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DLINFO(3) MidnightBSD Library Functions Manual DLINFO(3)

dlinfo — information about dynamically loaded object

Standard C Library (libc, -lc)

#include <link.h>
#include <dlfcn.h>

int
dlinfo(void * restrict handle, int request, void * restrict p);

The () function provides information about dynamically loaded object. The action taken by dlinfo() and exact meaning and type of p argument 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 returned from dlopen(3), the information returned by the () function 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:

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 argument. 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 following members:

caddr_t         l_base;    /* 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;
caddr_t         l_addr;     /* Load Offset of library */
const char      *l_refname; /* Object this one filters for */
l_base
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.
l_addr
The load offset of the object, that is, the difference between the actual load address and the base virtual address the object was linked at.
l_refname
A name of the object this object filters for, if any. If there are more then one filtee, a name from the first DT_FILTER dynamic entry is supplied.
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.

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).

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 retrieved via a call to dlerror(3).

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 library 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);
}

rtld(1), dladdr(3), dlopen(3), dlsym(3)

The dlinfo() function first appeared in the Solaris operating system. In FreeBSD, it first appeared in FreeBSD 4.8.

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>.

May 21, 2020 midnightbsd-3.1