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       dl_iterate_phdr - walk through list of shared objects


       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <link.h>

       int dl_iterate_phdr(
                 int (*callback) (struct dl_phdr_info *info,
                                  size_t size, void *data),
                 void *data);


       The  dl_iterate_phdr() function allows an application to inquire at run
       time to find out which shared objects it has loaded.

       The  dl_iterate_phdr()  function  walks  through   the   list   of   an
       application's  shared  objects and calls the function callback once for
       each object, until either all shared objects  have  been  processed  or
       callback returns a nonzero value.

       Each  call  to  callback  receives  three  arguments:  info, which is a
       pointer to a structure containing information about the shared  object;
       size,  which is the size of the structure pointed to by info; and data,
       which is a copy of whatever value was passed by the calling program  as
       the second argument (also named data) in the call to dl_iterate_phdr().

       The info argument is a structure of the following type:

           struct dl_phdr_info {
               ElfW(Addr)        dlpi_addr;  /* Base address of object */
               const char       *dlpi_name;  /* (Null-terminated) name of
                                                object */
               const ElfW(Phdr) *dlpi_phdr;  /* Pointer to array of
                                                ELF program headers
                                                for this object */
               ElfW(Half)        dlpi_phnum; /* # of items in dlpi_phdr */

       (The ElfW() macro definition turns its argument into the name of an ELF
       data type suitable for the hardware architecture.  For  example,  on  a
       32-bit  platform,  ElfW(Addr)  yields  the  data  type name Elf32_Addr.
       Further information on these types can be  found  in  the  <elf.h>  and
       <link.h> header files.)

       The  dlpi_addr  field  indicates  the base address of the shared object
       (i.e., the difference between the virtual memory address of the  shared
       object  and  the  offset  of  that object in the file from which it was
       loaded).  The dlpi_name field is a null-terminated  string  giving  the
       pathname from which the shared object was loaded.

       To  understand  the  meaning of the dlpi_phdr and dlpi_phnum fields, we
       need to be aware that an ELF shared object  consists  of  a  number  of
       segments,  each  of which has a corresponding program header describing
       the segment.  The dlpi_phdr field is a  pointer  to  an  array  of  the
       program headers for this shared object.  The dlpi_phnum field indicates
       the size of this array.

       These program headers are structures of the following form:

           typedef struct {
               Elf32_Word  p_type;    /* Segment type */
               Elf32_Off   p_offset;  /* Segment file offset */
               Elf32_Addr  p_vaddr;   /* Segment virtual address */
               Elf32_Addr  p_paddr;   /* Segment physical address */
               Elf32_Word  p_filesz;  /* Segment size in file */
               Elf32_Word  p_memsz;   /* Segment size in memory */
               Elf32_Word  p_flags;   /* Segment flags */
               Elf32_Word  p_align;   /* Segment alignment */
           } Elf32_Phdr;

       Note that we can calculate the location of a particular program header,
       x, in virtual memory using the formula:

         addr == info->dlpi_addr + info->dlpi_phdr[x].p_vaddr;


       The  dl_iterate_phdr()  function returns whatever value was returned by
       the last call to callback.


       dl_iterate_phdr() has been supported in glibc since version 2.2.4.


       For  an  explanation  of  the  terms  used   in   this   section,   see

       │InterfaceAttributeValue   │
       │dl_iterate_phdr() │ Thread safety │ MT-Safe │


       The  dl_iterate_phdr()  function  is  not  specified  in  any standard.
       Various other systems provide a  version  of  this  function,  although
       details of the returned dl_phdr_info structure differ.  On the BSDs and
       Solaris,  the  structure  includes  the  fields  dlpi_addr,  dlpi_name,
       dlpi_phdr,  and dlpi_phnum in addition to other implementation-specific


       Future versions of  the  C  library  may  add  further  fields  to  the
       dl_phdr_info  structure;  in  that  event, the size argument provides a
       mechanism for the callback function to discover whether it  is  running
       on a system with added fields.


       The  following  program  displays  a  list  of  pathnames of the shared
       objects it has loaded.  For each shared object, the program  lists  the
       virtual addresses at which the object's ELF segments are loaded.

       #define _GNU_SOURCE
       #include <link.h>
       #include <stdlib.h>
       #include <stdio.h>

       static int
       callback(struct dl_phdr_info *info, size_t size, void *data)
           int j;

           printf("name=%s (%d segments)\n", info->dlpi_name,

           for (j = 0; j < info->dlpi_phnum; j++)
                printf("\t\t header %2d: address=%10p\n", j,
                    (void *) (info->dlpi_addr + info->dlpi_phdr[j].p_vaddr));
           return 0;

       main(int argc, char *argv[])
           dl_iterate_phdr(callback, NULL);



       ldd(1), objdump(1), readelf(1), dladdr(3), dlopen(3), elf(5),

       Executable  and  Linking  Format  Specification,  available  at various
       locations online.


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