What does the undefined reference to varName in C mean

Question

I have 2 files  a c and b c  In a c I am sending a signal to a function located in b c  signal SIGUSR1 doSomething     On top of the a c file  I have   extern void doSomething  int    sig     When I compile  however  I get an error       tmp ccCw9Yun o  In function main     a c   text 0xba   undefined reference todoSomething    collect2  ld returned 1 exit status   The following headers are included    include  lt stdlib h gt   include  lt stdio h gt   include  lt signal h gt   include  lt unistd h gt   include  lt sys types h gt   include  lt sys wait h gt    How do I fix this

User · Answer

make sure your doSomething function is not static

User · Answer

An initial reaction to this would be to ask and ensure that the two object files are being linked together  This is done at the compile stage by compiling both files at the same time   gcc -o programName a c b c   Or if you want to compile separately  it would be   gcc -c a c gcc -c b c gcc -o programName a o b o

User · Answer

You need to link both a o and b o   gcc -o program a c b c     If you have a main   in each file  you cannot link them together   However  your a c file contains a reference to doSomething   and expects to be linked with a source file that defines doSomething   and does not define any function that is defined in a c  such as main      You cannot call a function in Process B from Process A   You cannot send a signal to a function  you send signals to processes  using the kill   system call   The signal   function specifies which function in your current process  program  is going to handle the signal when your process receives the signal   You have some serious work to do understanding how this is going to work - how ProgramA is going to know which process ID to send the signal to   The code in b c is going to need to call signal   with dosomething as the signal handler   The code in a c is simply going to send the signal to the other process

User · Answer

It is very bad style to define external interfaces in  c files       You should do this   a h      extern void doSomething  int    sig     a c      void doSomething  int    sig                   do stuff          b c   include  a h        signal SIGNAL  doSomething

User · Answer

You need to compile and then link the object files like this   gcc -c a c   gcc -c b c   gcc a o b o -o prog

User · Answer

You re getting a linker error  so your extern is working  the compiler compiled a c without a problem   but when it went to link the object files together at the end it couldn t resolve your extern -- void doSomething int   wasn t actually found anywhere  Did you mess up the extern  Make sure there s actually a doSomething defined in b c that takes an int and returns void  and make sure you remembered to include b c in your file list  i e  you re doing something like gcc a c b c  not just gcc a c

[c] What does the "undefined reference to varName" in C mean?

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