Undefined reference to sqrt or other mathematical functions

Question

I have this simple code   max    int  sqrt  number     and in the header I have    include  lt math h gt    But application still says undefined reference to sqrt  Do you see any problem here  It looks like everything should be okay

User · Accepted Answer

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.

User · Answer

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  lt math h gt    secondly  if you read manual page of sqrt you will notice this line Link with -lm    include  lt math h gt     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  amp  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

User · Answer

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

User · Answer

Just adding the  include  lt math h gt  in c source file and -lm  in Makefile at the end will work for me       gcc -pthread -o p3 p3 c -lm

User · Answer

I suppose you have imported math h with  include  lt math h gt   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

[c] Undefined reference to sqrt (or other mathematical functions)

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