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DLSYM(3)                                                                Linux Programmer's Manual                                                               DLSYM(3)

NAME
       dlsym, dlvsym - obtain address of a symbol in a shared object or executable

SYNOPSIS
       #include <dlfcn.h>

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

       #define _GNU_SOURCE
       #include <dlfcn.h>

       void *dlvsym(void *restrict handle, const char *restrict symbol,
                    const char *restrict version);

       Link with -ldl.

DESCRIPTION
       The  function  dlsym() takes a "handle" of a dynamic loaded shared object returned by dlopen(3) along with a null-terminated symbol name, and returns the address
       where that symbol is loaded into memory.  If the symbol is not found, in the specified object or any of the shared objects  that  were  automatically  loaded  by
       dlopen(3)  when  that object was loaded, dlsym() returns NULL.  (The search performed by dlsym() is breadth first through the dependency tree of these shared ob‐
       jects.)

       In unusual cases (see NOTES) the value of the symbol could actually be NULL.  Therefore, a NULL return from dlsym() need not indicate an error.  The correct  way
       to  distinguish  an  error from a symbol whose value is NULL is to call dlerror(3) to clear any old error conditions, then call dlsym(), and then call dlerror(3)
       again, saving its return value into a variable, and check whether this saved value is not NULL.

       There are two special pseudo-handles that may be specified in handle:

       RTLD_DEFAULT
              Find the first occurrence of the desired symbol using the default shared object search order.  The search will include global symbols  in  the  executable
              and its dependencies, as well as symbols in shared objects that were dynamically loaded with the RTLD_GLOBAL flag.

       RTLD_NEXT
              Find  the  next  occurrence of the desired symbol in the search order after the current object.  This allows one to provide a wrapper around a function in
              another shared object, so that, for example, the definition of a function in a preloaded shared object (see LD_PRELOAD in ld.so(8)) can  find  and  invoke
              the  "real" function provided in another shared object (or for that matter, the "next" definition of the function in cases where there are multiple layers
              of preloading).

       The _GNU_SOURCE feature test macro must be defined in order to obtain the definitions of RTLD_DEFAULT and RTLD_NEXT from <dlfcn.h>.

       The function dlvsym() does the same as dlsym() but takes a version string as an additional argument.

RETURN VALUE
       On success, these functions return the address associated with symbol.  On failure, they return NULL; the cause of the error can be diagnosed using dlerror(3).

VERSIONS
       dlsym() is present in glibc 2.0 and later.  dlvsym() first appeared in glibc 2.1.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       β”Œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”
       β”‚Interface                                                                                                                             β”‚ Attribute     β”‚ Value   β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚dlsym(), dlvsym()                                                                                                                     β”‚ Thread safety β”‚ MT-Safe β”‚
       β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”˜

CONFORMING TO
       POSIX.1-2001 describes dlsym().  The dlvsym() function is a GNU extension.

NOTES
       There are several scenarios when the address of a global symbol is NULL.  For example, a symbol can be placed at zero address by the linker, via a linker  script
       or  with  --defsym  command-line  option.   Undefined  weak symbols also have NULL value.  Finally, the symbol value may be the result of a GNU indirect function
       (IFUNC) resolver function that returns NULL as the resolved value.  In the latter case, dlsym() also returns NULL without error.  However, in the former two cas‐
       es,  the  behavior of GNU dynamic linker is inconsistent: relocation processing succeeds and the symbol can be observed to have NULL value, but dlsym() fails and
       dlerror() indicates a lookup error.

   History
       The dlsym() function is part of the dlopen API, derived from SunOS.  That system does not have dlvsym().

EXAMPLES
       See dlopen(3).

SEE ALSO
       dl_iterate_phdr(3), dladdr(3), dlerror(3), dlinfo(3), dlopen(3), ld.so(8)

Linux                                                                          2021-03-22                                                                       DLSYM(3)