dlsym(3) [osf1 man page]
dlsym(3) Library Functions Manual dlsym(3) NAME
dlsym - Obtain the address of a symbol from a dlopen() object SYNOPSIS
#include <dlfcn.h> void *dlsym(void *handle, const char *name) PARAMETERS
The value returned from a call to dlopen() (and which has not since been released by a call to dlclose()). The name (character string) of the symbol being sought. DESCRIPTION
The dlsym function allows a process to obtain the address of a symbol defined within an object made accessible by a dlopen() call. The dlsym function will search for the named symbol in all objects loaded automatically as a result of loading the object referenced by handle (see dlopen(3)). Load ordering is used in dlsym() operations upon the global symbol object. The symbol resolution algorithm used will be in dependency order as described in dlopen(). RETURN VALUE
The dlsym() function will return NULL, 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. More detailed diagnostic information is available through dlerror(). ERRORS
No errors are defined. EXAMPLES
The following example shows how one can use dlopen() and dlsym() 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.1", RTLD_LAZY); /* find the address of function and data objects */ fptr = (int (*)(int))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: 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(3)). 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 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. NOTES
Use of the dlsym routine is the preferred mechanism for retrieving symbol addresses. This routine reliably returns the current address of a symbol at any point in the program, while the dynamic symbol resolution described previously might not function as expected due to com- piler optimizations. For example, the address of a symbol might be saved in a register prior to a dlopen call, and the saved address might then be used after the dlopen call - even if the dlopen call changed the resolution of the symbol. RELATED INFORMATION
dlclose(3), dlerror(3), dlopen(3). delim off dlsym(3)
Check Out this Related Man Page
DLSYM(3P) POSIX Programmer's Manual DLSYM(3P) PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the correspond- ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux. NAME
dlsym - obtain the address of a symbol from a dlopen object SYNOPSIS
#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. The following sections are informative. 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 defini- tion 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 gener- ate 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> COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol- ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html . IEEE
/The Open Group 2003 DLSYM(3P)