I have read the other answers on this topic, and unfortunately they have not helped me. I am attempting to link several c programs together, and I am getting an error in response:
$ gcc -o runexp.o scd.o data_proc.o -lm -fopenmp
/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: ld returned 1 exit status
make: * [runexp] Error 1
I have exactly one main function and it is in runexp. The form is
int main(void) {
...;
return 0;
}
Any thoughts on why I might get this error? Thanks!
You should provide output file name after -o option. In your case runexp.o is treated as output file name, not input object file and thus your main function is undefined.
You're not including the C file that contains main() when compiling, so the linker isn't seeing it.
You need to add it:
$ gcc -o runexp runexp.c scd.o data_proc.o -lm -fopenmp
You are overwriting your object file runexp.o by running this command :
gcc -o runexp.o scd.o data_proc.o -lm -fopenmp
In fact, the -o is for the output file.
You need to run :
gcc -o runexp.out runexp.o scd.o data_proc.o -lm -fopenmp
runexp.out will be you binary file.
Generally you compile most .c files in the following way:
gcc foo.c -o foo. It might vary depending on what #includes you used or if you have any external .h files. Generally, when you have a C file, it looks somewhat like the following:
#include <stdio.h>
/* any other includes, prototypes, struct delcarations... */
int main(){
*/ code */
}
When I get an 'undefined reference to main', it usually means that I have a .c file that does not have int main() in the file. If you first learned java, this is an understandable manner of confusion since in Java, your code usually looks like the following:
//any import statements you have
public class Foo{
int main(){}
}
I would advise looking to see if you have int main() at the top.
Related
I have following two source codes and want to link them.
// test.c
#include <stdio.h>
void lib2();
void lib1(){
lib2();
return 0;
}
// main.c
#include <stdio.h>
int main() {
return 0;
}
I've used gcc -c main.c and gcc -c test.c to generate objects files
$ ls *.o
main.o test.o
and I've used ar rcs test.a test.o command to generate static library(test.a) from object file test.o
Then, I tried to build executable by linking main.o with test.a or test.o. As far as I know, a static library file(.a extension) is a kind of simple collection of object files(.o). so I expected both would give same result: error or success. but it didn't.
Linking with the object file gives undefined reference error.
$ gcc -o main main.o test.o
/usr/bin/ld: test.o: in function `lib1':
test.c:(.text+0xe): undefined reference to `lib2'
collect2: error: ld returned 1 exit status
$
but linking with the static library doesn't give any error and success on compilation.
$ gcc -o main main.o test.a
$
Why is this happening? and how can I get undefined reference errors even when linking with static libraries?
If your code contains a function call expression then the language standard requires a function definition exists. (See C11 6.9/3). If you don't provide a definition then it is undefined behaviour with no diagnostic required .
The rule was written this way so that implementation vendors aren't forced to perform analysis to determine if a function is ever called or not; for example in your library scenario the compiler isn't forced to dig around in the library if none of the rest of the code contains anything that references that library.
It's totally up to the implementation what to do, and in your case it decides to give an error in one case and not the other. To avoid this, you can provide definitions for all the functions you call.
You might be able to modify the behaviour in the first case by using linker options such as elimination of unused code sections. Another thing you can do is call lib1() from main() -- this is still not guaranteed to produce an error but is more likely to.
Force the linker to do some work use -flto option and the error will go away.
ld does not search libraries for objects which are not used it only searches for symbols used in object files. Imagine that you have a library where some functions require defined callbacks. If you do not have them in every program you link against the library even if you do not use those functions.
I expected both would give same result: error or success. but it didn't.
Your expectation is incorrect. A good explanation of the difference between .o and .a with respect to linking is here.
I'm getting the following error and can't for the life of me figure out what I'm doing wrong.
$ gcc main.c -o main
Undefined symbols:
"_wtf", referenced from:
_main in ccu2Qr2V.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
main.c:
#include <stdio.h>
#include "wtf.h"
main(){
wtf();
}
wtf.h:
void wtf();
wtf.c:
void wtf(){
printf("I never see the light of day.");
}
Now, if I include the entire function in the header file instead of just the signature, it complies fine so I know wtf.h is being included. Why doesn't the compiler see wtf.c? Or am I missing something?
Regards.
You need to link wtf with your main. Easiest way to compile it together - gcc will link 'em for you, like this:
gcc main.c wtf.c -o main
Longer way (separate compilation of wtf):
gcc -c wtf.c
gcc main.c wtf.o -o main
Even longer (separate compilation and linking)
gcc -c wtf.c
gcc -c main.c
gcc main.o wtf.o -o main
Instead of last gcc call you can run ld directly with the same effect.
You are missing the fact that merely including a header doesn't tell the compiler anything about where the actual implementation (the definitions) of the things declared in the header are.
They could be in a C file next to the one doing the include, they could come from a pre-compiled static link library, or a dynamic library loaded by the system linker when reading your executable, or they could come at run-time user programmer-controlled explicit dynamic loading (the dlopen() family of function in Linux, for instance).
C is not like Java, there is no implicit rule that just because a C file includes a certain header, the compiler should also do something to "magically" find the implementation of the things declared in the header. You need to tell it.
In my header interface.h I have a function,
extern void printb(char *, unsigned short, char *);
Which is called in main.c as follows,
printb(" available", EXTRACT_16BITS(ptr), LLDP_CAP_BITS);
And I have included the the heder file in my main.c as follows.
#include "interface.h"
However, when I try to compile, I get the error stating the following,
undefined reference to 'printb' collect2: ld returned 1 exit status
I tried to do some research on this and came up empty handed. any help to solve this issue is greatly appreciated :)
Thank you :)
Firstly,You need to find out where the definition of function printb() is. If it is in some other static library ,then you need to link to that lib while compiling. For example, if your printb function is in printb library, then compile your source as below.
gcc -o prog main.c -l printb
If your printb function is in another standalone printb.c file, then compile your code as
gcc -o prog main.c printb.c
Either of these should work for you.
(I'm on Windows.)
I'm testing a DLL I have compiled (libsox) with a C program which looks this way:
#include <stdio.h>
#include "sox.h"
int main(void) {
char const * versionText = sox_version();
printf(versionText);
return 0;
}
The function that is defined in the DLL has the following prototype in sox.h (something of this contains cdecl):
LSX_RETURN_VALID_Z LSX_RETURN_PURE
char const *
LSX_API
sox_version(void);
Here's the problem: When I try to build the file with gcc -lsox -o test.exe test.c I get the following error:
C:\DOKUME~1\ADMINI~1\LOKALE~1\Temp\ccSS2h2z.o:test.c:(.text+0xf): undefined reference to `sox_version'
collect2: ld returned 1 exit status
A word to -lsox: I have the library file "libsox.dll.a" in MinGW's lib folder. If I write -lsoxnonsense, then it says there is no library. That means in the shown case it finds the library. So why doesn't it want to create a link.
gcc -lsox -o test.exe test.c
You have to put your source file first:
gcc test.c -lsox -o test.exe
It is because the linker goes through the input files in order, finding undefined references and satisfying references that it saw before. So in your command line, it reads libsox.a (or something like that), finds undefined references (there would be none). Then, it goes to your test.c, finds undefined references in there, but there are no further libraries to satisfy them.
See this answer for more info.
I've just organized my code by using headers, but just as I've done this, I got a warning that turned into an error when linking.
I have a code(use of a function that is inside a header) in test.c that is like this:
#include "test1.h"
/* Some code */
main()
{
Testing();
}
And my test1.h header is like this:
void Testing();
void print(int, int, int, const char*);
And at test1.c
void Testing()
{
print(0xF9, 27, 5, "\xC9\\xBB");
}
void print(int colour, int x, int y, const char *string)
{
volatile char *video=(volatile char*)0xB8000 + y*160 + x*2;
while(*string != 0)
{
*video=*string;
string++;
video++;
*video=colour;
video++;
}
}
When I try to compile the code, I got this:
ubuntu#eeepc:~/Development/Test$ gcc -o test.o -c test.c -Wall -Wextra -nostdlib -nostartfiles -nodefaultlibs
test.c: In function ‘main’:
test.c:11: warning: implicit declaration of function ‘Testing’
ubuntu#eeepc:~/Development/Test$
At the time it's just a simple warning, but when I try to link it...
ubuntu#eeepc:~/Development/Test$ ld -T linker.ld -o kernel.bin loader.o test.o
test.o: In function main':
test.c:(.text+0xfc): undefined reference toTesting'
What I need to do?
Edit: To reflect the OP's question I have struck out some lines of my answer despite being upvoted...
Why is kernel.c flagged up in the compiler, even though you don't have it mentioned here? Am I missing something...
gcc -o test.o -c test.c -Wall -Wextra -nostdlib -nostartfiles -nodefaultlibs
kernel.c: In function ‘main’:
kernel.c:11: warning: implicit declaration of function ‘Testing’
ubuntu#eeepc:~/Development/Test$
maybe you need to do it this way somewhere in your header file as I'm judging you want kernel to access this function:
extern void Testing();
And, take out all your functions and place them in a separate .c file, they should not be in there in the first place... for example:
Testing.c
/* all your functions here */
Testing.h
/* Only global, external variables and function prototypes */
Hope this helps,
Best regards,
Tom.
I can't recreate your problem. This works as expected when I try to compile your code on an Ubuntu machine (Which based on your paste, I assume you're using.)
Are you sure the #include is happening correctly?
Try using -E instead of -c to see what the whole code the compiler is trying to compile looks like.
Somewhat of a shot in the dark here, since my C is a bit rusty, but does C allow you to put function bodies in a header? I don't recall that it does. Try moving the definition of Testing() and print() into a .c file? You could also try compiling as C++ as see if that fixes it, if you don't need/want C.
You included test.h into main.c, while your declarations, according to what your wrote, are in test1.h. Note the 1 in the name.
In addition to that, you are compiling test.c and linking test.o, while in reality the name of your file is test1.c. Again, note the 1 in the name.
Edit: Now you edited the name of the file included into main.c. After the edit it is safe to assert that most of the symptoms you describe are not possble with the current versions of the files. Re-verify what you are doing, post updated disgnostic information and/or post real code.
Still, you compiler and linker lines are referring to old file names.
i donno whats causing this , but i had this problem just now .
try to delete the .h file and put the declarations of the functions on the top of .c file itself .
in this case , delete the test1.h and put the declarations of functions of test1.c in test1.c.
and include test1.c in test.c
you wont get that warning message any more , nor the following linking errors .