mirror of https://github.com/mkerrisk/man-pages
178 lines
5.0 KiB
Groff
178 lines
5.0 KiB
Groff
.\" Copyright 1995 Yggdrasil Computing, Incorporated.
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.\" and Copyright 2003, 2015 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, see
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.\" <http://www.gnu.org/licenses/>.
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.\" %%%LICENSE_END
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.\"
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.TH DLSYM 3 2020-06-09 "Linux" "Linux Programmer's Manual"
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.SH NAME
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dlsym, dlvsym \- obtain address of a symbol in a shared object or executable
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.SH SYNOPSIS
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.B #include <dlfcn.h>
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.PP
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.BI "void *dlsym(void *" handle ", const char *" symbol );
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.PP
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.B #define _GNU_SOURCE
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.br
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.B #include <dlfcn.h>
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.PP
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.BI "void *dlvsym(void *" handle ", char *" symbol ", char *" version );
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.PP
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Link with \fI\-ldl\fP.
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.SH DESCRIPTION
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The function
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.BR dlsym ()
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takes a "handle" of a dynamic loaded shared object returned by
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.BR dlopen (3)
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along with a null-terminated symbol name,
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and returns the address where that symbol is
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loaded into memory.
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If the symbol is not found, in the specified
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object or any of the shared objects that were automatically loaded by
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.BR dlopen (3)
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when that object was loaded,
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.BR dlsym ()
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returns NULL.
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(The search performed by
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.BR dlsym ()
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is breadth first through the dependency tree of these shared objects.)
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.PP
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In unusual cases (see NOTES) the value of the symbol could actually be NULL.
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Therefore, a NULL return from
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.BR dlsym ()
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need not indicate an error.
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The correct way to distinguish an error from a symbol whose value is NULL
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is to call
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.BR dlerror (3)
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to clear any old error conditions, then call
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.BR dlsym (),
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and then call
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.BR dlerror (3)
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again, saving its return value into a variable, and check whether
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this saved value is not NULL.
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.PP
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There are two special pseudo-handles that may be specified in
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.IR handle :
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.TP
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.B RTLD_DEFAULT
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Find the first occurrence of the desired symbol
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using the default shared object search order.
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The search will include global symbols in the executable
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and its dependencies,
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as well as symbols in shared objects that were dynamically loaded with the
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.BR RTLD_GLOBAL
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flag.
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.TP
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.BR RTLD_NEXT
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Find the next occurrence of the desired symbol in the search order
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after the current object.
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This allows one to provide a wrapper
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around a function in another shared object, so that, for example,
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the definition of a function in a preloaded shared object
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(see
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.B LD_PRELOAD
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in
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.BR ld.so (8))
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can find and invoke the "real" function provided in another shared object
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(or for that matter, the "next" definition of the function in cases
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where there are multiple layers of preloading).
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.PP
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The
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.B _GNU_SOURCE
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feature test macro must be defined in order to obtain the
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definitions of
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.B RTLD_DEFAULT
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and
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.B RTLD_NEXT
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from
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.IR <dlfcn.h> .
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.PP
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The function
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.BR dlvsym ()
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does the same as
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.BR dlsym ()
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but takes a version string as an additional argument.
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.SH RETURN VALUE
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On success,
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these functions return the address associated with
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.IR symbol .
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On failure, they return NULL;
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the cause of the error can be diagnosed using
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.BR dlerror (3).
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.SH VERSIONS
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.BR dlsym ()
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is present in glibc 2.0 and later.
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.BR dlvsym ()
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first appeared in glibc 2.1.
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.TS
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allbox;
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lb lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR dlsym (),
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.BR dlvsym ()
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T} Thread safety MT-Safe
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.TE
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.SH CONFORMING TO
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POSIX.1-2001 describes
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.BR dlsym ().
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The
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.BR dlvsym ()
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function is a GNU extension.
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.SH NOTES
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There are several scenarios when the address of a global symbol is NULL.
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For example, a symbol can be placed at zero address by the linker, via
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a linker script or with
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.I \-\-defsym
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command-line option. Undefined weak symbols also have NULL value.
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Finally, the symbol value may be the result of
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a GNU indirect function (IFUNC) resolver function that returns
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NULL as the resolved value. In the latter case,
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.BR dlsym ()
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also returns NULL without error. However, in the former two cases, the
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behavior of GNU dynamic linker is inconsistent: relocation processing
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succeeds and the symbol can be observed to have NULL value, but
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.BR dlsym ()
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fails and
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.BR dlerror ()
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indicates a lookup error.
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.\"
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.SS History
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The
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.BR dlsym ()
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function is part of the dlopen API, derived from SunOS.
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That system does not have
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.BR dlvsym ().
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.SH EXAMPLES
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See
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.BR dlopen (3).
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.SH SEE ALSO
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.BR dl_iterate_phdr (3),
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.BR dladdr (3),
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.BR dlerror (3),
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.BR dlinfo (3),
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.BR dlopen (3),
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.BR ld.so (8)
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