Undefined reference to... when linking library header - c

Question

I'm trying to compile a C program that uses libvncserver but no matter what I do I keep getting undefined reference errors, the library I'm having troubles with is rfb/rfb.h.
vnc.c code (copied from here):
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <rfb/rfb.h>
#define WIDTH 640
#define HEIGHT 480
#define BPP 4
/* 15 frames per second (if we can) */
#define PICTURE_TIMEOUT (1.0/15.0)
/*
* throttle camera updates
*/
int TimeToTakePicture() {
static struct timeval now={0,0}, then={0,0};
double elapsed, dnow, dthen;
gettimeofday(&now,NULL);
dnow = now.tv_sec + (now.tv_usec /1000000.0);
dthen = then.tv_sec + (then.tv_usec/1000000.0);
elapsed = dnow - dthen;
if (elapsed > PICTURE_TIMEOUT)
memcpy((char *)&then, (char *)&now, sizeof(struct timeval));
return elapsed > PICTURE_TIMEOUT;
}
/*
* simulate grabbing a picture from some device
*/
int TakePicture(unsigned char *buffer)
{
static int last_line=0, fps=0, fcount=0;
int line=0;
int i,j;
struct timeval now;
/*
* simulate grabbing data from a device by updating the entire framebuffer
*/
for(j=0;j<HEIGHT;++j) {
for(i=0;i<WIDTH;++i) {
buffer[(j*WIDTH+i)*BPP+0]=(i+j)*128/(WIDTH+HEIGHT); /* red */
buffer[(j*WIDTH+i)*BPP+1]=i*128/WIDTH; /* green */
buffer[(j*WIDTH+i)*BPP+2]=j*256/HEIGHT; /* blue */
}
buffer[j*WIDTH*BPP+0]=0xff;
buffer[j*WIDTH*BPP+1]=0xff;
buffer[j*WIDTH*BPP+2]=0xff;
}
/*
* simulate the passage of time
*
* draw a simple black line that moves down the screen. The faster the
* client, the more updates it will get, the smoother it will look!
*/
gettimeofday(&now,NULL);
line = now.tv_usec / (1000000/HEIGHT);
if (line>HEIGHT) line=HEIGHT-1;
memset(&buffer[(WIDTH * BPP) * line], 0, (WIDTH * BPP));
/* frames per second (informational only) */
fcount++;
if (last_line > line) {
fps = fcount;
fcount = 0;
}
last_line = line;
fprintf(stderr,"%03d/%03d Picture (%03d fps)\r", line, HEIGHT, fps);
/* success! We have a new picture! */
return (1==1);
}
/*
* Single-threaded application that interleaves client servicing with taking
* pictures from the camera. This way, we do not update the framebuffer
* while an encoding is working on it too (banding, and image artifacts).
*/
int main(int argc,char** argv)
{
long usec;
rfbScreenInfoPtr server=rfbGetScreen(&argc,argv,WIDTH,HEIGHT,8,3,BPP);
if(!server)
return 0;
server->desktopName = "Live Video Feed Example";
server->frameBuffer=(char*)malloc(WIDTH*HEIGHT*BPP);
server->alwaysShared=(1==1);
/* Initialize the server */
rfbInitServer(server);
/* Loop, processing clients and taking pictures */
while (rfbIsActive(server)) {
if (TimeToTakePicture())
if (TakePicture((unsigned char *)server->frameBuffer))
rfbMarkRectAsModified(server,0,0,WIDTH,HEIGHT);
usec = server->deferUpdateTime*1000;
rfbProcessEvents(server,usec);
}
return(0);
}
Compiler output:
sudo gcc -g -Wall -Wextra -O2 vnc.c
/tmp/cc7dpMCs.o: In function `main':
/home/arcm/Projects/c/vnc.c:77: undefined reference to `rfbGetScreen'
/home/arcm/Projects/c/vnc.c:84: undefined reference to `rfbInitServerWithPthreadsAndZRLE'
/home/arcm/Projects/c/vnc.c:91: undefined reference to `rfbProcessEvents'
/home/arcm/Projects/c/vnc.c:86: undefined reference to `rfbIsActive'
/home/arcm/Projects/c/vnc.c:89: undefined reference to `rfbMarkRectAsModified'
collect2: error: ld returned 1 exit status
I've got libvncserver0 and libvncserver-dev installed and i'm using ubuntu 14.04.
I tried:
sudo gcc -g -Wall -Wextra -O2 vnc.c -lm
sudo gcc -g -Wall -Wextra -O2 vnc.c -ldl
sudo gcc -g -Wall -Wextra -O2 -ldl vnc.c
sudo gcc -g -Wall -Wextra -O2 -I/usr/include/rfb -L/usr/include/rbf/rfb.h vnc.c
sudo gcc -g -Wall -Wextra -O2 -I/usr/include/rfb vnc.c
sudo gcc -g -Wall -Wextra -O2 -L/usr/include/rbf/rfb.h vnc.c
sudo gcc -g -Wall -Wextra -O2 /usr/include/rbf/rfb.h vnc.c
sudo gcc -g -Wall -Wextra -O2 -L/usr/include/rbf/rfb.h -ldl vnc.c
But I get the same errors everytime. What am I doing wrong and how can I fix it?

Answer 1

You don't "link" a library header, you include it so the compiler sees the library's declarations at compile time and knows that rfbGetScreen() is a function that takes so-and-so many arguments of this-and-that type and returns a rfbScreenInfoPtr. How it does this (the definition of the function) is not important to the compiler. It just adds a reference to that function, which is left for the linker to resolve. (Note the vocabulary here.)
You link your compiled code to the library binary. This is done by the linker, in a different (and later) step that just happens to be supported by the same frontend as compiling your source (gcc). In this step, any of the library functions that your code actually uses (references) are resolved by linking them in from the specified libraries.
This here...
sudo gcc -g -Wall -Wextra -O2 vnc.c
...links only the standard library and runtime, as there are no specific linking instructions in there.
This here...
-L/usr/include/rbf/rfb.h
...is nonsense, as -L is for giving directories where library binaries should be looked for (and is not necessary if the library in question is installed in the standard location).
The actual link instruction is -l. If you state -lfoo, then the library libfoo is searched for any undefined references.
This here...
-ldl
...is linking libdl, and from that you should be able to deduce that...
-lvncserver
...is what you're looking for (assuming that <rfb/rfb.h> actually does refer to libvncserver, which I don't know).
Note that the linker is processing libraries in the sequence they are given on the command line, so you need to state -lvncserver after vnc.c, because only then does the linker know which undefined references it ought to be looking for in libvncserver.
And never, ever run a compiler as sudo. Why in {.....}'s name do you think this would be necessary?