man3/dlsym.3

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.TH DLSYM 3 2020-06-09 "Linux" "Linux Programmer's Manual"
.SH NAME
dlsym, dlvsym \- obtain address of a symbol in a shared object or executable
.SH SYNOPSIS
.B #include <dlfcn.h>
.PP
.BI "void *dlsym(void *" handle ", const char *" symbol );
.PP
.B #define _GNU_SOURCE
.br
.B #include <dlfcn.h>
.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 <dlfcn.h> .
.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
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
.I \-\-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,
.BR dlsym ()
also returns NULL without error. However, in the former two cases, the
behavior of GNU dynamic linker is inconsistent: relocation processing
succeeds and the symbol can be observed to have NULL value, but
.BR dlsym ()
fails and
.BR dlerror ()
indicates a lookup error.
.\"
.SS History
The
.BR dlsym ()
function is part of the dlopen API, derived from SunOS.
That system does not have
.BR dlvsym ().
.SH EXAMPLES
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)