dlinfo

- dynamic load information

Synopsis

#include <dlfcn.h>
#include <link.h>
#include <limits.h>
#include <sys/mman.h>

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

Description

The dlinfo() function sets or extracts information from the runtime linker ld.so.1(1). This function is loosely modeled after the ioctl(2) function. The request argument and a third argument of varying type are passed to dlinfo(). The action taken by dlinfo() depends on the value of the request that is provided.

The handle argument is either the value that is returned from a dlopen(3C) or dlmopen() call, or the special handle RTLD_SELF. A handle argument is required for all requests except RTLD_DI_CONFIGADDR, RTLD_DI_GETSIGNAL, and RTLD_DI_SETSIGNAL. If handle is the value that is returned from a dlopen() or dlmopen() call, the information returned by the dlinfo() call pertains to the specified object. If handle is the special handle RTLD_SELF, the information returned by the dlinfo() call pertains to the caller.

The request argument can take the following values:

RTLD_DI_ARGSINFO

Obtain process argument information. The p argument is a pointer (Dl_argsinfo_t *p). The following elements from this structure are initialized:

dla_argc

The number of arguments passed to the process.

dla_argv

The argument array passed to the process.

dla_envp

The active environment variable array that is available to the process. This element initially points to the environment variable array that is made available to exec(2). This element can be updated should an alternative environment be established by the process. See putenv(3C).

dla_auxv

The auxiliary vector array passed to the process.

A process can be established from executing the runtime linker directly from the command line. See ld.so.1(1). The Dl_argsinfo_t information reflects the information that is made available to the application regardless of how the runtime linker has been invoked.

RTLD_DI_CONFIGADDR

Obtain the configuration file information. The p argument is a Dl_info_t pointer (Dl_info_t *p). The following elements from this structure are initialized:

dli_fname

The full name of the configuration file.

dli_fbase

The base address of the configuration file loaded into memory.

RTLD_DI_LINKMAP

Obtain the Link_map for the handle that is specified. The p argument points to a Link_map pointer (Link_map **p). The actual storage for the Link_map structure is maintained by ld.so.1.

The Link_map structure includes the following members:

    unsigned long l_addr;      /* base address */
    char          *l_name;     /* object name */
    Elf32_Dyn     *l_ld;       /* .dynamic section */
    Link_map      *l_next;     /* next link object */
    Link_map      *l_prev;     /* previous link object */
    char          *l_refname;  /* filter reference name */

l_addr

The base address of the object loaded into memory.

l_name

The full name of the loaded object. This full name is the filename of the object as referenced by ld.so.1.

l_ld

Points to the SHT_DYNAMIC structure.

l_next

The next Link_map on the link-map list. Other objects on the same link-map list as the current object can be examined by following the l_next and l_prev members.

l_prev

The previous Link_map on the link-map list.

l_refname

If the object that is referenced is a filter, this member points to the name of the object being filtered. If the object is not a filter, this member is 0. See the Linker and Libraries Guide.

RTLD_DI_LMID

Obtain the ID for the link-map list upon which the handle is loaded. The p argument is a Lmid_t pointer (Lmid_t *p).

RTLD_DI_MMAPCNT

Determine the number of segment mappings for the handle that is specified, for use in a RTLD_DI_MMAPS request. The p argument is a uint_t pointer (uint_t *p). On return from a RTLD_DI_MMAPCNT request, the uint_t value that is pointed to by p contains the number of segment mappings that the associated object uses.

To obtain the complete mapping information for an object, a mmapobj_result_t array for RTLD_DI_MMAPCNT entries must be provided. This array is assigned to the dlm_maps member, and the number of entries available in the array are assigned to the dlm_acnt member. This initialized structure is then passed to a RTLD_DI_MMAPS request. See EXAMPLES.

RTLD_DI_MMAPS

Obtain segment mapping information for the handle that is specified. The p argument is a Dl_mapinfo_t pointer (Dl_mapinfo_t *p). This structure can be initialized from the mapping count obtained from a previous RTLD_DI_MMAPCNT request.

Segment mapping information is provided in an array of mmapobj_result_t structures that originate from the mmapobj(2) of the associated object. The dlm_acnt member, typically initialized from a previous RTLD_DI_MMAPCNT request, indicates the number of entries in a mmapobj_result_t array. This array is assigned to the dlm_maps member. This initialized structure is then passed to a RTLD_DI_MMAPS request, where the segment mapping information is copied to the mmapobj_result_t array. The dlm_rcnt member indicates the number of mmapobj_result_t element entries that are returned. See EXAMPLES.

RTLD_DI_SERINFO

Obtain the library search paths for the handle that is specified. The p argument is a Dl_serinfo_t pointer (Dl_serinfo_t *p). A user must first initialize the Dl_serinfo_t structure with a RTLD_DI_SERINFOSIZE request. See EXAMPLES.

The returned Dl_serinfo_t structure contains dls_cnt Dl_serpath_t entries. Each entry's dlp_name member points to the search path. The corresponding dlp_info member contains one of more flags indicating the origin of the path. See the LA_SER_* flags that are defined in <link.h>.

RTLD_DI_SERINFOSIZE

Initialize a Dl_serinfo_t structure for the handle that is specified, for use in a RTLD_DI_SERINFO request. Both the dls_cnt and dls_size members are returned. The dls_cnt member indicates the number of search paths that are applicable to the handle. The dls_size member indicates the total size of a Dl_serinfo_t buffer required to hold dls_cnt Dl_serpath_t entries and the associated search path strings. The p argument is a Dl_serinfo_t pointer (Dl_serinfo_t *p).

To obtain the complete path information, a new Dl_serinfo_t buffer of size dls_size should be allocated. This new buffer should be initialized with the dls_cnt and dls_size entries. The initialized buffer is then passed to a RTLD_DI_SERINFO request. See EXAMPLES.

RTLD_DI_ORIGIN

Obtain the origin of the dynamic object that is associated with the handle. The p argument is a char pointer (char *p). The dirname(3C) of the associated object's realpath(3C), which can be no larger than {PATH_MAX}, is copied to the pointer p.

RTLD_DI_GETSIGNAL

Obtain the numeric signal number used by the runtime linker to kill the process in the event of a fatal runtime error. The p argument is an int pointer (int *p). The signal number is copied to the pointer p.

By default, the signal used by the runtime linker to terminate a process is SIGKILL. See thr_kill(3C). This default can be changed by calling dlinfo() with RTLD_DI_SETSIGNAL or by setting the environment variable LD_SIGNAL. See ld.so.1(1).

RTLD_DI_SETSIGNAL

Provide a numeric signal number used by the runtime linker to kill the process in the event of a fatal runtime error. The p argument is an int pointer (int *p). The value pointed to by p is established as the terminating signal value.

The current signal number used by the runtime linker to terminate a process can be obtained from dlinfo() using RTLD_DI_GETSIGNAL. Use of the RTLD_DI_SETSIGNAL option is equivalent to setting the environment variable LD_SIGNAL. See ld.so.1(1).

RTLD_DI_DEFERRED

Assign a new dependency name to an existing deferred dependency. The p argument is a Dl_definfo_t pointer (Dl_definfo *p). The dlv_refname field defines an existing dependency name. The dlv_depname field defines the new dependency name.

Dependency names are defined by DT_NEEDED dynamic entries, which can be displayed using the -d option of elfdump(1). Individual dependencies can be tagged as deferred. See the -z deferred option of ld(1). Deferred dependencies are only loaded during process execution, when the first binding to a deferred reference is made. Prior to a deferred dependency being loaded, the dependency name can be changed with RTLD_DI_DEFERRED. See also RTLD_DI_DEFERRED_SYM.

Once a deferred dependency is loaded, any attempt to change the dependency name with dlinfo() results in an error return of -1.

RTLD_DI_DEFERRED_SYM

Assign a new dependency name to an existing deferred symbol, using a symbol reference that exists to the dependency. The p argument is a Dl_definfo_t pointer (Dl_definfo *p). The dlv_refname field defines a symbol reference to the deferred dependency. The dlv_depname field defines the new dependency name.

RTLD_DI_DEFERRED_SYM provides an alternative means of modifying a deferred dependency to using RTLD_DI_DEFERRED. One, or more symbol references can be associated with a deferred dependency. RTLD_DI_DEFERRED_SYM allows one of these deferred symbol references to be used to select the associated deferred dependency. Prior to a deferred dependency being loaded, the dependency name can be changed with RTLD_DI_DEFERRED_SYM. See EXAMPLES.

Once a deferred dependency is loaded, any attempt to change the dependency name with dlinfo() results in an error return of -1.

Return Values

The dlinfo() function returns -1 if the request is invalid, the parameter p is NULL, or the Dl_serinfo_t structure is uninitialized for a RTLD_DI_SERINFO request. dlinfo() also returns -1 if the handle argument does not refer to a valid object opened by dlopen(), or is not the special handle RTLD_SELF. Detailed diagnostic information is available with dlerror(3C).

Examples

Example 1 Use dlinfo() to obtain library search paths.

The following example demonstrates how a dynamic object can inspect the library search paths that would be used to locate a simple filename with dlopen(). For simplicity, error checking has been omitted.

    Dl_serinfo_t   _info, *info = &_info;
    Dl_serpath_t   *path;
    uint_t         cnt;

    /* determine search path count and required buffer size */
    dlinfo(RTLD_SELF, RTLD_DI_SERINFOSIZE, info);

    /* allocate new buffer and initialize */
    info = malloc(_info.dls_size);
    info->dls_size = _info.dls_size;
    info->dls_cnt = _info.dls_cnt;

    /* obtain search path information */
    dlinfo(RTLD_SELF, RTLD_DI_SERINFO, info);

    path = &info->dls_serpath[0];

    for (cnt = 1; cnt <= info->dls_cnt; cnt++, path++) {
        (void) printf("%2d: %s\n", cnt, path->dls_name);
    }

Example 2 Use dlinfo() to obtain segment information.

The following example demonstrates how a dynamic object can inspect its segment mapping information. For simplicity, error checking has been omitted

    Dl_mapinfo_t   mi;
    uint_t         cnt;

    /* determine the number of segment mappings */
    dlinfo(RTLD_SELF, RTLD_DI_MMAPCNT, &mi.dlm_acnt);

    /* allocate the appropriate mapping array */
    mi.dlm_maps = malloc(mi.dlm_acnt *
      sizeof (mmapobj_result_t));

    /* obtain the mapping information */
    dlinfo(RTLD_SELF, RTLD_DI_MMAPS, &mi);

    for (cnt = 0; cnt < mi.dlm_rcnt; cnt++) {
        (void) printf("addr=%x - memory size=%x\n",
          mi.dlm_maps[cnt].mr_addr,
          mi.dlm_maps[cnt].mr_msize);
    }

Example 3 Use dlinfo() to change a deferred dependency.

The following program defines a deferred dependency, foo.so, and an associated deferred symbol reference, foo().

$ elfdump -d main | egrep "NEEDED|POSFLAG
    [0]  POSFLAG_1         0x5             [ LAZY DEFERRED ]
    [1]  NEEDED            0x17e           foo.so
$ elfdump -y main | fgrep foo
    [12]  DBLP             [1] foo.so      foo

The program probes the existence of the symbol foo() to verify that an associated deferred dependency exists. If the dependency does not exist, and hence the symbol can not be found, the program exchanges the deferred dependency associated with the symbol for a new dependency named bar.so. Following this exchange, the program once more probes for the existence of the symbol foo()to verify that the new dependency can be loaded, and the symbol can be found.

    if (dlsym(RTLD_PROBE, "foo") == NULL) {
        Dl_definfo_t    info;

        info.dld_refname = "foo";
        info.dld_depname = "bar.so";

        if (dlinfo(RTLD_SELF, RTLD_DI_DEFERRED_SYM,
          &info) == -1)
            return (1);
        if (dlsym(RTLD_PROBE, "foo") == NULL)
            return (1);
    }
    foo();

A deferred dependency can only be exchanged before the dependency is loaded. If the dependency exists, then any probe would cause the dependency to be loaded, and any following exchange attempts would fail. To successfully exchange a deferred dependency that is expected to exist, a program must not probe for the symbol before making the exchange.

    Dl_definfo_t    info;

    info.dld_refname = "foo";
    info.dld_depname = "bar.so";

    if (dlinfo(RTLD_SELF, RTLD_DI_DEFERRED_SYM, &info) == -1)
        (void) printf("Using original dependency\n");
    else
        (void) printf("Using new dependency: bar.so\n");

Usage

The dlinfo() function is one of a family of functions that give the user direct access to the dynamic linking facilities. These facilities are available to dynamically-linked processes only. See the Linker and Libraries Guide.

Attributes

See attributes(5) for descriptions of the following attributes:

See Also

elfdump(1), ld(1), ld.so.1(1), exec(2), ioctl(2), mmapobj(2), dirname(3C), dlclose(3C), dldump(3C), dlerror(3C), dlopen(3C), dlsym(3C), putenv(3C), realpath(3C), thr_kill(3C), attributes(5).

Linker and Libraries Guide