GCC: compile-time library path and runtime library path

Assume a program with the following dependencies:

m depends on a

a depends on b

//a.c#include void b();void a(){printf("Here is a call b\n");b();}

//b.c#include void b(){printf("Here is b\n");}

//m.c#include void a();int main(){a();return 0;}

Compile the dynamic libraries liba.so and libb.so:

gcc -fpic -shared a.c -o liba.so

gcc -fpic -shared b.c -o libb.so

Compiling and linking program m may encounter the following problems:

1. Directly compile and link m

gcc -o m m.c

Because the function a defined in the dynamic library liba.so that m directly depends on cannot be found, an error is reported:
/usr/bin/ld: /tmp/ccpoXOmH.o: in function `main':
m.c:(.text+0xe): undefined reference to `a'
collect2: error: ld returned 1 exit status

2. Compile the specified dynamic libraries a and b:

gcc -o m m.c -la -lb

Since the default library file path does not contain the current directory, an error is reported:
/usr/bin/ld: cannot find -la
/usr/bin/ld: cannot find -lb
collect2: error: ld returned 1 exit status

It can be solved in two ways:

• Use the compile-time parameter -L to specify to find library files in the current directory

gcc -o m m.c -la -lb -L .

• By setting LIBRARY_PATH

      export LIBRARY_PATH=./:$LIBRARY_PATH

      gcc -o m m.c -la -lb

3. Runtime error:

./m
./m: error while loading shared libraries: liba.so: cannot open shared object file: No such file or directory

This is because the library cannot be connected at runtime when the program m is loaded, so an error is reported.

Runtime can be resolved by settingLD_LIBRARY_PATH,Connection error:

export LD_LIBRARY_PATH=./:$LD_LIBRARY_PATH

./m
Here is a call b
Here is b