Question

The following make output is showing an undefined reference, and I am not sure what is causing it. Could someone help?

"/usr/bin/make" -f nbproject/Makefile-Debug.mk QMAKE= SUBPROJECTS= .build-conf
make[1]: Entering directory `/cygdrive/g/workspace/c_cpp/MongoDriverTest'
"/usr/bin/make"  -f nbproject/Makefile-Debug.mk dist/Debug/Cygwin_4.x-Windows/mongodrivertest.exe
make[2]: Entering directory `/cygdrive/g/workspace/c_cpp/MongoDriverTest'
mkdir -p build/Debug/Cygwin_4.x-Windows
rm -f build/Debug/Cygwin_4.x-Windows/main.o.d
gcc -std=c99   -c -g -I../mongodb-mongo-c-driver/src/\*.c -MMD -MP -MF build/Debug/Cygwin_4.x-Windows/main.o.d -o build/Debug/Cygwin_4.x-Windows/main.o main.c
mkdir -p dist/Debug/Cygwin_4.x-Windows
gcc -std=c99    -o dist/Debug/Cygwin_4.x-Windows/mongodrivertest build/Debug/Cygwin_4.x-Windows/main.o  
nbproject/Makefile-Debug.mk:61: recipe for target `dist/Debug/Cygwin_4.x-Windows/mongodrivertest.exe' failed
make[2]: Leaving directory `/cygdrive/g/workspace/c_cpp/MongoDriverTest'
nbproject/Makefile-Debug.mk:58: recipe for target `.build-conf' failed
make[1]: Leaving directory `/cygdrive/g/workspace/c_cpp/MongoDriverTest'
nbproject/Makefile-impl.mk:39: recipe for target `.build-impl' failed
build/Debug/Cygwin_4.x-Windows/main.o: In function `main':
/cygdrive/g/workspace/c_cpp/MongoDriverTest/main.c:19: undefined reference to `_mongo_connect'
collect2: ld returned 1 exit status
make[2]: *** [dist/Debug/Cygwin_4.x-Windows/mongodrivertest.exe] Error 1
make[1]: *** [.build-conf] Error 2
make: *** [.build-impl] Error 2

And this is the content of my main.c:

#include <stdio.h>
#include <stdlib.h>
#include "../mongodb-mongo-c-driver/src/mongo.h"

int main(int argc, char** argv) {
    int status;
    mongo conn[1];

    status=mongo_connect(conn, "127.0.0.1", 27017);
    return EXIT_SUCCESS;
}  

It was working two days ago, I reinstalled the OS and now it’s not working anymore and I don’t seem to find the cause. mongo.h exists, mongo.o is there as well. mongo_connect is in mongo.c. Any idea?

Answer 1

Your link line is:

gcc -std=c99 -o dist/Debug/Cygwin_4.x-Windows/mongodrivertest build/Debug/Cygwin_4.x-Windows/main.o  

It doesn’t tell GCC where to collect mongo_connect() from. You need to specify the Mongo library on the command line.

Given the include line in your source code:

#include "../mongodb-mongo-c-driver/src/mongo.h"

You might add options:

-L../mongodb-mongo-c-driver/lib -lmongo

to the link line. Both the location and the library name are guesses. That would pick up libmongo.dll or libmongo.lib from the specified directory.

If you can’t find the library under the ../mongodb-mongo-c-driver directory, somewhere, you may have to build and install it. Alternatively, it may already be installed and you simply need to ensure you are referencing the correct locations where it is installed.

---

Also, as a general rule, avoid pathnames like that in the source code. You should specify:

#include "mongo.h"

and supply a compile line option to specify where to look for it:

-I../mongodb-mongo-c-driver/src

See also: What are the benefits of a relative path such as #include "../include/header.h" for a header?.