In the last chapter, we saw how to link to a program with functions in the C math library libm.a, using the short-cut option -lm and the header file math.h.
A common problem when compiling a program using library header files is the error:
FILE.h: No such file or directory
This occurs if a header file is not present in the standard include file
directories used by gcc
. A similar problem can occur for
libraries:
/usr/bin/ld: cannot find library
This happens if a library used for linking is not present in the
standard library directories used by gcc
.
By default, gcc
searches the following directories for header
files:
/usr/local/include/ /usr/include/
and the following directories for libraries:
/usr/local/lib/ /usr/lib/
The list of directories for header files is often referred to as the include path, and the list of directories for libraries as the library search path or link path.
The directories on these paths are searched in order, from first to last in the two lists above.7 For example, a header file found in /usr/local/include takes precedence over a file with the same name in /usr/include. Similarly, a library found in /usr/local/lib takes precedence over a library with the same name in /usr/lib.
When additional libraries are installed in other directories it is necessary to extend the search paths, in order for the libraries to be found. The compiler options -I and -L add new directories to the beginning of the include path and library search path respectively.
• Search path example | ||
• Environment variables | ||
• Extended search paths |
The default search paths may also include additional system-dependent or site-specific directories, and directories in the GCC installation itself. For example, on 64-bit platforms additional /usr/lib/x86_64-linux-gnu or /usr/lib64 directories may also be searched by default. see Multi-architecture support