Python module installation fails because the gcc command is missing a flag...that the gcc command has

Question

I am attempting to build a (very simple) Python extension from C code and have run into a snag in the compilation.

(For the record: my C skills are very out-of-date and my code is probably awful.)

As directed in the docs and on another site I found, I created a C file that exposes the functionality I need built into a Python module:

#include "strcrypto.h"
#include <stdlib.h>

#define CRYPTO_POOLNUMBER 15

int main() {
    return 0;
}

char* encryptString(char* string) {
    COMM* comm;
    char* encrypted_string = malloc(sizeof(char) * 4096);
    int res;

    comm = init_client(CRYPTO_PORT);

    if (comm == NULL) {
        return NULL;
    }

    res = StrEncrypt(comm, string, encrypted_string, CRYPTO_POOLNUMBER);

    end_client(comm);
    if (res != 1) {
        return NULL;
    }
    else {
        return encrypted_string;
    }
}

char* decrypt(char* string) {
    COMM* comm;
    char* decrypted_string = malloc(sizeof(char) * 4096);
    int res;

    comm = init_client(CRYPTO_PORT);

    if (comm == NULL) {
        return NULL;
    }

    res = StrDecrypt(comm, string, decrypted_string);

    end_client(comm);
    if (res != 1) {
        return NULL;
    }
    else {
        return decrypted_string;
    }
}

char* randomToken(char* string) {
    COMM* comm;
    char* token = malloc(sizeof(char) * 4096);
    int res;

    comm = init_client(CRYPTO_PORT);

    if (comm == NULL) {
        return NULL;
    }

    res = getPBToken(comm, string, token);

    end_client(comm);
    if (res != 1) {
        return NULL;
    }
    else {
        return token;
    }
}

char* fixedToken(char* string) {
    COMM* comm;
    char* token = malloc(sizeof(char) * 4096);
    int res;

    comm = init_client(CRYPTO_PORT);

    if (comm == NULL) {
        return NULL;
    }

    res = getFixedToken(comm, string, token);

    end_client(comm);
    if (res != 1) {
        return NULL;
    }
    else {
        return token;
    }
}

I also created the C file that wraps the methods for Python:

#include <Python.h>

static PyObject* crypto_encrypt(PyObject* self, PyObject* args) {
    char* original_string;
    char* encrypted_string;
    PyObject* return_value;

    if (!PyArg_ParseTuple(args, "s", &original_string)) {
        return NULL;    // Could not parse string or no string was passed.
    }

    encrypted_string = encryptString(original_string);

    if (encrypted_string == NULL) {
        return NULL;
    }
    else {
        return_value = PyString_FromString(encrypted_string);
        free(encrypted_string);
        return return_value;
    }
}

static PyObject* crypto_decrypt(PyObject* self, PyObject* args) {
    char* original_string;
    char* decrypted_string;
    PyObject* return_value;

    if (!PyArg_ParseTuple(args, "s", &original_string)) {
        return NULL;    // Could not parse string or no string was passed.
    }

    decrypted_string = decryptString(original_string);

    if (decrypted_string == NULL) {
        return NULL;
    }
    else {
        return_value = PyString_FromString(decrypted_string);
        free(decrypted_string);
        return return_value;
    }
}

static PyObject* crypto_fixedToken(PyObject* self, PyObject* args) {
    char* original_string;
    char* token;
    PyObject* return_value;

    if (!PyArg_ParseTuple(args, "s", &original_string)) {
        return NULL;    // Could not parse string or no string was passed.
    }

    token = fixedToken(original_string);

    if (token == NULL) {
        return NULL;
    }
    else {
        return_value = PyString_FromString(token);
        free(token);
        return return_value;
    }
}

static PyObject* crypto_randomToken(PyObject* self, PyObject* args) {
    char* original_string;
    char* token;
    PyObject* return_value;

    if (!PyArg_ParseTuple(args, "s", &original_string)) {
        return NULL;    // Could not parse string or no string was passed.
    }

    token = randomToken(original_string);

    if (token == NULL) {
        return NULL;
    }
    else {
        return_value = PyString_FromString(token);
        free(token);
        return return_value;
    }
}

static PyMethodDef CryptoMethods[] = {
    { "encrypt", crypto_encrypt, METH_VARARGS, "Encrypt text using a Crypto server." },
    { "decrypt", crypto_decrypt, METH_VARARGS, "Decrypt text encrypted using a Crypto server." },
    { "getFixedToken", crypto_fixedToken, METH_VARARGS, "Return a fixed PKCS5 token from Crypto." },
    { "getRandomToken", crypto_randomToken, METH_VARARGS, "Return a random PKCS5 token from Crypto." },
    { NULL, NULL, 0, NULL }
};

PyMODINIT_FUNC initcrypto(void) {
    (void) Py_InitModule("crypto", CryptoMethods);
}

Finally, I created a setup.py file that creates the extension and calls setup() on the extension:

from distutils.core import setup, Extension

ext = Extension("crypto",
    ["cryptomodule.c", "crypto.c"],
    libraries = ["strcrypto",],
    library_dirs = ["/usr/local/lib",]
)

setup(name = "crypto", ext_modules = [ext,])

The library "strcrypto" is actually "libstrcrypto.a" and was provided to me; it wasn't something I wrote. Unfortunately, that is also what appears to be causing the error when I go to install the file:

running install
running build
running build_ext
building 'crypto' extension
x86_64-linux-gnu-gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/include/python2.7 -c cryptomodule.c -o build/temp.linux-x86_64-2.7/cryptomodule.o
cryptomodule.c: In function ‘crypto_encrypt’:
cryptomodule.c:15:5: warning: implicit declaration of function ‘encryptString’ [-Wimplicit-function-declaration]
     encrypted_string = encryptString(original_string);
     ^
cryptomodule.c:15:22: warning: assignment makes pointer from integer without a cast [enabled by default]
     encrypted_string = encryptString(original_string);
                      ^
cryptomodule.c: In function ‘crypto_decrypt’:
cryptomodule.c:36:5: warning: implicit declaration of function ‘decryptString’ [-Wimplicit-function-declaration]
     decrypted_string = decryptString(original_string);
     ^
cryptomodule.c:36:22: warning: assignment makes pointer from integer without a cast [enabled by default]
     decrypted_string = decryptString(original_string);
                      ^
cryptomodule.c: In function ‘crypto_fixedToken’:
cryptomodule.c:57:5: warning: implicit declaration of function ‘fixedToken’ [-Wimplicit-function-declaration]
     token = fixedToken(original_string);
     ^
cryptomodule.c:57:11: warning: assignment makes pointer from integer without a cast [enabled by default]
     token = fixedToken(original_string);
           ^
cryptomodule.c: In function ‘crypto_randomToken’:
cryptomodule.c:78:5: warning: implicit declaration of function ‘randomToken’ [-Wimplicit-function-declaration]
     token = randomToken(original_string);
     ^
cryptomodule.c:78:11: warning: assignment makes pointer from integer without a cast [enabled by default]
     token = randomToken(original_string);
           ^
x86_64-linux-gnu-gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/include/python2.7 -c crypto.c -o build/temp.linux-x86_64-2.7/crypto.o
crypto.c:6:5: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
 int main() {
     ^
x86_64-linux-gnu-gcc -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions -Wl,-Bsymbolic-functions -Wl,-z,relro -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -D_FORTIFY_SOURCE=2 -g -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security build/temp.linux-x86_64-2.7/cryptomodule.o build/temp.linux-x86_64-2.7/crypto.o -L/usr/local/lib -lstrcrypto -o build/lib.linux-x86_64-2.7/crypto.so
/usr/bin/ld: /usr/local/lib/libstrcrypto.a(strcrypto.o): relocation R_X86_64_32 against `.rodata.str1.1' can not be used when making a shared object; recompile with -fPIC
/usr/local/lib/libstrcrypto.a: error adding symbols: Bad value
collect2: error: ld returned 1 exit status
error: command 'x86_64-linux-gnu-gcc' failed with exit status 1

I don't know what that error means, because as you can see, the gcc calls are indeed using -fPIC.

Can someone please point out what I'm doing wrong here? Thanks in advance.

sterin · Accepted Answer · 2015-03-20 04:35:24Z

1

The Python extension you are building is a shared library. The code in a shared library

must be compiled to be position independent. This is done by compiling with -fPIC.

Your problem is that your libstrcrypto.a is not compiled with -fPIC and therefore cannot be used inside a shared library. You have several alternatives:

Ask for a version of libstrcrypto.a that is compiled with -fPIC.
Ask for a shared library version of libstrcrypto.a.
Link your extension statically to produce a new Python executable that contains your extension built in.
Split the extension into an executable that links with the library and a python extension that somehow communicates with the executable.

edited Mar 20, 2015 at 4:35

answered Mar 15, 2015 at 1:35

sterin

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2 Comments

TrevorNT Over a year ago

Thanks for the reply. My coworker has to go back to the vendor and ask about the version of strcrypto that is compiled with -fPIC. In the event that he's unable to do so, what are my options on getting the strcrypto library to work in Python?

TrevorNT Over a year ago

Nevermind, the vendor delivered the updated module. Thanks again for your help!

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