using c executables / methods in an android ndk project

Question

I'm trying to use canutils in an android ndk-project. the package canutils usually compiles to executable files, but i didnt find a way yet to inlude these executables in an ndk-project. so what im doing at the moment is just loading the shared libraries like this:

static{
System.loadLibrary("cansend");
}
public native void cansend();

that for I've changes the android-mk to build shared libraries instead.

still my c-code looks like this cansend.c as an example :

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <net/if.h>
#include <sys/ioctl.h>
#include <sys/socket.h>

#include <linux/can.h>
#include <linux/can/raw.h>

#include "lib.h"

int main(int argc, char **argv)
{
int s; /* can raw socket */ 
int required_mtu;
int mtu;
int enable_canfd = 1;
struct sockaddr_can addr;
struct canfd_frame frame;
struct ifreq ifr;

/* check command line options */
if (argc != 3) {
    fprintf(stderr, "Usage: %s <device> <can_frame>.\n", argv[0]);
    return 1;
}

/* parse CAN frame */
required_mtu = parse_canframe(argv[2], &frame);
if (!required_mtu){
    fprintf(stderr, "\nWrong CAN-frame format! Try:\n\n");
    fprintf(stderr, "    <can_id>#{R|data}          for CAN 2.0 frames\n");
    fprintf(stderr, "    <can_id>##<flags>{data}    for CAN FD frames\n\n");
    fprintf(stderr, "<can_id> can have 3 (SFF) or 8 (EFF) hex chars\n");
    fprintf(stderr, "{data} has 0..8 (0..64 CAN FD) ASCII hex-values (optionally");
    fprintf(stderr, " seperated by '.')\n");
    fprintf(stderr, "<flags> a single ASCII Hex value (0 .. F) which defines");
    fprintf(stderr, " canfd_frame.flags\n\n");
    fprintf(stderr, "e.g. 5A1#11.2233.44556677.88 / 123#DEADBEEF / 5AA# / ");
    fprintf(stderr, "123##1 / 213##311\n     1F334455#1122334455667788 / 123#R ");
    fprintf(stderr, "for remote transmission request.\n\n");
    return 1;
}

/* open socket */
if ((s = socket(PF_CAN, SOCK_RAW, CAN_RAW)) < 0) {
    perror("socket");
    return 1;
}

addr.can_family = AF_CAN;

strcpy(ifr.ifr_name, argv[1]);
if (ioctl(s, SIOCGIFINDEX, &ifr) < 0) {
    perror("SIOCGIFINDEX");
    return 1;
}
addr.can_ifindex = ifr.ifr_ifindex;

if (required_mtu > CAN_MTU) {

    /* check if the frame fits into the CAN netdevice */
    if (ioctl(s, SIOCGIFMTU, &ifr) < 0) {
        perror("SIOCGIFMTU");
        return 1;
    }
    mtu = ifr.ifr_mtu;

    if (mtu != CANFD_MTU) {
        printf("CAN interface ist not CAN FD capable - sorry.\n");
        return 1;
    }

    /* interface is ok - try to switch the socket into CAN FD mode */
    if (setsockopt(s, SOL_CAN_RAW, CAN_RAW_FD_FRAMES,
               &enable_canfd, sizeof(enable_canfd))){
        printf("error when enabling CAN FD support\n");
        return 1;
    }

    /* ensure discrete CAN FD length values 0..8, 12, 16, 20, 24, 32, 64 */
    frame.len = can_dlc2len(can_len2dlc(frame.len));
}

/* disable default receive filter on this RAW socket */
/* This is obsolete as we do not read from the socket at all, but for */
/* this reason we can remove the receive list in the Kernel to save a */
/* little (really a very little!) CPU usage.                          */
setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, NULL, 0);

if (bind(s, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
    perror("bind");
    return 1;
}

/* send frame */
if (write(s, &frame, required_mtu) != required_mtu) {
    perror("write");
    return 1;
}

close(s);

return 0;
}

I want to be able to use this cansend method in my android-ndk-project. Do I need to adjust the c-code and make a shared library out of the code or do i need to use the executable and call and include it in my project in a certain way to be able to use it?

Turning it into a shared library might be the easier way, but both should be possible. — thejh
– thejh, Commented Jun 11, 2013 at 11:02
but if i turn it into a shared library, do i need to change the whole c-code and can i use this c-code as a shared libary and call cansend somehow? — DrDieHard
– DrDieHard, Commented Jun 11, 2013 at 11:10
i'm not sure if i understand you. can i just rename my main89 for example to cansend() and it should work as a shared library? — DrDieHard
– DrDieHard, Commented Jun 12, 2013 at 8:26

Robert Wang · Accepted Answer · 2015-03-29 10:05:41Z

1

Rename your main function or put another wrapper around it. If you want your native function to be named as cansend(), your wrapper function should be something like this:

#ifdef __cplusplus
extern "C" {
#endif

JNIEXPORT 
void
Java_com_aaa_bbb_ccc_cansend( JNIEnv* env, jobject thiz);

#ifdef __cplusplus
}
#endif

Here, com_aaa_bbb_ccc comes from your package name of your java code which contains public native void cansend();.

For example, if your package name is com.example.test, your function name will be:

Java_com_example_test_cansend();

answered Mar 29, 2015 at 10:05

Robert Wang

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