dlinfo
man page
Linux
dlinfo(3) - Linux manual page
dlinfo(3) Library Functions Manual dlinfo(3)
NAME top
dlinfo - obtain information about a dynamically loaded object
LIBRARY top
Dynamic linking library (libdl, -ldl)
SYNOPSIS top
int dlinfo(void *restrict handle, int request, void *restrict info);
DESCRIPTION top
The dlinfo() function obtains information about the dynamically
loaded object referred to by handle (typically obtained by an
earlier call to dlopen(3) or dlmopen(3)). The request argument
specifies which information is to be returned. The info argument
is a pointer to a buffer used to store information returned by
the call; the type of this argument depends on request.
The following values are supported for request (with the
corresponding type for info shown in parentheses):
RTLD_DI_LMID (Lmid_t *)
Obtain the ID of the link-map list (namespace) in which
handle is loaded.
RTLD_DI_LINKMAP (struct link_map **)
Obtain a pointer to the link_map structure corresponding
to handle. The info argument points to a pointer to a
link_map structure, defined in <link.h> as:
struct link_map {
ElfW(Addr) l_addr; /* Difference between the
address in the ELF file and
the address in memory */
char *l_name; /* Absolute pathname where
object was found */
ElfW(Dyn) *l_ld; /* Dynamic section of the
shared object */
struct link_map *l_next, *l_prev;
/* Chain of loaded objects */
/* Plus additional fields private to the
implementation */
};
RTLD_DI_ORIGIN (char *)
Copy the pathname of the origin of the shared object
corresponding to handle to the location pointed to by
info.
RTLD_DI_SERINFO (Dl_serinfo *)
Obtain the library search paths for the shared object
referred to by handle. The info argument is a pointer to
a Dl_serinfo that contains the search paths. Because the
number of search paths may vary, the size of the structure
pointed to by info can vary. The RTLD_DI_SERINFOSIZE
request described below allows applications to size the
buffer suitably. The caller must perform the following
steps:
(1) Use a RTLD_DI_SERINFOSIZE request to populate a
Dl_serinfo structure with the size (dls_size) of the
structure needed for the subsequent RTLD_DI_SERINFO
request.
(2) Allocate a Dl_serinfo buffer of the correct size
(dls_size).
(3) Use a further RTLD_DI_SERINFOSIZE request to populate
the dls_size and dls_cnt fields of the buffer
allocated in the previous step.
(4) Use a RTLD_DI_SERINFO to obtain the library search
paths.
The Dl_serinfo structure is defined as follows:
typedef struct {
size_t dls_size; /* Size in bytes of
the whole buffer */
unsigned int dls_cnt; /* Number of elements
in 'dls_serpath' */
Dl_serpath dls_serpath[1]; /* Actually longer,
'dls_cnt' elements */
} Dl_serinfo;
Each of the dls_serpath elements in the above structure is
a structure of the following form:
typedef struct {
char *dls_name; /* Name of library search
path directory */
unsigned int dls_flags; /* Indicates where this
directory came from */
} Dl_serpath;
The dls_flags field is currently unused, and always
contains zero.
RTLD_DI_SERINFOSIZE (Dl_serinfo *)
Populate the dls_size and dls_cnt fields of the Dl_serinfo
structure pointed to by info with values suitable for
allocating a buffer for use in a subsequent
RTLD_DI_SERINFO request.
RTLD_DI_TLS_MODID (size_t *, since glibc 2.4)
Obtain the module ID of this shared object's TLS (thread-
local storage) segment, as used in TLS relocations. If
this object does not define a TLS segment, zero is placed
in *info.
RTLD_DI_TLS_DATA (void **, since glibc 2.4)
Obtain a pointer to the calling thread's TLS block
corresponding to this shared object's TLS segment. If
this object does not define a PT_TLS segment, or if the
calling thread has not allocated a block for it, NULL is
placed in *info.
RETURN VALUE top
On success, dlinfo() returns 0. On failure, it returns -1; the
cause of the error can be diagnosed using dlerror(3).
ATTRIBUTES top
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼─────────┤
│ dlinfo() │ Thread safety │ MT-Safe │
└─────────────────────────────────────┴───────────────┴─────────┘
VERSIONS top
The sets of requests supported by the various implementations
overlaps only partially.
STANDARDS top
GNU.
HISTORY top
glibc 2.3.3. Solaris.
EXAMPLES top
The program below opens a shared objects using dlopen(3) and then
uses the RTLD_DI_SERINFOSIZE and RTLD_DI_SERINFO requests to
obtain the library search path list for the library. Here is an
example of what we might see when running the program:
$ ./a.out /lib64/libm.so.6
dls_serpath[0].dls_name = /lib64
dls_serpath[1].dls_name = /usr/lib64
Program source
#define _GNU_SOURCE
#include <dlfcn.h>
#include <link.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
void *handle;
Dl_serinfo serinfo;
Dl_serinfo *sip;
if (argc != 2) {
fprintf(stderr, "Usage: %s <libpath>\n", argv[0]);
exit(EXIT_FAILURE);
}
/* Obtain a handle for shared object specified on command line. */
handle = dlopen(argv[1], RTLD_NOW);
if (handle == NULL) {
fprintf(stderr, "dlopen() failed: %s\n", dlerror());
exit(EXIT_FAILURE);
}
/* Discover the size of the buffer that we must pass to
RTLD_DI_SERINFO. */
if (dlinfo(handle, RTLD_DI_SERINFOSIZE, &serinfo) == -1) {
fprintf(stderr, "RTLD_DI_SERINFOSIZE failed: %s\n", dlerror());
exit(EXIT_FAILURE);
}
/* Allocate the buffer for use with RTLD_DI_SERINFO. */
sip = malloc(serinfo.dls_size);
if (sip == NULL) {
perror("malloc");
exit(EXIT_FAILURE);
}
/* Initialize the 'dls_size' and 'dls_cnt' fields in the newly
allocated buffer. */
if (dlinfo(handle, RTLD_DI_SERINFOSIZE, sip) == -1) {
fprintf(stderr, "RTLD_DI_SERINFOSIZE failed: %s\n", dlerror());
exit(EXIT_FAILURE);
}
/* Fetch and print library search list. */
if (dlinfo(handle, RTLD_DI_SERINFO, sip) == -1) {
fprintf(stderr, "RTLD_DI_SERINFO failed: %s\n", dlerror());
exit(EXIT_FAILURE);
}
for (size_t j = 0; j < serinfo.dls_cnt; j++)
printf("dls_serpath[%zu].dls_name = %s\n",
j, sip->dls_serpath[j].dls_name);
exit(EXIT_SUCCESS);
}
SEE ALSO top
dl_iterate_phdr(3), dladdr(3), dlerror(3), dlopen(3), dlsym(3),
ld.so(8)
COLOPHON top
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Linux man-pages 6.9.1 2024-06-15 dlinfo(3)
FreeBSD
dlinfo(3)
DLINFO(3) Library Functions Manual DLINFO(3)
NAME
dlinfo -- information about dynamically loaded object
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
int
dlinfo(void * restrict handle, int request, void * restrict p);
DESCRIPTION
The dlinfo() function provides information about dynamically loaded ob-
ject. The action taken by dlinfo() and exact meaning and type of p ar-
gument depend on value of the request argument provided by caller.
The handle argument is either the value returned from the dlopen(3)
function call or special handle RTLD_SELF. If handle is the value re-
turned from dlopen(3), the information returned by the dlinfo() func-
tion pertains to the specified object. If handle is the special handle
RTLD_SELF, the information returned pertains to the caller itself.
Possible values for the request argument are:
RTLD_DI_LINKMAP
Retrieve the Link_map (struct link_map) structure pointer for
the specified handle. On successful return, the p argument is
filled with the pointer to the Link_map structure (Link_map
**p) describing a shared object specified by the handle argu-
ment. The Link_map structures are maintained as a doubly
linked list by ld.so(1), in the same order as dlopen(3) and
dlclose(3) are called. See "EXAMPLES", example 1.
The Link_map structure is defined in <link.h> and has the fol-
lowing members:
caddr_t l_addr; /* Base Address of library */
const char *l_name; /* Absolute Path to Library */
const void *l_ld; /* Pointer to .dynamic in memory */
struct link_map *l_next, /* linked list of mapped libs */
*l_prev;
l_addr The base address of the object loaded into memory.
l_name The full name of the loaded shared object.
l_ld The address of the dynamic linking information segment
(PT_DYNAMIC) loaded into memory.
l_next The next Link_map structure on the link-map list.
l_prev The previous Link_map structure on the link-map list.
RTLD_DI_SERINFO
Retrieve the library search paths associated with the given
handle argument. The p argument should point to Dl_serinfo
structure buffer (Dl_serinfo *p). The Dl_serinfo structure
must be initialized first with the RTLD_DI_SERINFOSIZE request.
The returned Dl_serinfo structure contains dls_cnt Dl_serpath
entries. Each entry's dlp_name field points to the search
path. The corresponding dlp_info field contains one of more
flags indicating the origin of the path (see the LA_SER_* flags
defined in the <link.h> header file). See "EXAMPLES", example
2, for a usage example.
RTLD_DI_SERINFOSIZE
Initialize a Dl_serinfo structure for use in a RTLD_DI_SERINFO
request. Both the dls_cnt and dls_size fields are returned to
indicate the number of search paths applicable to the handle,
and the total size of a Dl_serinfo buffer required to hold
dls_cnt Dl_serpath entries and the associated search path
strings. See "EXAMPLES", example 2, for a usage example.
RTLD_DI_ORIGIN
Retrieve the origin of the dynamic object associated with the
handle. On successful return, p argument is filled with the
char pointer (char *p).
RETURN VALUES
The dlinfo() function returns 0 on success, or -1 if an error occurred.
Whenever an error has been detected, a message detailing it can be re-
trieved via a call to dlerror(3).
EXAMPLES
Example 1: Using dlinfo() to retrieve Link_map structure.
The following example shows how dynamic library can detect the list of
shared libraries loaded after caller's one. For simplicity, error
checking has been omitted.
Link_map *map;
dlinfo(RTLD_SELF, RTLD_DI_LINKMAP, &map);
while (map != NULL) {
printf("%p: %s\n", map->l_addr, map->l_name);
map = map->l_next;
}
Example 2: Using dlinfo() to retrieve the library search paths.
The following example shows how a dynamic object can inspect the li-
brary search paths that would be used to locate a simple filename with
dlopen(3). For simplicity, error checking has been omitted.
Dl_serinfo _info, *info = &_info;
Dl_serpath *path;
unsigned int cnt;
/* determine search path count and required buffer size */
dlinfo(RTLD_SELF, RTLD_DI_SERINFOSIZE, (void *)info);
/* allocate new buffer and initialize */
info = malloc(_info.dls_size);
info->dls_size = _info.dls_size;
info->dls_cnt = _info.dls_cnt;
/* obtain sarch path information */
dlinfo(RTLD_SELF, RTLD_DI_SERINFO, (void *)info);
path = &info->dls_serpath[0];
for (cnt = 1; cnt <= info->dls_cnt; cnt++, path++) {
(void) printf("%2d: %s\n", cnt, path->dls_name);
}
SEE ALSO
rtld(1), dladdr(3), dlopen(3), dlsym(3)
HISTORY
The dlinfo() function first appeared in the Solaris operating system.
In FreeBSD, it first appeared in FreeBSD 4.8.
AUTHORS
The FreeBSD implementation of the dlinfo() function was originally
written by Alexey Zelkin <phantom@FreeBSD.org> and later extended and
improved by Alexander Kabaev <kan@FreeBSD.org>.
The manual page for this function was written by Alexey Zelkin
<phantom@FreeBSD.org>.
FreeBSD 13.2 February 14, 2003 DLINFO(3)