#include <dlfcn.h>
void *dlsym(void *restrict handle, const char *restrict
name);
The dlsym() function shall obtain the address of a symbol defined within an object made accessible through a dlopen() call. The handle argument is the value returned from a call to dlopen() (and which has not since been released via a call to dlclose()), and name is the symbol's name as a character string.
The dlsym() function shall search for the named symbol in all objects loaded automatically as a result of loading the object referenced by handle (see dlopen()). Load ordering is used in dlsym() operations upon the global symbol object. The symbol resolution algorithm used shall be dependency order as described in dlopen().
The RTLD_DEFAULT and RTLD_NEXT flags are reserved for future use.
If handle does not refer to a valid object opened by dlopen(), or if the named symbol cannot be found within any of the objects associated with handle, dlsym() shall return NULL. More detailed diagnostic information shall be available through dlerror() .
No errors are defined.
The following sections are informative.
The following example shows how dlopen() and dlsym() can be used to access either function or data objects. For simplicity, error checking has been omitted.
void *handle;
int *iptr, (*fptr)(int);
/* open the needed object */
handle = dlopen("/usr/home/me/libfoo.so", RTLD_LOCAL | RTLD_LAZY);
/* find the address of function and data objects */
*(void **)(&fptr) = dlsym(handle, "my_function");
iptr = (int *)dlsym(handle, "my_object");
/* invoke function, passing value of integer as a parameter */
(*fptr)(*iptr);
Special purpose values for handle are reserved for future use. These values and their meanings are:
The RTLD_NEXT flag is useful to navigate an intentionally created hierarchy of multiply-defined symbols created through interposition. For example, if a program wished to create an implementation of malloc() that embedded some statistics gathering about memory allocations, such an implementation could use the real malloc() definition to perform the memory allocation-and itself only embed the necessary logic to implement the statistics gathering function.
The ISO C standard does not require that pointers to functions can be cast back and forth to pointers to data. Indeed, the ISO C standard does not require that an object of type void * can hold a pointer to a function. Implementations supporting the XSI extension, however, do require that an object of type void * can hold a pointer to a function. The result of converting a pointer to a function into a pointer to another data type (except void *) is still undefined, however. Note that compilers conforming to the ISO C standard are required to generate a warning if a conversion from a void * pointer to a function pointer is attempted as in:
fptr = (int (*)(int))dlsym(handle, "my_function");
Due to the problem noted here, a future version may either add a new function to return function pointers, or the current interface may be deprecated in favor of two new functions: one that returns data pointers and the other that returns function pointers.
dlclose(), dlerror(), dlopen(), the Base Definitions volume of IEEE Std 1003.1-2001, <dlfcn.h>