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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, and the order in which they were 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 following fields were added in glibc 2.4, after the first
                  version of this structure was available.  Check the size
                  argument passed to the dl_iterate_phdr callback to determine
                  whether or not each later member is available.  */

               unsigned long long dlpi_adds;
                               /* Incremented when a new object may
                                  have been added */
               unsigned long long dlpi_subs;
                               /* Incremented when an object may
                                  have been removed */
               size_t dlpi_tls_modid;
                               /* If there is a PT_TLS segment, its module
                                  ID as used in TLS relocations, else zero */
               void  *dlpi_tls_data;
                               /* The address of the calling thread's instance
                                  of this module's PT_TLS segment, if it has
                                  one and it has been allocated in the calling
                                  thread, otherwise a null pointer */

       (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;

       Possible values for p_type include the following (see <elf.h> for further details):

           #define PT_LOAD         1    /* Loadable program segment */
           #define PT_DYNAMIC      2    /* Dynamic linking information */
           #define PT_INTERP       3    /* Program interpreter */
           #define PT_NOTE         4    /* Auxiliary information */
           #define PT_SHLIB        5    /* Reserved */
           #define PT_PHDR         6    /* Entry for header table itself */
           #define PT_TLS          7    /* Thread-local storage segment */
           #define PT_GNU_EH_FRAME 0x6474e550 /* GCC .eh_frame_hdr segment */
           #define PT_GNU_STACK  0x6474e551 /* Indicates stack executability */
           #define PT_GNU_RELRO  0x6474e552 /* Read-only after relocation */


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


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


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

       │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 fields.


       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 first object visited by callback is the main  program.   For  the  main  program,  the
       dlpi_name field will be an empty string.


       The  following  program  displays a list of pathnames of the shared objects it has loaded.
       For each shared object, the program lists some information (virtual address, size,  flags,
       and type) for each of the objects ELF segments.

       The  following  shell session demonstrates the output produced by the program on an x86-64
       system.  The first shared object for which output is displayed (where the name is an empty
       string) is the main program.

           $ ./a.out
           Name: "" (9 segments)
                0: [      0x400040; memsz:    1f8] flags: 0x5; PT_PHDR
                1: [      0x400238; memsz:     1c] flags: 0x4; PT_INTERP
                2: [      0x400000; memsz:    ac4] flags: 0x5; PT_LOAD
                3: [      0x600e10; memsz:    240] flags: 0x6; PT_LOAD
                4: [      0x600e28; memsz:    1d0] flags: 0x6; PT_DYNAMIC
                5: [      0x400254; memsz:     44] flags: 0x4; PT_NOTE
                6: [      0x400970; memsz:     3c] flags: 0x4; PT_GNU_EH_FRAME
                7: [         (nil); memsz:      0] flags: 0x6; PT_GNU_STACK
                8: [      0x600e10; memsz:    1f0] flags: 0x4; PT_GNU_RELRO
           Name: "" (4 segments)
                0: [0x7ffc6edd1000; memsz:    e89] flags: 0x5; PT_LOAD
                1: [0x7ffc6edd1360; memsz:    110] flags: 0x4; PT_DYNAMIC
                2: [0x7ffc6edd17b0; memsz:     3c] flags: 0x4; PT_NOTE
                3: [0x7ffc6edd17ec; memsz:     3c] flags: 0x4; PT_GNU_EH_FRAME
           Name: "/lib64/" (10 segments)
                0: [0x7f55712ce040; memsz:    230] flags: 0x5; PT_PHDR
                1: [0x7f557145b980; memsz:     1c] flags: 0x4; PT_INTERP
                2: [0x7f55712ce000; memsz: 1b6a5c] flags: 0x5; PT_LOAD
                3: [0x7f55716857a0; memsz:   9240] flags: 0x6; PT_LOAD
                4: [0x7f5571688b80; memsz:    1f0] flags: 0x6; PT_DYNAMIC
                5: [0x7f55712ce270; memsz:     44] flags: 0x4; PT_NOTE
                6: [0x7f55716857a0; memsz:     78] flags: 0x4; PT_TLS
                7: [0x7f557145b99c; memsz:   544c] flags: 0x4; PT_GNU_EH_FRAME
                8: [0x7f55712ce000; memsz:      0] flags: 0x6; PT_GNU_STACK
                9: [0x7f55716857a0; memsz:   3860] flags: 0x4; PT_GNU_RELRO
           Name: "/lib64/" (7 segments)
                0: [0x7f557168f000; memsz:  20828] flags: 0x5; PT_LOAD
                1: [0x7f55718afba0; memsz:   15a8] flags: 0x6; PT_LOAD
                2: [0x7f55718afe10; memsz:    190] flags: 0x6; PT_DYNAMIC
                3: [0x7f557168f1c8; memsz:     24] flags: 0x4; PT_NOTE
                4: [0x7f55716acec4; memsz:    604] flags: 0x4; PT_GNU_EH_FRAME
                5: [0x7f557168f000; memsz:      0] flags: 0x6; PT_GNU_STACK
                6: [0x7f55718afba0; memsz:    460] flags: 0x4; PT_GNU_RELRO

   Program source

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

       static int
       callback(struct dl_phdr_info *info, size_t size, void *data)
           char *type;
           int p_type;

           printf("Name: \"%s\" (%d segments)\n", info->dlpi_name,

           for (int j = 0; j < info->dlpi_phnum; j++) {
               p_type = info->dlpi_phdr[j].p_type;
               type =  (p_type == PT_LOAD) ? "PT_LOAD" :
                       (p_type == PT_DYNAMIC) ? "PT_DYNAMIC" :
                       (p_type == PT_INTERP) ? "PT_INTERP" :
                       (p_type == PT_NOTE) ? "PT_NOTE" :
                       (p_type == PT_INTERP) ? "PT_INTERP" :
                       (p_type == PT_PHDR) ? "PT_PHDR" :
                       (p_type == PT_TLS) ? "PT_TLS" :
                       (p_type == PT_GNU_EH_FRAME) ? "PT_GNU_EH_FRAME" :
                       (p_type == PT_GNU_STACK) ? "PT_GNU_STACK" :
                       (p_type == PT_GNU_RELRO) ? "PT_GNU_RELRO" : NULL;

               printf("    %2d: [%14p; memsz:%7jx] flags: %#jx; ", j,
                       (void *) (info->dlpi_addr + info->dlpi_phdr[j].p_vaddr),
                       (uintmax_t) info->dlpi_phdr[j].p_memsz,
                       (uintmax_t) info->dlpi_phdr[j].p_flags);
               if (type != NULL)
                   printf("%s\n", type);
                   printf("[other (%#x)]\n", p_type);

           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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