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
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 cannot make rpath work properly and make my binary to search for the library in the specified folder:
I have 3 very simple files:
main.c
#include <stdio.h>
#include <func.h>
int main() {
testing();
return 1;
}
func.h
void testing();
func.c
#include "func.h"
void testing(){
printf(testing\n");
}
Then I proceed to create a shared library as it follows:
gcc -c -fpic func.c -o ../release/func.o
gcc -shared -o ../release/lib/lib_func.so ../release/func.o
And then compile the program:
gcc main.c ../release/lib/lib_time_mgmt.so -Wl,-rpath=/home/root/ -o ../release/main
I receive the next warning:
main.c:7:2: warning: implicit declaration of function ‘testing’ [-Wimplicit-function-declaration]
testing();
But besides it, the program works fine.
However, my problem is that if now I want to move the library to /home/root (as specified in rpath) it does not work and the library is still searched only in the path specified when I compiled the main.c file which is ../release/lib/lib_time_mgmt.so
What am I doing wrong?
EDIT: After accepting the answer, I leave here the exact line as I used it and made it work for whoever might find it useful:
gcc main.c -L/home/root -Wl,-rpath,'/home/root/' -l:libtime_mgmt -o ${OUT_FILE}
Note: the rpath was used with the path betwen simple '. Not sure if that was the reason why it was not working before, but it worked this way now.
rpath is not used at compile time, but rather at link/runtime... thus you probably need to use both of these:
-L /home/root - to link correctly at build time
-Wl,-rpath=/home/root - to link correctly at run-time
You should use the -l ${lib} flag to link with libraries, don't specify their path as an input.
In addition to this, convention states that the libraries are named libNAME.so - e.g:
-l func will try to link with libfunc.so
-l time_mgmt will try to link with libtime_mgmt.so
Once you've addressed the above points, try the following:
gcc main.c -L/home/root -Wl,-rpath=/home/root -lfunc -ltime_mgmt -o ${OUT_FILE}
As a final point, I'd advise that you try not to use rpath, and instead focus on installing libraries in the correct places.
Unrelated to your question, but worth noting. Your use of #include <...> vs #include "..." is questionable. See: What is the difference between #include <filename> and #include "filename"?
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.
I have this trivial C program, but just can't it to link correctly.
here is the program,
#include <gsl/gsl_cdf.h>
#include <stdio.h>
int main()
{
double bottom_tail = gsl_cdf_gaussian_P(-1.96, 1);
printf("Area between [-1.96, 1.96]: %g\n", 1-2*bottom_tail);
}
at the shell, I'm doing:
gcc gsl_erf.c -o gslTest -I/usr/local/include/gsl -L/usr/local/lib
I know for sure that the gsl_cdf.h header file is in /usr/local/include/gsl, similarly I know that the .sos are in /usr/local/lib
the linker, gcc backend?, complains that I have an undefined reference to gsl_cdf_gaussian_P
I thought my order was incorrect, so I also tried:
gcc -I/usr/local/include/gsl -L/usr/local/lib gsl_erf.c -o gslTest
but this craps out as well. What am I doing wrong? :(
You have to tell the compiler to actually link with the library you're using: you need an -l option, probably something like -lgsl. (Take the name of the .so file, remove the .so suffix and lib prefix, and that's what to put after -l.)
The -L option tells the linker where to find libraries, but doesn't direct it to actually link with anything — just like the -I option tells the compiler where to find headers, but doesn't actually #include any code.
You don't actually ask for the libraries on the compile command. You provide a search path with "-L" but you don't actually request the libraries. You need something like "-lgsl" as well (assuming the library is libgsl.so).