This question already has answers here:
Undefined reference to sqrt (or other mathematical functions)
(5 answers)
Closed 4 years ago.
I'm very new to C and I have this code:
#include <stdio.h>
#include <math.h>
int main(void)
{
double x = 0.5;
double result = sqrt(x);
printf("The square root of %lf is %lf\n", x, result);
return 0;
}
But when I compile this with:
gcc test.c -o test
I get an error like this:
/tmp/cc58XvyX.o: In function `main':
test.c:(.text+0x2f): undefined reference to `sqrt'
collect2: ld returned 1 exit status
Why does this happen? Is sqrt() not in the math.h header file? I get the same error with cosh and other trigonometric functions. Why?
The math library must be linked in when building the executable. How to do this varies by environment, but in Linux/Unix, just add -lm to the command:
gcc test.c -o test -lm
The math library is named libm.so, and the -l command option assumes a lib prefix and .a or .so suffix.
You need to link the with the -lm linker option
You need to compile as
gcc test.c -o test -lm
gcc (Not g++) historically would not by default include the mathematical functions while linking. It has also been separated from libc onto a separate library libm. To link with these functions you have to advise the linker to include the library -l linker option followed by the library name m thus -lm.
This is a likely a linker error.
Add the -lm switch to specify that you want to link against the standard C math library (libm) which has the definition for those functions (the header just has the declaration for them - worth looking up the difference.)
Because you didn't tell the linker about location of math library. Compile with gcc test.c -o test -lm
Add header:
#include<math.h>
Note: use abs(), sometimes at the time of evaluation sqrt() can take negative values which leave to domain error.
abs()- provides absolute values;
example, abs(-3) =3
Include -lm at the end of your command during compilation time:
gcc <filename.extension> -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'm attempting to get the Octave C++ code here to compile in g++ (Ubuntu/Linaro 4.6.4-1ubuntu1~12.04) 4.6.4).
This trimmed version of the above will compile in g++:
#include <iostream>
#include <octave/oct.h>
using namespace std;
int main() {
// Matrix L=Matrix(2,2);
return 0;
}
but if I unremark out the line Matrix L=Matrix(2,2); then compile with g++ temp.cpp it gives the error message:
/tmp/ccTa3Am5.o: In function `Array2<double>::~Array2()':
temp.cpp:(.text._ZN6Array2IdED2Ev[_ZN6Array2IdED5Ev]+0x1f): undefined reference to `Array<double>::~Array()'
/tmp/ccTa3Am5.o: In function `Array<double>::Array(dim_vector const&)':
temp.cpp:(.text._ZN5ArrayIdEC2ERK10dim_vector[_ZN5ArrayIdEC5ERK10dim_vector]+0x26): undefined reference to `Array<double>::get_size(dim_vector const&)'
/tmp/ccTa3Am5.o:(.rodata._ZTV5ArrayIdE[vtable for Array<double>]+0x10): undefined reference to `Array<double>::~Array()'
/tmp/ccTa3Am5.o:(.rodata._ZTV5ArrayIdE[vtable for Array<double>]+0x18): undefined reference to `Array<double>::~Array()'
collect2: ld returned 1 exit status
I'm unsure why. Perhaps I'm missing an #include, perhaps I don't have an appropriate file installed, perhaps I'm not linking to the appropriate library, or perhaps I'm misusing Octave in some way.
Question: Why is this failing to compile? How can I fix it?
It compiles fine if I use mkoctfile --link-stand-alone temp.cpp as indicated at the above site, however, I'd like to use g++ if possible, since I eventually want to be able to call Octave functions from another program I've written in C++.
As indicated in my comment a simple example can be found in this Howto. So in your case a simple way to achieve compilation will be creating a makefile as follows:
makefile:
all: temp
clean:
-rm temp.o temp
temp: temp.o
mkoctfile --link-stand-alone -o temp temp.o
temp.o: temp.cpp
g++ -c -I$(OCTAVE_INCLUDE)
-I$(OCTAVE_INCLUDE)octave -o temp.o temp.cpp
$(OCTAVE_INCLUDE) is an environment variable that should be set to your octave include path (e.g. /usr/include/octave-x.x.xx). Then you can simply compile and link your test application using the command make all.
You need to link to the octave library. If the library is octave.a:
g++ -loctave temp.cpp
Add the library directories to your link command:
your-local-path\octave-x.x.xx\lib\
your-local-path\octave-x.x.xx\lib\octave\x.x.xx\
mkoctfile -L"\your-path\octave-x.x.xx\lib" -L"\your-path\octave-x.x.x\lib\octave\x.x.xx" --link-stand-alone temp.cpp
For cxx11 linker errors:
Converting std::__cxx11::string to std::string
"If you get linker errors about undefined references to symbols that involve types in the std::__cxx11 namespace or the tag [abi:cxx11] then it probably indicates that you are trying to link together object files that were compiled with different values for the _GLIBCXX_USE_CXX11_ABI macro. This commonly happens when linking to a third-party library that was compiled with an older version of GCC."
Defining the following macro before including any standard library headers should fix your problem:
#define _GLIBCXX_USE_CXX11_ABI 0
This question already has answers here:
Linker errors when compiling against glib...?
(2 answers)
Closed 10 years ago.
I am getting very strange error whenever I am trying to compile a C++ program with FFTW3 implementation.
I am compiling as follows
g++ -O3 -lm -lfftw3 myFile.cpp -o myFileFFTW
I also included my headers file as follows
#include <math.h> #include "fftw3.h"
The error is as follows
(.text+0x63): undefined reference to `fftw_malloc'
Any suggestions?
Edit:
the suggestion by hmjd worked for me.
Linker errors when compiling against glib...?
I guess one should not work for straight 3 days otherwise mind does not work!!
Special thanks hmjd!! you saved my day and I could finish my project on time !!
I guess problem is -lfftw3 not present on your system and you are also not specifying libs correctly.
Libraries at the end of the compiler command:
gcc -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include re.c -o re -lglib-2.0
From GCC Link Options:
-llibrary
-l library
Search the library named library when linking.
(The second alternative with the library as a separate argument
is only for POSIX compliance and is not recommended.)
It makes a difference where in the command you write this option;
the linker searches and processes libraries and object files in the
order they are specified.
Thus, `foo.o -lz bar.o' searches library `z' after file foo.o but
before bar.o. If bar.o refers to functions in `z', those functions
may not be loaded.
snnippet from Linker errors when compiling against glib...?
I have two simple files:
runner.cpp:
#define BOOST_TEST_DYN_LINK
#define BOOST_TEST_MODULE Main
#include <boost/test/unit_test.hpp>
and test1.cpp:
#define BOOST_TEST_DYN_LINK
#ifdef STAND_ALONE
# define BOOST_TEST_MODULE Main
#endif
#include <boost/test/unit_test.hpp>
BOOST_AUTO_TEST_SUITE( Foo)
BOOST_AUTO_TEST_CASE( TestSomething )
{
BOOST_CHECK( true );
}
BOOST_AUTO_TEST_SUITE_END()
To compile, I'm using:
$ g++ -I/e/code/boost_1_52_0 -o runner -lboost_unit_test_framework runner.cpp test1.cpp
I get the following error:
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ccU0cDSz.o:runner.cpp:(.text+0x8c): multiple definition of `main'
c:/pdev/mingw/bin/../lib/gcc/i686-pc-mingw32/4.7.2/../../../libboost_unit_test_framework.a(unit_test_main.o):unit_test_main.cpp:(.text.startup+0x0): first defined here
c:/pdev/mingw/bin/../lib/gcc/i686-pc-mingw32/4.7.2/../../../libboost_unit_test_framework.a(unit_test_main.o):unit_test_main.cpp:(.text.startup+0x14): undefined reference to `init_unit_test_suite(int, char**)'
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ccU0cDSz.o:runner.cpp:(.text+0x52): undefined reference to `_imp___ZN5boost9unit_test9framework17master_test_suiteEv'
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ccU0cDSz.o:runner.cpp:(.text+0xb0): undefined reference to `_imp___ZN5boost9unit_test14unit_test_mainEPFbvEiPPc'
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ccU0cDSz.o:runner.cpp:(.text$_ZN5boost9unit_test13test_observerD2Ev[__ZN5boost9unit_test13test_observerD2Ev]+0xe): undefined reference to `_imp___ZTVN5boost9unit_test13test_observerE'
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ccU0cDSz.o:runner.cpp:(.text$_ZN5boost9unit_test13test_observerC2Ev[__ZN5boost9unit_test13test_observerC2Ev]+0xe): undefined reference to `_imp___ZTVN5boost9unit_test13test_observerE'
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ccU0cDSz.o:runner.cpp:(.text$_ZN5boost9unit_test15unit_test_log_tC1Ev[__ZN5boost9unit_test15unit_test_log_tC1Ev]+0x22): undefined reference to `_imp___ZTVN5boost9unit_test15unit_test_log_tE'
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\cciSdkmB.o:test1.cpp:(.text+0x88): undefined reference to `_imp___ZN5boost9unit_test15unit_test_log_t14set_checkpointENS0_13basic_cstringIKcEEjS4_'
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\cciSdkmB.o:test1.cpp:(.text+0x136): undefined reference to `_imp___ZN5boost10test_tools9tt_detail10check_implERKNS0_16predicate_resultERKNS_9unit_test12lazy_ostreamENS5_13basic_cstringIKcEEjNS1_10tool_levelENS1_10check_typeEjz'
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\cciSdkmB.o:test1.cpp:(.text+0x21d): undefined reference to `_imp___ZN5boost9unit_test9ut_detail24auto_test_unit_registrarC1ENS0_13basic_cstringIKcEE'
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\cciSdkmB.o:test1.cpp:(.text+0x284): undefined reference to `_imp___ZN5boost9unit_test9ut_detail24auto_test_unit_registrarC1EPNS0_9test_caseEm'
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\cciSdkmB.o:test1.cpp:(.text+0x2a4): undefined reference to `_imp___ZN5boost9unit_test9ut_detail24auto_test_unit_registrarC1Ei'
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\cciSdkmB.o:test1.cpp:(.text$_ZN5boost9unit_test14make_test_caseERKNS0_9callback0INS0_9ut_detail6unusedEEENS0_13basic_cstringIKcEE[__ZN5boost9unit_test14make_test_caseERKNS0_9callback0INS0_9ut_detail6unusedEEENS0_13basic_cstringIKcEE]+0x1d): undefined reference to `_imp___ZN5boost9unit_test9ut_detail24normalize_test_case_nameENS0_13basic_cstringIKcEE'
C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\cciSdkmB.o:test1.cpp:(.text$_ZN5boost9unit_test14make_test_caseERKNS0_9callback0INS0_9ut_detail6unusedEEENS0_13basic_cstringIKcEE[__ZN5boost9unit_test14make_test_caseERKNS0_9callback0INS0_9ut_detail6unusedEEENS0_13basic_cstringIKcEE]+0x5b): undefined reference to `_imp___ZN5boost9unit_test9test_caseC1ENS0_13basic_cstringIKcEERKNS0_9callback0INS0_9ut_detail6unusedEEE'
collect2.exe: error: ld returned 1 exit status
I'm using g++ 4.7.2 on MinGW, with boost 1.52.0.
I get the same errors when only trying to compile test1.cpp - except the "multiple main definition" one.
I perused the official documentation for quite a while, but its scarce on details regarding linking options. When I compiled the boost libs, besides unit_test_framework, I also got prg_exec_monitor and test_exec_monitor; perhaps I should link these somehow ? I tried many combinations, but all resulted in some kind of undefined reference linker error.
Complete list of boost generated libraries - I have them all in the project root:
libboost_prg_exec_monitor-mgw47-mt-1_52.a
libboost_prg_exec_monitor-mgw47-mt-1_52.dll
libboost_prg_exec_monitor-mgw47-mt-1_52.dll.a
libboost_prg_exec_monitor-mgw47-mt-d-1_52.a
libboost_prg_exec_monitor-mgw47-mt-d-1_52.dll
libboost_prg_exec_monitor-mgw47-mt-d-1_52.dll.a
libboost_test_exec_monitor-mgw47-mt-1_52.a
libboost_test_exec_monitor-mgw47-mt-d-1_52.a
libboost_unit_test_framework-mgw47-mt-1_52.a
libboost_unit_test_framework-mgw47-mt-1_52.dll
libboost_unit_test_framework-mgw47-mt-1_52.dll.a
libboost_unit_test_framework-mgw47-mt-d-1_52.a
libboost_unit_test_framework-mgw47-mt-d-1_52.dll
libboost_unit_test_framework-mgw47-mt-d-1_52.dll.a
With help from #llonesmiz, a number of issues were identified.
1. Libraries need to be specified after objects and sources which use them.
As described here:
The traditional behavior of linkers is to search for external functions from
left to right in the libraries specified on the command line. This means that a
library containing the definition of a function should appear after any source
files or object files which use it. This includes libraries specified with the
short-cut -l option, as shown in the following command:
$ gcc -Wall calc.c -lm -o calc (correct order)
With some linkers the opposite ordering (placing the -lm option before the file
which uses it) would result in an error,
$ cc -Wall -lm calc.c -o calc (incorrect order)
main.o: In function 'main':
main.o(.text+0xf): undefined reference to 'sqrt'
because there is no library or object file containing sqrt after ‘calc.c’. The
option -lm should appear after the file ‘calc.c’
2. Library paths should be explicitly specified.
If no lib paths are specified, the linker might look for the libs in a series
of default folders, thus loading a different library then intended. This is what
happened in my case - I wanted to link boost_unit_test_framework, but did not
specify a path because I assumed the linker would look in the current folder.
That's what happens at runtime, after all - if the dll is in the same folder
with the exe, it will find it.
I found it a little bit strange the linker would find the lib, since it was
named ibboost_unit_test_framework-mgw47-mt-1_52.dll. When I tried to link to
a non-existing lib, the linker complained though, so I assumed this isn't an
issue, and MinGW 's linker ignores those suffixes.
After some more research, I found this article about MinGW library paths.
The folders MinGW searches for libs can be found in the output of gcc -print-search-dirs.
The article also contains some bash magic to make sense of that output:
gcc -print-search-dirs | sed '/^lib/b 1;d;:1;s,/[^/.][^/]*/\.\./,/,;t 1;s,:[^=]*=,:;,;s,;,; ,g' | tr \; \\012 | grep -v '^ */'
This will print a nice list of those folders. gcc will not, by default,
look in the current directory for libs. I looked in each of them, and found the
lib that was being loaded - libboost_unit_test_framework.a, a static lib.
This brings into light another issue worth mentioning:
3. Static versus dynamic linking
I did not specify whether I want boost_unit_test_framework linked statically or dynamically.
In this case, gcc prefers dynamic linking:
Because of these advantages gcc compiles programs to use shared libraries by
default on most systems, if they are available. Whenever a static library
‘libNAME.a’ would be used for linking with the option -lNAME the compiler
first checks for an alternative shared library with the same name and a ‘.so’
extension.
(so is the extension for dynamic libraries on Unix - on Windows, the equivalent is dll.)
So, what happened is that gcc looked for libboost_unit_test_framework.dll
in all it's default folders, but couldn't find it. Then it looked for
libboost_unit_test_framework.a, and statically linked that. This resulted in
linking errors because the sources have #define BOOST_TEST_DYN_LINK, and
therefore expect to have the lib dynamically linked.
To enforce static or dynamic linking, the -Wl,-Bstatic and -Wl,-Bdynamic
linker options come into play, described here.
If I tell the linker that I want dynamic linking:
$ g++ -I/e/code/boost_1_52_0 runner.cpp test1.cpp -o runner -Wl,Bdynamic -lboost_unit_test_framework
This will fail, because the linker will not be able to find the dll.
4.Summary
The issues were:
libraries where specified before the sources which used them
the lib path wasn't specified
the type of linking wasn't specified
the name of the library was not correct
Final, working command:
$ g++ -I/e/code/boost_1_52_0 -o runner runner.cpp test1.cpp -L. -Wl,-Bdynamic -lboost_unit_test_framework-mgw47-mt-1_52
I am just learning C++. Compiling with g++ version 3.2.3, "g++ hworld.cpp":
double sqrt(double);
int main(){
double x = sqrt(1515.15);
return 0;
}
That compiles fine, but if we were to replace sqrt with "sqrtfoo" the compiler would say sqrtfoo cannot be used as a function. I thought I would have to include cmath, but I guess not? Can someone please explain what my program has access to before any includes? For comparison, gcc does not allow me to do this, saying "undefined reference to 'sqrt'." Thank you.
You don't need to include cmath because your code has a prototype for sqrt in it already, the very first line.
As the existing answers explain, the double sort(double) provides a prototype to let the compiler know that the function exists.
But you also mentioned that this doesn't work under GCC. When you build a C or C++ program, the source code is compiled into object format. The object files are then linked together to form an executable.
To see this in action, try
gcc -c hello.c
This tells GCC to compile (-c) the source file hello.c. Assuming that hello.c exists and has no errors, you'll find hello.o in the current directory. Now try
gcc -o hello hello.o
This tells GCC to link hello.o with the appropriate system libraries, and to generate an output file called "hello". If hello.c uses math functions, you'll also need to link in the math library:
gcc -o hello hello.o -lm
"-l" is used to tell gcc to include extra libraries (beyond the default "libc" C library). "m" refers to "libm", which is the math library containing sqrt. If your program uses only one source file it's common to ask implicitly GCC to compile and link in a single command:
gcc -o hello hello.c -lm
Now to your question. GCC won't compile the above code because you haven't asked it to link in the math library. But g++ is okay with it. There's a very similar question already on Stack Overflow. According to its accepted answer,
the C++ runtime libstdc++ requres libm, so if you compile a C++
program with GCC (g++), you will automatically get libm linked in.
Since "libstdc++" is the C++ language runtime library, it's included by g++ by default. And as it depends on libm, the linker automatically loads libm while producing the final binary program.
Header files hold only declarations (signatures), and you've included one in the first line (prototype: double sqrt(double)).
The compiler compiles it just fine, because you've stated that somewhere this function is defined. The step that occurs after compiling is responsible for actually looking for that function definition. It's called linking, and during that phase linker lookups those definitions. In case of sqrtfoo it cannot find anything, whereas in case of sqrt it finds it in some standard library (I do not know the details here).