clinkerrouteropenwrtgpsd

Compiling against gpsd on OpenWRT - linking fails

I'm trying to compile a tool that uses gps.h, but my compilation seems to fail each time when it tries to link to libgps. The error message I receive is:

/opt/openwrt-sdk/staging_dir/toolchain-arm_cortex-a9+vfpv3_gcc-7.3.0_musl_eabi/bin/../lib/gcc/arm-openwrt-linux-muslgnueabi/7.3.0/../../../../arm-openwrt-linux-muslgnueabi/bin/ld: cannot find -lgps

This is the command I'm compiling with:

arm-openwrt-linux-gcc -o ./bin/eagle src/main.c -I./src -I/opt/openwrt-sdk/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include -static -L/opt/openwrt-sdk/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/lib  -lpthread -lgps

Basic code for reference:

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

int main(void)
{
  int rc;
  struct gps_data_t gps_data;
  if ((rc = gps_open("localhost", "2947", &gps_data)) == -1)
  {
    printf("code: %d, reason: %s\n", rc, gps_errstr(rc));
    return 1;
  }
  gps_stream(&gps_data, WATCH_ENABLE | WATCH_JSON, NULL);

  return 0;
}

And some directory listings in my toolchain - as far as I can tell, libgps has compiled successfully:

# ls -lah /opt/openwrt-sdk/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/include/ | grep gps

-rw-rw-r-- 1 root root  80K Sep  7  2017 gps.h

# ls -lah /opt/openwrt-sdk/staging_dir/target-arm_cortex-a9+vfpv3_musl_eabi/usr/lib/ | grep gps

lrwxrwxrwx 1 root root   16 Oct 17 18:46 libgps.so -> libgps.so.23.0.0
lrwxrwxrwx 1 root root   16 Oct 17 18:46 libgps.so.23 -> libgps.so.23.0.0
-rwxr-xr-x 1 root root 101K Oct 17 18:46 libgps.so.23.0.0

Many thanks in advance for any help.

Solution

  • Your link command line contains '-static', which prevents linking against *.so shared libraries (aka shared objects, hence 'so'), but still permits linking against *.a static-link libraries (aka archives). See: https://gcc.gnu.org/onlinedocs/gcc/Link-Options.html

    In the directory whose contents you listed, and which the link command line is directed towards using the '-L' flag, there is a shared object libgps.so, but there is no static-link library libgps.a .

    This leaves the linker no way to satisfy the link-time dependency on libgps. The only way to satisfy it, using libgps.so, has been disabled using '-static'.

    To fix, either:

    1. Modify the tool's compile recipe, so as to remove '-static' from the link command line, so as to enable use of the shared object, or
    2. Modify gpsd's compile recipe, so as to cause a static-link library libgps.a to built, either in addition to or instead of libgps.so.