man3/dlsym.3

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  25 .TH DLSYM 3 2017-07-13 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 dlsym, dlvsym \- obtain address of a symbol in a shared object or executable
  28 .SH SYNOPSIS
  29 .B #include <dlfcn.h>
  30 .sp
  31 .BI "void *dlsym(void *" handle ", const char *" symbol );
  32 .sp
  33 .B #define _GNU_SOURCE
  34 .br
  35 .B #include <dlfcn.h>
  36 .sp
  37 .BI "void *dlvsym(void *" handle ", char *" symbol ", char *" version );
  38 .sp
  39 Link with \fI\-ldl\fP.
  40 .SH DESCRIPTION
  41 The function
  42 .BR dlsym ()
  43 takes a "handle" of a dynamic loaded shared object returned by
  44 .BR dlopen (3)
  45 along with a null-terminated symbol name,
  46 and returns the address where that symbol is
  47 loaded into memory.
  48 If the symbol is not found, in the specified
  49 object or any of the shared objects that were automatically loaded by
  50 .BR dlopen (3)
  51 when that object was loaded,
  52 .BR dlsym ()
  53 returns NULL.
  54 (The search performed by
  55 .BR dlsym ()
  56 is breadth first through the dependency tree of these shared objects.)
  57
  58 Since the value of the symbol could actually be NULL (so that a
  59 NULL return from
  60 .BR dlsym ()
  61 need not indicate an error), the correct way to test for an error
  62 is to call
  63 .BR dlerror (3)
  64 to clear any old error conditions, then call
  65 .BR dlsym (),
  66 and then call
  67 .BR dlerror (3)
  68 again, saving its return value into a variable, and check whether
  69 this saved value is not NULL.
  70 .PP
  71 There are two special pseudo-handles that may be specified in
  72 .IR handle :
  73 .TP
  74 .B RTLD_DEFAULT
  75 Find the first occurrence of the desired symbol
  76 using the default shared object search order.
  77 The search will include global symbols in the executable
  78 and its dependencies,
  79 as well as symbols in shared objects that were dynamically loaded with the
  80 .BR RTLD_GLOBAL
  81 flag.
  82 .TP
  83 .BR RTLD_NEXT
  84 Find the next occurrence of the desired symbol in the search order
  85 after the current object.
  86 This allows one to provide a wrapper
  87 around a function in another shared object, so that, for example,
  88 the definition of a function in a preloaded shared object
  89 (see
  90 .B LD_PRELOAD
  91 in
  92 .BR ld.so (8))
  93 can find and invoke the "real" function provided in another shared object
  94 (or for that matter, the "next" definition of the function in cases
  95 where there are multiple layers of preloading).
  96 .PP
  97 The
  98 .B _GNU_SOURCE
  99 feature test macro must be defined in order to obtain the
 100 definitions of
 101 .B RTLD_DEFAULT
 102 and
 103 .B RTLD_NEXT
 104 from
 105 .IR <dlfcn.h> .
 106
 107 .PP
 108 The function
 109 .BR dlvsym ()
 110 does the same as
 111 .BR dlsym ()
 112 but takes a version string as an additional argument.
 113 .SH RETURN VALUE
 114 On success,
 115 these functions return the address associated with
 116 .IR symbol .
 117 On failure, they return NULL;
 118 the cause of the error can be diagnosed using
 119 .BR dlerror (3).
 120 .SH VERSIONS
 121 .BR dlsym ()
 122 is present in glibc 2.0 and later.
 123 .BR dlvsym ()
 124 first appeared in glibc 2.1.
 125 .SH ATTRIBUTES
 126 For an explanation of the terms used in this section, see
 127 .BR attributes (7).
 128 .TS
 129 allbox;
 130 lb lb lb
 131 l l l.
 132 Interface       Attribute       Value
 133 T{
 134 .BR dlsym (),
 135 .BR dlvsym ()
 136 T}      Thread safety   MT-Safe
 137 .TE
 138 .SH CONFORMING TO
 139 POSIX.1-2001 describes
 140 .BR dlsym ().
 141 The
 142 .BR dlvsym ()
 143 function is a GNU extension.
 144 .SH NOTES
 145 .SS History
 146 The
 147 .BR dlsym ()
 148 function is part of the dlopen API, derived from SunOS.
 149 That system does not have
 150 .BR dlvsym ().
 151 .SH EXAMPLE
 152 See
 153 .BR dlopen (3).
 154 .SH SEE ALSO
 155 .BR dl_iterate_phdr (3),
 156 .BR dladdr (3),
 157 .BR dlerror (3),
 158 .BR dlinfo (3),
 159 .BR dlopen (3),
 160 .BR ld.so (8)