I try to compile a library in C that need "math.h", here is the begining of the .c file:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>
#include "sparse_matrix.h"
...
and I compile with this command:
gcc -c ./sparse_matrix.c -o sparse_matrix.o -lm -Wall -pedantic -std=c99 -g -O
But even with the #include done and the flag -lm after the file (I've tried at the end of the line but nothing changed) I still get the error:
undefined reference to « sqrt »
collect2: error: ld returned 1 exit status
I don't get it after an hour of googling the issue.
I'm working with gcc 4.9 under ubuntu 14.10 (utopic unicorn).
Thank for any help in advance!
I don't think that is the command you're running (well, it may be one of them, but it's certainly not the one causing your error).
The -c option to gcc tells it to only create the object files (and you're specifically sending the output to sparse_matrix.o, an object file rather than an executable one).
In that case, the linker should not be called at all.
In fact, with a dummy sparse_matrix.c of:
#include <math.h>
int main(void) {
return (int)(sqrt(16.0));
}
your command works fine and, when I complete the process with:
pax> gcc -o sparse_matrix sparse_matrix.o -lm
pax> ./sparse_matrix
pax> echo $?
4
you can see that it also runs just fine.
It may be that you're leaving off the linker flags (such as -lm) from the actual link stage, which would cause this problem. They should have no effect on the compilation stage (unless they affect both compile and link stages but -l isn't one of those).
And, by "leaving off", I also include the possibility of "misplacing". Some linkers are positional in the way they handle libraries in that they will only extract objects from libraries if they satisfy an undefined symbol at the point where they're listed.
So, the command:
linker sparse_matrix.o -lm ...
would work because the .o file introduces an unsatisfied reference to sqrt, which is satisfied by libm. If your linker is positional, then:
linker -lm sparse_matrix.o ...
wouldn't work because, at the time of processing libm, there were no unsatisfied symbols so nothing was extracted. The undefined reference to sqrt is then introduced after that point and there are no other objects or libraries to satisfy it.
Whether ld or the gcc linker stage has that limitation, I don't know, I'm just raising the possibility as something to watch out for.
Related
I have this simple code:
max = (int) sqrt (number);
and in the header I have:
#include <math.h>
But application still says undefined reference to sqrt. Do you see any problem here? It looks like everything should be okay.
You may find that you have to link with the math libraries on whatever system you're using, something like:
gcc -o myprog myprog.c -L/path/to/libs -lm
^^^ - this bit here.
Including headers lets a compiler know about function declarations but it does not necessarily automatically link to the code required to perform that function.
Failing that, you'll need to show us your code, your compile command and the platform you're running on (operating system, compiler, etc).
The following code compiles and links fine:
#include <math.h>
int main (void) {
int max = sqrt (9);
return 0;
}
Just be aware that some compilation systems depend on the order in which libraries are given on the command line. By that, I mean they may process the libraries in sequence and only use them to satisfy unresolved symbols at that point in the sequence.
So, for example, given the commands:
gcc -o plugh plugh.o -lxyzzy
gcc -o plugh -lxyzzy plugh.o
and plugh.o requires something from the xyzzy library, the second may not work as you expect. At the point where you list the library, there are no unresolved symbols to satisfy.
And when the unresolved symbols from plugh.o do appear, it's too late.
I suppose you have imported math.h with #include <math.h>
So the only other reason I can see is a missing linking information. You must link your code with the -lm option.
If you're simply trying to compile one file with gcc, just add -lm to your command line, otherwise, give some informations about your building process.
Just adding the #include <math.h> in c source file and -lm in Makefile at the end will work for me.
gcc -pthread -o p3 p3.c -lm
Here are my observation, firstly you need to include the header math.h as sqrt() function declared in math.h header file. For e.g
#include <math.h>
secondly, if you read manual page of sqrt you will notice this line Link with -lm.
#include <math.h> /* header file you need to include */
double sqrt(double x); /* prototype of sqrt() function */
Link with -lm. /* Library linking instruction */
But application still says undefined reference to sqrt. Do you see any
problem here?
Compiler error is correct as you haven't linked your program with library lm & linker is unable to find reference of sqrt(), you need to link it explicitly. For e.g
gcc -Wall -Wextra -Werror -pedantic test.c -lm
I had the same issue, but I simply solved it by adding -lm after the command that runs my code.
Example.
gcc code.c -lm
I have this simple code:
max = (int) sqrt (number);
and in the header I have:
#include <math.h>
But application still says undefined reference to sqrt. Do you see any problem here? It looks like everything should be okay.
You may find that you have to link with the math libraries on whatever system you're using, something like:
gcc -o myprog myprog.c -L/path/to/libs -lm
^^^ - this bit here.
Including headers lets a compiler know about function declarations but it does not necessarily automatically link to the code required to perform that function.
Failing that, you'll need to show us your code, your compile command and the platform you're running on (operating system, compiler, etc).
The following code compiles and links fine:
#include <math.h>
int main (void) {
int max = sqrt (9);
return 0;
}
Just be aware that some compilation systems depend on the order in which libraries are given on the command line. By that, I mean they may process the libraries in sequence and only use them to satisfy unresolved symbols at that point in the sequence.
So, for example, given the commands:
gcc -o plugh plugh.o -lxyzzy
gcc -o plugh -lxyzzy plugh.o
and plugh.o requires something from the xyzzy library, the second may not work as you expect. At the point where you list the library, there are no unresolved symbols to satisfy.
And when the unresolved symbols from plugh.o do appear, it's too late.
I suppose you have imported math.h with #include <math.h>
So the only other reason I can see is a missing linking information. You must link your code with the -lm option.
If you're simply trying to compile one file with gcc, just add -lm to your command line, otherwise, give some informations about your building process.
Just adding the #include <math.h> in c source file and -lm in Makefile at the end will work for me.
gcc -pthread -o p3 p3.c -lm
Here are my observation, firstly you need to include the header math.h as sqrt() function declared in math.h header file. For e.g
#include <math.h>
secondly, if you read manual page of sqrt you will notice this line Link with -lm.
#include <math.h> /* header file you need to include */
double sqrt(double x); /* prototype of sqrt() function */
Link with -lm. /* Library linking instruction */
But application still says undefined reference to sqrt. Do you see any
problem here?
Compiler error is correct as you haven't linked your program with library lm & linker is unable to find reference of sqrt(), you need to link it explicitly. For e.g
gcc -Wall -Wextra -Werror -pedantic test.c -lm
I had the same issue, but I simply solved it by adding -lm after the command that runs my code.
Example.
gcc code.c -lm
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.
I have this simple code:
max = (int) sqrt (number);
and in the header I have:
#include <math.h>
But application still says undefined reference to sqrt. Do you see any problem here? It looks like everything should be okay.
You may find that you have to link with the math libraries on whatever system you're using, something like:
gcc -o myprog myprog.c -L/path/to/libs -lm
^^^ - this bit here.
Including headers lets a compiler know about function declarations but it does not necessarily automatically link to the code required to perform that function.
Failing that, you'll need to show us your code, your compile command and the platform you're running on (operating system, compiler, etc).
The following code compiles and links fine:
#include <math.h>
int main (void) {
int max = sqrt (9);
return 0;
}
Just be aware that some compilation systems depend on the order in which libraries are given on the command line. By that, I mean they may process the libraries in sequence and only use them to satisfy unresolved symbols at that point in the sequence.
So, for example, given the commands:
gcc -o plugh plugh.o -lxyzzy
gcc -o plugh -lxyzzy plugh.o
and plugh.o requires something from the xyzzy library, the second may not work as you expect. At the point where you list the library, there are no unresolved symbols to satisfy.
And when the unresolved symbols from plugh.o do appear, it's too late.
I suppose you have imported math.h with #include <math.h>
So the only other reason I can see is a missing linking information. You must link your code with the -lm option.
If you're simply trying to compile one file with gcc, just add -lm to your command line, otherwise, give some informations about your building process.
Just adding the #include <math.h> in c source file and -lm in Makefile at the end will work for me.
gcc -pthread -o p3 p3.c -lm
Here are my observation, firstly you need to include the header math.h as sqrt() function declared in math.h header file. For e.g
#include <math.h>
secondly, if you read manual page of sqrt you will notice this line Link with -lm.
#include <math.h> /* header file you need to include */
double sqrt(double x); /* prototype of sqrt() function */
Link with -lm. /* Library linking instruction */
But application still says undefined reference to sqrt. Do you see any
problem here?
Compiler error is correct as you haven't linked your program with library lm & linker is unable to find reference of sqrt(), you need to link it explicitly. For e.g
gcc -Wall -Wextra -Werror -pedantic test.c -lm
I had the same issue, but I simply solved it by adding -lm after the command that runs my code.
Example.
gcc code.c -lm
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.