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

NAME

     dlinfo - obtain information about a dynamically loaded object

SYNOPSIS

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

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

     Link with -ldl.

DESCRIPTION

     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 infor-
     mation 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 argu-
     ment 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 han-
            dle.  The info argument points to a pointer to a link_map struc-
            ture, 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 correspond-
            ing 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  per-
            form 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  allo-
            cated a block for it, NULL is placed in *info.

RETURN VALUE

     On  success,  dlinfo() returns 0.  On failure, it returns -1; the cause
     of the error can be diagnosed using dlerror(3).

VERSIONS

     dlinfo() first appeared in glibc 2.3.3.

ATTRIBUTES

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

     +----------+---------------+---------+
     |Interface | Attribute     | Value   |
     +----------+---------------+---------+
     |dlinfo()  | Thread safety | MT-Safe |
     +----------+---------------+---------+

CONFORMING TO

     This function is a nonstandard GNU extension.

NOTES

     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.

EXAMPLE

     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;
         int j;

         if (argc != 2) {
             fprintf(stderr, "Usage: %s <libpath>\n", argv[0]);
             exit(EXIT_FAILURE);
         }

         /* Obtain a handle for shared objects 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 (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); }