dlinfo(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

DLINFO(3)               Linux Programmer's Manual              DLINFO(3)

NAME         top

       dlinfo - obtain information about a dynamically loaded object

SYNOPSIS         top

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

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

       Link with -ldl.

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).

VERSIONS         top

       dlinfo() first appeared in glibc 2.3.3.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌──────────────────────────────────────┬───────────────┬─────────┐
       │Interface                             Attribute     Value   │
       ├──────────────────────────────────────┼───────────────┼─────────┤
       │dlinfo()                              │ Thread safety │ MT-Safe │
       └──────────────────────────────────────┴───────────────┴─────────┘

CONFORMING TO         top

       This function is a nonstandard GNU extension.

NOTES         top

       This function derives from the Solaris function of the same name
       and also appears on some other systems.  The sets of requests
       supported by the various implementations overlaps only partially.

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 (int j = 0; j < serinfo.dls_cnt; j++)
               printf("dls_serpath[%d].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

       This page is part of release 5.13 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                      DLINFO(3)

Pages that refer to this page: dladdr(3)dlerror(3)dlopen(3)dlsym(3)