NAME

dlsym - obtain the address of a symbol from a dlopen object

SYNOPSIS

[XSI] #include <dlfcn.h>

void *dlsym(void *restrict handle, const char *restrict name);

DESCRIPTION

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.

RETURN VALUE

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().

ERRORS

No errors are defined.

EXAMPLES

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);

APPLICATION USAGE

Special purpose values for handle are reserved for future use. These values and their meanings are:

RTLD_DEFAULT

The symbol lookup happens in the normal global scope; that is, a search for a symbol using this handle would find the same definition as a direct use of this symbol in the program code.

RTLD_NEXT

Specifies the next object after this one that defines name. This one refers to the object containing the invocation of dlsym(). The next object is the one found upon the application of a load order symbol resolution algorithm (see dlopen()). The next object is either one of global scope (because it was introduced as part of the original process image or because it was added with a dlopen() operation including the RTLD_GLOBAL flag), or is an object that was included in the same dlopen() operation that loaded this one.

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.

RATIONALE

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.

FUTURE DIRECTIONS

None.

SEE ALSO

dlclose(), dlerror(), dlopen(), the Base Definitions volume of IEEE Std 1003.1-2001, <dlfcn.h>

CHANGE HISTORY

First released in Issue 5.

Issue 6

The restrict keyword is added to the dlsym() prototype for alignment with the ISO/IEC 9899:1999 standard.

The RTLD_DEFAULT and RTLD_NEXT flags are reserved for future use.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/14 is applied, correcting an example, and adding text to the RATIONALE describing issues related to conversion of pointers to functions and back again.