# How to call a C code (or a compiled library) from Sage ?¶

If you have some C/C++ code that you would like to call from Sage for your own use, this document is for you.

• Do you want to contibute to Sage by adding your interface to its code? The (more complex) instructions are available here.

## Calling “hello_world()” from hello.c¶

Let us suppose that you have a file named ~/my_dir/hello.c containing:

#include <stdio.h>

void hello_world(){
printf("Hello World\n");
}


In order to call this function from Sage, you must create a Cython file (i.e. a file whose extension is .pyx). Here, ~/my_dir/hello_sage.pyx contains a header describing the signature of the function that you want to call:

cdef extern from "hello.c":
void hello_world()

def my_bridge_function():
hello_world() # This is the C function from hello.c


You can now load this file in Sage, and call the C code though the Python function my_bridge_function:

sage: %runfile hello_sage.pyx
Compiling ./hello_sage.pyx...
sage: my_bridge_function()
Hello World


## Arguments and return value¶

The following example defines a function taking and returning int * pointers, and involves some memory allocation. The C code defines a function whose purpose is to return the sum of two vectors as a third vector.

The C file (double_vector.c)

#include <string.h>

int * sum_of_two_vectors(int n, int * vec1, int * vec2){
/*
INPUT : two arrays vec1,vec2 of n integers
OUTPUT: an array of size n equal to vec1+vec2
*/
int * sum = (int *) malloc(n*sizeof(int));
int i;

for(i=0;i<n;i++)
sum[i] = vec1[i] + vec2[i];
return sum;
}


The Cython file (double_vector_sage.pyx)

cdef extern from "double_vector.c":
int * sum_of_two_vectors(int n, int * vec1, int * vec2)

from libc.stdlib cimport malloc, free

def sage_sum_of_vectors(n, list1, list2):
cdef int * vec1 = <int *> malloc(n*sizeof(int))
cdef int * vec2 = <int *> malloc(n*sizeof(int))

# Fill the vectors
for i in range(n):
vec1[i] = list1[i]
vec2[i] = list2[i]

# Call the C function
cdef int * vec3 = sum_of_two_vectors(n,vec1,vec2)

# Save the answer in a Python object
answer = [vec3[i] for i in range(n)]

free(vec1)
free(vec2)
free(vec3)



Call from Sage:

sage: %runfile double_vector_sage.pyx
Compiling ./double_vector_sage.pyx...
sage: sage_sum_of_vectors(3,[1,1,1],[2,3,4])
[3, 4, 5]


## Calling code from a compiled library¶

The procedure is very similar again. For our purposes, we build a library from the file ~/my_dir/hello.c:

 #include <stdio.h>

void hello_world(){
printf("Hello World\n");
}


We also need a ~/my_dir/hello.h header file:

void hello_world();


We can now compile it as a library:

[[email protected] ~/my_dir/] gcc -c -Wall -Werror -fpic hello.c
[[email protected] ~/my_dir/] gcc -shared -o libhello.so hello.o


The only files that we need now are hello.h and libhello.so (you can remove the others if you like). We must now indicate the location of the .so and .h files in the header of our ~/my_dir/hello_sage.pyx file:

# distutils: libraries = /home/username/my_dir/hello

cdef extern from "hello.h":
void hello_world()

def my_bridge_function():
hello_world() # This is the C function from hello.c


Note

The instruction # distutils: libraries = /home/username/my_dir/hello indicates that the library is actually named /home/username/my_dir/hello. Change it according to your needs. For more information about these instructions, see http://cython.readthedocs.io/en/latest/src/reference/compilation.html#configuring-the-c-build

We can now load this file in Sage and call the function:

sage: %runfile hello_sage.pyx
Compiling ./hello_sage.pyx...
sage: my_bridge_function()
Hello World