plucky (3) dl_iterate_phdr.3.gz

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NAME

       dl_iterate_phdr - walk through list of shared objects

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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);

DESCRIPTION

       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 */

RETURN VALUE

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

ATTRIBUTES

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

       ┌──────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├──────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │dl_iterate_phdr()                                                             │ Thread safety │ MT-Safe │
       └──────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

VERSIONS

       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.

STANDARDS

       None.

HISTORY

       glibc 2.2.4.

NOTES

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

EXAMPLES

       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: "linux-vdso.so.1" (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/libc.so.6" (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/ld-linux-x86-64.so.2" (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 <stdint.h>
       #include <stdio.h>
       #include <stdlib.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,
                  info->dlpi_phnum);

           for (size_t 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("    %2zu: [%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);
               else
                   printf("[other (%#x)]\n", p_type);
           }

           return 0;
       }

       int
       main(void)
       {
           dl_iterate_phdr(callback, NULL);

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       ldd(1), objdump(1), readelf(1), dladdr(3), dlopen(3), elf(5), ld.so(8)

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