I had a function in a file "draw.h":
void TileSystem() {
// Some code here.
}
And in the "main.c" file, I call it, because I have #included "draw.h" in the file. The function works nicely well!!
But then, I decide to rename the function to
void CreateTileSystem() {
// Some code here.
}
And I get the following output:
gcc main.c -std=c99 -o main `pkg-config --cflags --libs allegro-5.0 allegro_acodec-5.0 allegro_audio-5.0 allegro_color-5.0 allegro_dialog-5.0 allegro_font-5.0 allegro_image-5.0 allegro_main-5.0 allegro_memfile-5.0 allegro_physfs-5.0 allegro_primitives-5.0 allegro_ttf-5.0`
main.c: In function ‘main’:
main.c:217:12: warning: implicit declaration of function ‘CreateTileSystem’ [-Wimplicit-function-declaration]
/tmp/cclNEg6q.o: In function `main':
main.c:(.text+0xb1e): undefined reference to `CreateTileSystem'
collect2: error: ld returned 1 exit status
make: *** [main] Error 1
And then, I just rename it back to TileSystem and works well. I have no other references in the entire code. It makes no sense at all! I want to rename, in order to use verbs in functions (A more "correct" standard, I think). Well, I would really like to know what happens. I really CAN'T find errors, and the fact that when I rename it, it backs to work, frustrates me even more.
Thank you very much!
You have to also change the name in the header file, draw.h.
That error/warning indicates that there was no forward declaration for the function, which in older C dialects results in an assumption that the function is int fn(int) (if memory serves).
It works when you change it back because the header was never modified, so it's still providing a forward declaration of the old name.
I tried creating a couple of simple files like you've described here, but I don't get any problems. So:
are you sure you used the same name for the new function in both draw.h and main.c (really sure)?
I don't know about allegro: does it happen to have a function called TileSystem in it?
Beyond those, I would recommend starting with a bare bones setup (so just the include, the main function, and the call in main.c, and just the declaration in draw.h), make sure that compiles with gcc main.c -std=c99 -o main, and then build your application up to what it is currently, piece by piece, until the error starts happening again. That should help you identify the source of the problem.
I recreated the header file, without altering anything, and it worked. It really had something to do with me never altering the file (I mean, renaming the file or excluding it). Some very underground compiling issue happened.
Related
Why does the compiler sometimes not give line number of the error? Where is the use case of that "undefined reference". I've already included everything as header files I myself wrote so it needs to give a specific line number. It is not closed source. Have I changed some setting of the compiler by accident or is it another thing whatever that another thing is?
D:\Projects\DanceOfPixels\GLEW>gcc main.c glad.c -IC:\mingw_dev_lib\include\SDL2 -LC:\mingw_dev_lib\lib -lmingw32 -lopengl32 -lSDL2main -lSDL2 -lSDL2_image -o main.exe -ansi -std=c89 -pedantic -w
C:\Users\user\AppData\Local\Temp\ccMooHZm.o:main.c:(.text+0x126ce): undefined reference to `drawImagePartScaledHW'
collect2.exe: error: ld returned 1 exit status
Edit: I have solved the problem. I have included two different versions of the draw.h, one coming from software renderer, other from OpenGL renderer. Since they use same
#ifndef DRAW_H
#define DRAW_H
...
#endif
structure for both files; the compiler didn't include the second. Once I've changed the DRAW_H to DRAW_HW I managed to compile and run the application.
That error comes from the "linker" (ld), not the compiler proper.
Typically, the compiler compiles each source file into its own, individual object file, containing just the code and data from that source file. Then, the linker combines one or more object files together, and also links in any needed library functions.
Crucially, there's no problem if a single source file (a single object file) calls an undefined function -- that's normal, if the definition of the function is in another source file, or a library. So that's why it's the linker (not the compiler) that finally discovers that there's not a definition for a function anywhere, that it's truly undefined.
But since the linker is working with object files, typically it doesn't know which source file line numbers the functions were originally called on.
(Some C compilers work more closely with their linkers, so that these "undefined external" error messages can, more usefully, contain actual source file line numbers, but that's a relatively recent innovation. For this to work it may be important to compile with debugging enabled, e.g. by using the -g flag, so that the compiler includes source line number information in its object files.)
When I include "stdio.h" and try to use "printf" it works fine.
In the same way when I use "krb5.h", its function "krb5_get_init_creds_password" does not get resolved by its own.
Error faced :
bash-4.1$ gcc krb.c
krb.c: In function ‘main’:
krb.c:6: warning: incompatible implicit declaration of built-in function ‘printf’
/tmp/ccTK4DJM.o: In function main':
krb.c:(.text+0x53): undefined reference tokrb5_get_init_creds_password'
collect2: ld returned 1 exit status
I had to compile using -lkrb5 which resolved the issue.
Can someone let me know why do I need to use gcc option "-lkrb5" to get that krb5 function resolved ?
I am using C.
The header file you include only contains the declaration of the function, the actual function definition (the implementation) is in the actual library (usually a file named (using your example) libkrb5.a or libkrb5.so).
The option you pass to the linker is -l (lower-case L) which tells the linker to find and use whatever library you pass as argument to the -l option.
If you have create programs consisting of multiple source files yourself, then this should not have been much of a surprise really. If you have two source files a.c and b.c, where in a.c you call a function defined in b.c you need to declare the function for it to be available in a.c. And then when you link the executable program you need to link with the object file created by b.c to be able to find the definition.
Using a library is really not much different from that.
Here is the error I get from the gcc call:
gcc -o rr4 shells2.c graph1.c rng.c;
Undefined symbols:
"_getdisc", referenced from:
_main in cckR7zjP.o
ld: symbol(s) not found
The "cckR7zjP.o" keeps changing every time I call the compiler. The code for the method is in the file graph1.c; its header file is called graph2.h, and I am importing it to the file with the main method called shells2.c using:
#include "graph2.h"
The method or function definition is:
int getdisc(int i){ return disc[i];}
which attempts to return the ith member of the array disc created by
static int *disc;
that I already initialized in some other method! I think the problematic call is:
for (iter = 0; iter < n; iter++) {
if (getdisc(iter) == cln)
avgbtwn += get_betweenness(iter);
}
This seems like a linker problem I checked with some other questions, and I think I am linking my method properly (and am using the same method elsewhere in the code) but I still can't figure this out.
Edit: So I switched the order of the command in linux to
gcc -o rr4 graph1.c rng.c shells2.c
as per Soren's suggestion and the function compiled as normal, does anyone know why?
Further it seems when i put a trailing line break in the file graph1.c alleviates the problem.
There used to be a issue in the old GCC 2.x compilers/linkers where the linker couldn't resolve linking when the symbols were not group together -- think of it as that the linker would only looks for symbols that is still needed, and it would drop symbols which were unused.
To most people the problem would manifest itself as a problem of the ordering of libraries (specified with -l or as .a).
I see from the comments that you use a mac, so it might just be that the mac version of the compiler/linker still has that problem -- anyway since reordering the source files solved the problem, then you certainly have some variation of this bug.
So possible solutions;
Group all your source files into larger files -- bad solution -- but the linker is less likely to fail with this symptom -- or
Try to compiler all the files to .o first and then link the .o files (using a makefile would usually do this, but may or may not resolve the problem) and possibly combine the .o into a single .a (man ar), or
Change the order of the source files to have the shells2.c last (which worked for you), or
See if upgrading your compiler helps
Sorry for the long laundry list, but this is clearly just a compiler bug which just need a simple work around.
That's definitely an error with getdisc not being visible to the linker but, if what you say is correct, that shouldn't happen.
The gcc command line you have includes graph1.c which you assure use contains the function.
Don't worry about the object file name, that's just a temprary name created by the compiler to pass to the linker.
Can you confirm (exact cut and paste) the gcc command line you're using, and show us the function definition with some context around it?
In addition, make sure that graph1.c is being compiled as expected by inserting immediately before the getdisc function, the following line:
xyzzy plugh twisty;
If your function is being seen by the compiler, that should cause an error first. It may be something like ifdef statements causing your code not to be compiled.
By way of testing, the following transcript shows that what you are trying to do works just fine:
pax> cat shells2.c
#include "graph2.h"
int main (void) {
int x = getdisc ();
return x;
}
pax> cat graph2.h
int getdisc (void);
pax> cat graph1.c
int getdisc (void) {
return 42;
}
pax> gcc -o rr4 shells2.c graph1.c
pax> ./rr4
pax> echo $?
42
We have to therefore assume that what you're actually doing is something different, and that's unusually tactful for me :-)
What you're experiencing is what would happen with something like:
pax> gcc -o rr4 shells2.c
/tmp/ccb4ZOpG.o: In function `main':
shells2.c:(.text+0xa): undefined reference to `getdisc'
collect2: ld returned 1 exit status
or if getdisc was not declared correctly in graph1.c.
That last case could be for many reasons including, but not limited to:
mis-spelling of getdisc.
#ifdef type statements meaning the definition is never seen (though you seem to have discounted that in a comment).
some wag using #define to change getdisc to something else (unlikely, but possible).
I'm trying to cross compile my application for the maemo environment (GNU).
When compiling the application normally, everything works fine, however when it's compiled through sb2 the following warning comes up:
$ sb2 gcc -D_GNU_SORCE -o app -Wall -g -I.......//don't think this is relevant
In file included from wifi_collector_menu.c:50:
wifi_collector_list.c: In function `show_net_apns':
wifi_collector_list.c:777: warning: implicit declaration of function `getline'
I am completely confused as to why this happens, there are other getlines that do work in the program, i have tried to define the variable _GNU_SOURCE both inside the code and in the compiler command (not at the same time)
This is the line of code which causes the warning apparently:
size_t bytesnum = MAX_ESSID;
size_t bytes_read;
char *netname = NULL;
printf("Enter name of selected network:");
bytes_read=getline(&netname,&bytesnum,stdin);//This line
Any help would be appreciated, thanks in advance.
Problem solved, all I had to do was add:
#define _GNU_SOURCE
In each header file, before stdio.h was included, very simple really.
I guess this info is assumed known between programmers as i was unable to find it anywhere online, and had to ask my C programming professor personally, and even then we had some trouble tracing the source.
Thanks anyway.
Change your compiler line to include the -E option and redirect the output. The compiler will only pre-proccess your file when this option is used. Do this for both versions, with and without sb2. getline() is normally found in stdio.h. By viewing the preprocessed output from both versions, you should be able to see where getline() is being included from.
I'm trying to compile the Neko VM on Mac OS X (10.5.7) using GCC 4.01 and I'm completely stuck, because it stops while compiling saying:
vm/threads.c:202: error: conflicting types for 'neko_thread_register'
vm/neko_vm.h:37: error: previous declaration of 'neko_thread_register' was here
I've tried googling this and some say it's because of lack of a "prototype" and some say it's because of a header include being done several times, and I can't really find any of those.
The affected line in threads.c:202 looks like this:
EXTERN bool neko_thread_register( bool t ) {
And the affected line in neko_vm.h:37 looks like this:
EXTERN bool neko_thread_register( bool t );
I can't see any difference in them, besides one of them being the implementation of the other.
The compiler command I'm using is:
cc -Wall -O3 -v -fPIC -fomit-frame-pointer -I vm -D_GNU_SOURCE -arch i386 -L/usr/local/lib -L/opt/local/lib -I/opt/local/include -o vm/threads.o -c vm/threads.c
I'd appreciate some ideas on what i might be able to do here, I don't really know where to go from here.
A mirror of the code for Neko which I'm trying to compile can be found here.
Thanks!
Have you tried compiling that file alone and outputting the preprocessed version? It could be that the scope or linkage macros are being modified somewhere in between the header file and the implementation file-- the same could be true of the 'bool' type, which is usually a macro defined by a system header.
According to the GCC 4.2 docs here, you should need to add the -E flag to the compilation line above, and you ought to change -o vm/threads.o to -o vm/threads.i so a file with the correct extension is created (.i means 'preprocessed file', essentially).
First, make sure you compile this as C, not C++.
Second, without seeing the code, it's pretty much impossible to say what the problem is.
But reading the error messages is often helpful (even before you google them):
Apparently neko_thread_register is declared twice, once in threads.c:202 and once in neko_vm.h:37, and the two declarations have different (conflicting) types. So look at the two declarations. If you can't see a problem with them, show us some code.
At the very least, seeing those two lines of code would be necessary. Most likely, the types are typedefs or macros or something similar, and then we'd need to see where they are defined as well.
Without seeing the code, all we can do is repeat the compiler error. "neko_thread_register has two conflicting definitions, at the lines specified."
Did you uncomment this line:
# For OSX
#
# MACOSX = 1 <-- this one
In the makefile?