.\" Copyright 1995 Yggdrasil Computing, Incorporated. .\" and Copyright 2003, 2015 Michael Kerrisk .\" .\" %%%LICENSE_START(GPLv2+_DOC_FULL) .\" This is free documentation; you can redistribute it and/or .\" modify it under the terms of the GNU General Public License as .\" published by the Free Software Foundation; either version 2 of .\" the License, or (at your option) any later version. .\" .\" The GNU General Public License's references to "object code" .\" and "executables" are to be interpreted as the output of any .\" document formatting or typesetting system, including .\" intermediate and printed output. .\" .\" This manual is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, see .\" . .\" %%%LICENSE_END .\" .TH DLSYM 3 2019-03-06 "Linux" "Linux Programmer's Manual" .SH NAME dlsym, dlvsym \- obtain address of a symbol in a shared object or executable .SH SYNOPSIS .B #include .PP .BI "void *dlsym(void *" handle ", const char *" symbol ); .PP .B #define _GNU_SOURCE .br .B #include .PP .BI "void *dlvsym(void *" handle ", char *" symbol ", char *" version ); .PP Link with \fI\-ldl\fP. .SH DESCRIPTION The function .BR dlsym () takes a "handle" of a dynamic loaded shared object returned by .BR 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 .BR dlopen (3) when that object was loaded, .BR dlsym () returns NULL. (The search performed by .BR dlsym () is breadth first through the dependency tree of these shared objects.) .PP In unusual cases (see NOTES) the value of the symbol could actually be NULL. Therefore, a NULL return from .BR dlsym () need not indicate an error. The correct way to distinguish an error from a symbol whose value is NULL is to call .BR dlerror (3) to clear any old error conditions, then call .BR dlsym (), and then call .BR dlerror (3) again, saving its return value into a variable, and check whether this saved value is not NULL. .PP There are two special pseudo-handles that may be specified in .IR handle : .TP .B 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 .BR RTLD_GLOBAL flag. .TP .BR 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 .B LD_PRELOAD in .BR 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). .PP The .B _GNU_SOURCE feature test macro must be defined in order to obtain the definitions of .B RTLD_DEFAULT and .B RTLD_NEXT from .IR . .PP .PP The function .BR dlvsym () does the same as .BR dlsym () but takes a version string as an additional argument. .SH RETURN VALUE On success, these functions return the address associated with .IR symbol . On failure, they return NULL; the cause of the error can be diagnosed using .BR dlerror (3). .SH VERSIONS .BR dlsym () is present in glibc 2.0 and later. .BR dlvsym () first appeared in glibc 2.1. .SH ATTRIBUTES For an explanation of the terms used in this section, see .BR attributes (7). .TS allbox; lb lb lb l l l. Interface Attribute Value T{ .BR dlsym (), .BR dlvsym () T} Thread safety MT-Safe .TE .SH CONFORMING TO POSIX.1-2001 describes .BR dlsym (). The .BR dlvsym () function is a GNU extension. .SH NOTES The value of a symbol returned by .BR dlsym () will never be NULL if the shared object is the result of normal compilation, since a global symbol is never placed at the NULL address. There are nevertheless cases where a lookup using .BR dlsym () may return NULL as the value of a symbol. For example, the symbol value may be the result of a GNU indirect function (IFUNC) resolver function that returns NULL as the resolved value. .\" .SS History The .BR dlsym () function is part of the dlopen API, derived from SunOS. That system does not have .BR dlvsym (). .SH EXAMPLE See .BR dlopen (3). .SH SEE ALSO .BR dl_iterate_phdr (3), .BR dladdr (3), .BR dlerror (3), .BR dlinfo (3), .BR dlopen (3), .BR ld.so (8) .SH COLOPHON This page is part of release 5.04 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.