Undefined reference to pow and floor

Question

I m trying to make a simple fibonacci calculator in C but when compiling gcc tells me that I m missing the pow and floor functions  What s wrong   Code    include  lt stdio h gt   include  lt math h gt   int fibo int n    int main             printf  Fib 4     d   fibo 4            return 0     int fibo int n            double phi   1 61803399           return  int  floor  float  pow phi  n    sqrt 5      5f        Output   gab testvm   work c fibo  gcc fib c -o fibo  tmp ccNSjm4q o  In function  fibo   fib c   text 0x4a   undefined reference to  pow  fib c   text 0x68   undefined reference to  floor  collect2  ld returned 1 exit status

User · Answer

Add -lm to your link options  since pow   and floor   are part of the math library   gcc fib c -o fibo -lm

User · Answer

All answers above are incomplete  the problem here lies in linker ld rather than compiler collect2  ld returned 1 exit status  When you are compiling your fib c to object     gcc -c fib c   nm fib o 0000000000000028 T fibo                  U floor                  U  GLOBAL OFFSET TABLE  0000000000000000 T main                  U pow                  U printf   Where nm lists symbols from object file  You can see that this was compiled without an error  but pow  floor  and printf functions have undefined references  now if I will try to link this to executable     gcc fib o fib o  In function  fibo   fib c   text 0x57   undefined reference to  pow  fib c   text 0x84   undefined reference to  floor  collect2  error  ld returned 1 exit status   Im getting similar output you get  To solve that  I need to tell linker where to look for references to pow  and floor  for this purpose I will use linker -l flag with m which comes from libm so library      gcc fib o -lm   nm a out 0000000000201010 B   bss start 0000000000201010 b completed 7697                  w   cxa finalize  GLIBC 2 2 5 0000000000201000 D   data start 0000000000201000 W data start 0000000000000620 t deregister tm clones 00000000000006b0 t   do global dtors aux 0000000000200da0 t    do global dtors aux fini array entry 0000000000201008 D   dso handle 0000000000200da8 d  DYNAMIC 0000000000201010 D  edata 0000000000201018 B  end 0000000000000722 T fibo 0000000000000804 T  fini                  U floor  GLIBC 2 2 5 00000000000006f0 t frame dummy 0000000000200d98 t   frame dummy init array entry 00000000000009a4 r   FRAME END   0000000000200fa8 d  GLOBAL OFFSET TABLE                   w   gmon start   000000000000083c r   GNU EH FRAME HDR 0000000000000588 T  init 0000000000200da0 t   init array end 0000000000200d98 t   init array start 0000000000000810 R  IO stdin used                  w  ITM deregisterTMCloneTable                  w  ITM registerTMCloneTable 0000000000000800 T   libc csu fini 0000000000000790 T   libc csu init                  U   libc start main  GLIBC 2 2 5 00000000000006fa T main                  U pow  GLIBC 2 2 5                  U printf  GLIBC 2 2 5 0000000000000660 t register tm clones 00000000000005f0 T  start 0000000000201010 D   TMC END     You can now see  functions pow  floor are linked to GLIBC 2 2 5   Parameters order is important too  unless your system is configured to use shared librares by default  my system is not  so when I issue     gcc -lm fib o fib o  In function  fibo   fib c   text 0x57   undefined reference to  pow  fib c   text 0x84   undefined reference to  floor  collect2  error  ld returned 1 exit status   Note -lm flag before object file  So in conclusion  add -lm flag after all other flags  and parameters  to be sure

User · Answer

To find the point where to add the -lm in Eclipse-IDE is really horrible  so it took me some time   If someone else also uses Edlipse  here s the way how to add the command   Project -  Properties -  C C   Build -  Settings -  GCC C Linker -  Miscelleaneous -  Linker flags  in this field add the command -lm

User · Answer

You need to compile with the link flag -lm  like this   gcc fib c -lm -o fibo   This will tell gcc to link your code against the math lib  Just be sure to put the flag after the objects you want to link

User · Answer

In regards to the answer provided by Fuzzy   I actually had to do something slightly different   Project -  Properties -  C C   Build -  Settings -  GCC C Linker -  Libraries  Click the little green add icon  type m and hit ok  Everything in this window automatically has -l applied to it since it is a library

User · Answer

For the benefit of anyone reading this later  you need to link against it as Fred said   gcc fib c -lm -o fibo   One good way to find out what library you need to link is by checking the man page if one exists  For example  man pow and man floor will both tell you   Link with -lm   An explanation for linking math library in C programming - Linking in C

[c] Undefined reference to `pow' and `floor'

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