_write’
add.f95:(.text+0x69): undefined reference to `_gfortran_transfer_real’
add.f95:(.text+0x87): undefined reference to `_gfortran_transfer_character’
add.f95:(.text+0xa4): undefined reference to `_gfortran_transfer_real’
add.f95:(.text+0xc2): undefined reference to `_gfortran_transfer_character’
add.f95:(.text+0xdf): undefined reference to `_gfortran_transfer_real’
add.f95:(.text+0xed): undefined reference to `_gfortran_st_write_done’
collect2: ld returned 1 exit status
Some Wrong with your PATH for you gcc library
OK! you are clever and has known how call fortran routines in the C progaram. Next part I show you how to call C routines in the Fortran program!
2, Fortran 调用 C 函数
编辑C源码add.c
1 #include <stdio.h>
2
3 void add_(float *a, float *b, float *c)
4 {
5 *c = *a + *b;
6 printf(\"%f + %f = %f\\n\", *a, *b, *c);
7 }
Notes The funtion name \"add_\"
编译源代码
# gcc -o add.o -c add.c
编辑fortran 主程序main.f95，编译和执行
1 PROGRAM MAIN
2 implicit none [Page]
3 real:: i, j, k
4 !
5 ! The first fortran program
6 !
7 i = 5.0
8 j = 8.0
9 call add(i, j, k)
10 stop
11 end program MAIN
NOTE 9 line
# gfortran -o main main.f95 add.o (or f95 -o main main.f95 add.o)
# ./main
5.000000 + 8.000000 = 13.000000
OK. If you reach here, congratulations, you have learned how to call routines between C and fortran!