undefined reference to `S_ISLINK'

This error occurs when the code is trying to use the S_ISLINK macro from the header file sys/stat.h, but the necessary library is not linked properly. The S_ISLINK macro is used to check if a file is a symbolic link.

To fix this error, you need to link the proper library by adding -l option followed by the library name to the compiler or linker command. For example, if you are using gcc compiler, you can add -lstat to the command as follows:

gcc -o myprogram myprogram.c -lstat

This will link the library libstat.a, which contains the necessary functions for the S_ISLINK macro. Make sure that the library is installed on your system and is in the search path of the compiler or linker.