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void *dlopen(const char *filename, int flag);
char *dlerror(void);
void *dlsym(void *handle, const char *symbol);
int dlclose(void *handle);
Link with -ldl.
If the library has dependencies on other shared libraries, then these are also automatically loaded by the dynamic linker using the same rules. (This process may occur recursively, if those libraries in turn have dependencies, and so on.)
One of the following two values must be included in flag:
Zero or more of the following values may also be ORed in flag:
If filename is NULL, then the returned handle is for the main program. When given to dlsym(), this handle causes a search for a symbol in the main program, followed by all shared libraries loaded at program startup, and then all shared libraries loaded by dlopen() with the flag RTLD_GLOBAL.
External references in the library are resolved using the libraries in that library’s dependency list and any other libraries previously opened with the RTLD_GLOBAL flag. If the executable was linked with the flag "-rdynamic" (or, synonymously, "--export-dynamic"), then the global symbols in the executable will also be used to resolve references in a dynamically loaded library.
If the same library is loaded again with dlopen(), the same library handle is returned. The dl library maintains reference counts for library handles, so a dynamic library is not deallocated until dlclose() has been called on it as many times as dlopen() has succeeded on it. The _init() routine, if present, is called only once. But a subsequent call with RTLD_NOW may force symbol resolution for a library earlier loaded with RTLD_LAZY.
If dlopen() fails for any reason, it returns NULL.
There are two special pseudo-handles, RTLD_DEFAULT and RTLD_NEXT. The former will find the first occurrence of the desired symbol using the default library search order. The latter will find the next occurrence of a function in the search order after the current library. This allows one to provide a wrapper around a function in another shared library.
The function dlclose() returns 0 on success, and nonzero on error.
Using these routines, or the gcc -nostartfiles or -nostdlib options, is not recommended. Their use may result in undesired behavior, since the constructor/destructor routines will not be executed (unless special measures are taken).
Instead, libraries should export routines using the __attribute__((constructor)) and __attribute__((destructor)) function attributes. See the gcc info pages for information on these. Constructor routines are executed before dlopen() returns, and destructor routines are executed before dlclose() returns.
#define _GNU_SOURCE /* See feature_test_macros(7) */#include <dlfcn.h> int dladdr(void *addr, Dl_info *info); void *dlvsym(void *handle, char *symbol, char *version);
The function dladdr() takes a function pointer and tries to resolve name and file where it is located. Information is stored in the Dl_info structure:
typedef struct {
const char *dli_fname; /* Pathname of shared object that
contains address */
void *dli_fbase; /* Address at which shared object
is loaded */
const char *dli_sname; /* Name of symbol whose definition
overlaps addr */
void *dli_saddr; /* Exact address of symbol named
in dli_sname */
} Dl_info;
If no symbol matching addr could be found, then dli_sname and dli_saddr are set to NULL.
dladdr() returns 0 on error, and nonzero on success.
The function dlvsym(), provided by glibc since version 2.1, does the same as dlsym() but takes a version string as an additional argument.
Since glibc 2.2.3, atexit(3) can be used to register an exit handler that is automatically called when a library is unloaded.
The problem is that the function pointer will still be resolved at compile time, but merely point to the plt (Procedure Linkage Table) section of the original object (which dispatches the call after asking the dynamic linker to resolve the symbol). To work around this, you can try to compile the code to be position-independent: then, the compiler cannot prepare the pointer at compile time anymore and today’s gcc(1) will generate code that just loads the final symbol address from the got (Global Offset Table) at run time before passing it to dladdr().
#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
int
main(int argc, char **argv)
{
void *handle;
double (*cosine)(double);
char *error;
handle = dlopen("libm.so", RTLD_LAZY);
if (!handle) {
fprintf(stderr, "%s\n", dlerror());
exit(EXIT_FAILURE);
}
dlerror(); /* Clear any existing error */
cosine = (double (*)(double)) dlsym(handle, "cos");
/* According to the ISO C standard, casting between function
pointers and ’void *’, as done above, produces undefined results.
POSIX.1-2003 and POSIX.1-2008 accepted this state of affairs and
proposed the following workaround:
*(void **) (&cosine) = dlsym(handle, "cos");
This (clumsy) cast conforms with the ISO C standard and will
avoid any compiler warnings.
The 2013 Technical Corrigendum to POSIX.1-2008 (a.k.a.
POSIX.1-2013) improved matters by requiring that conforming
implementations support casting ’void *’ to a function pointer.
Nevertheless, some compilers (e.g., gcc with the ’-pedantic’
option) may complain about the cast used in this program. */
error = dlerror();
if (error != NULL) {
fprintf(stderr, "%s\n", error);
exit(EXIT_FAILURE);
}
printf("%f\n", (*cosine)(2.0));
dlclose(handle);
exit(EXIT_SUCCESS);
}
If this program were in a file named "foo.c", you would build the program with the following command:
gcc -rdynamic -o foo foo.c -ldl
Libraries exporting _init() and _fini() will want to be compiled as follows, using bar.c as the example name:
gcc -shared -nostartfiles -o bar bar.c
ld.so info pages, gcc info pages, ld info pages