Libraries

Here are some notes on building an using libraries in their different flavours on POSIX systems.

the library

hello.h

typedef void (*HelloFunc)();

void hello();

hello.c

#include <stdio.h>

#include "hello.h"

void hello()
{
    printf("hello world\n");
}

building the library:

gcc -fPIC -c hello.c

gcc -shared -o libhello.o hello.o

binding on load time

main.c

#include "hello.h"

int main()
{
    hello();
    return 0;
}

building main

gcc -o main main.c -L. -lhello

running main

$ LD_LIBRARY_PATH=. ./main
hello world

binding on run time

main2.c

#include <dlfcn.h>
#include <stdio.h>
#include "hello.h"

int main()
{
    void* handle = dlopen("./libhello.so", RTLD_NOW);
    if (handle == NULL) {
        printf("dlopen failed: %s\n", dlerror());
        return 1;
    }

    void* symbol = dlsym(handle, "hello");
    if (symbol == NULL) {
        printf("dlsym failed: %s\n", dlerror());
        return 1;
    }

    HelloFunc helloFunc = (HelloFunc) symbol;
    helloFunc();

    if (dlclose(handle) != 0) {
        printf("dlclose failed: %s\n", dlerror());
        return 1;
    }

    return 0;
}

building main

gcc -o main2 main2.c -ldl

running main

$ ./main2
hello world

static binding

building the library

gcc -c hello.c
ar rc libhello.a hello.o
ranlib libhello.a

building main

gcc -o main main.o libhello.a

running main

$ ./main
hello world

further reading

tools:

• ldd - print shared library dependencies
• nm - list symbols from object files
• objdump - display information from object files
• readelf - Displays information about ELF files

More on libraries can be found here.

Update: Ian Lance Taylor has written a series of 20 articles on linkers and the ELF format. Here is the first one.