man3/dl_iterate_phdr.3

   1 .\" Copyright (c) 2003 by Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\"
   3 .\" %%%LICENSE_START(VERBATIM)
   4 .\" Permission is granted to make and distribute verbatim copies of this
   5 .\" manual provided the copyright notice and this permission notice are
   6 .\" preserved on all copies.
   7 .\"
   8 .\" Permission is granted to copy and distribute modified versions of this
   9 .\" manual under the conditions for verbatim copying, provided that the
  10 .\" entire resulting derived work is distributed under the terms of a
  11 .\" permission notice identical to this one
  12 .\"
  13 .\" Since the Linux kernel and libraries are constantly changing, this
  14 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  15 .\" responsibility for errors or omissions, or for damages resulting from
  16 .\" the use of the information contained herein.  The author(s) may not
  17 .\" have taken the same level of care in the production of this manual,
  18 .\" which is licensed free of charge, as they might when working
  19 .\" professionally.
  20 .\"
  21 .\" Formatted or processed versions of this manual, if unaccompanied by
  22 .\" the source, must acknowledge the copyright and authors of this work.
  23 .\" %%%LICENSE_END
  24 .\"
  25 .TH DL_ITERATE_PHDR 3 2015-07-23 "GNU" "Linux Programmer's Manual"
  26 .SH NAME
  27 dl_iterate_phdr \- walk through list of shared objects
  28 .SH SYNOPSIS
  29 .nf
  30 .BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
  31 .B #include <link.h>
  32
  33 .BI "int dl_iterate_phdr("
  34 .BI "          int (*" callback ") (struct dl_phdr_info *" info ,
  35 .BI "                           size_t " size ", void *" data "),"
  36 .BI "          void *" data ");"
  37 .fi
  38 .SH DESCRIPTION
  39 The
  40 .BR dl_iterate_phdr ()
  41 function allows an application to inquire at run time to find
  42 out which shared objects it has loaded.
  43
  44 The
  45 .BR dl_iterate_phdr ()
  46 function walks through the list of an
  47 application's shared objects and calls the function
  48 .I callback
  49 once for each object,
  50 until either all shared objects have been processed or
  51 .I callback
  52 returns a nonzero value.
  53
  54 Each call to
  55 .I callback
  56 receives three arguments:
  57 .IR info ,
  58 which is a pointer to a structure containing information
  59 about the shared object;
  60 .IR size ,
  61 which is the size of the structure pointed to by
  62 .IR info ;
  63 and
  64 .IR data ,
  65 which is a copy of whatever value was passed by the calling
  66 program as the second argument (also named
  67 .IR data )
  68 in the call to
  69 .BR dl_iterate_phdr ().
  70
  71 The
  72 .I info
  73 argument is a structure of the following type:
  74
  75 .in +4n
  76 .nf
  77 struct dl_phdr_info {
  78     ElfW(Addr)        dlpi_addr;  /* Base address of object */
  79     const char       *dlpi_name;  /* (Null-terminated) name of
  80                                      object */
  81     const ElfW(Phdr) *dlpi_phdr;  /* Pointer to array of
  82                                      ELF program headers
  83                                      for this object */
  84     ElfW(Half)        dlpi_phnum; /* # of items in \fIdlpi_phdr\fP */
  85 };
  86 .fi
  87 .in
  88
  89 (The
  90 .IR ElfW ()
  91 macro definition turns its argument into the name of an ELF data
  92 type suitable for the hardware architecture.
  93 For example, on a 32-bit platform,
  94 .I ElfW(Addr)
  95 yields the data type name
  96 .IR Elf32_Addr .
  97 Further information on these types can be found in the
  98 .IR <elf.h> " and " <link.h>
  99 header files.)
 100
 101 The
 102 .I dlpi_addr
 103 field indicates the base address of the shared object
 104 (i.e., the difference between the virtual memory address of
 105 the shared object and the offset of that object in the file
 106 from which it was loaded).
 107 The
 108 .I dlpi_name
 109 field is a null-terminated string giving the pathname
 110 from which the shared object was loaded.
 111
 112 To understand the meaning of the
 113 .I dlpi_phdr
 114 and
 115 .I dlpi_phnum
 116 fields, we need to be aware that an ELF shared object consists
 117 of a number of segments, each of which has a corresponding
 118 program header describing the segment.
 119 The
 120 .I dlpi_phdr
 121 field is a pointer to an array of the program headers for this
 122 shared object.
 123 The
 124 .I dlpi_phnum
 125 field indicates the size of this array.
 126
 127 These program headers are structures of the following form:
 128 .in +4n
 129 .nf
 130
 131 typedef struct {
 132     Elf32_Word  p_type;    /* Segment type */
 133     Elf32_Off   p_offset;  /* Segment file offset */
 134     Elf32_Addr  p_vaddr;   /* Segment virtual address */
 135     Elf32_Addr  p_paddr;   /* Segment physical address */
 136     Elf32_Word  p_filesz;  /* Segment size in file */
 137     Elf32_Word  p_memsz;   /* Segment size in memory */
 138     Elf32_Word  p_flags;   /* Segment flags */
 139     Elf32_Word  p_align;   /* Segment alignment */
 140 } Elf32_Phdr;
 141 .fi
 142 .in
 143
 144 Note that we can calculate the location of a particular program header,
 145 .IR x ,
 146 in virtual memory using the formula:
 147
 148 .nf
 149   addr == info\->dlpi_addr + info\->dlpi_phdr[x].p_vaddr;
 150 .fi
 151 .SH RETURN VALUE
 152 The
 153 .BR dl_iterate_phdr ()
 154 function returns whatever value was returned by the last call to
 155 .IR callback .
 156 .SH VERSIONS
 157 .BR dl_iterate_phdr ()
 158 has been supported in glibc since version 2.2.4.
 159 .SH ATTRIBUTES
 160 For an explanation of the terms used in this section, see
 161 .BR attributes (7).
 162 .TS
 163 allbox;
 164 lb lb lb
 165 l l l.
 166 Interface       Attribute       Value
 167 T{
 168 .BR dl_iterate_phdr ()
 169 T}      Thread safety   MT-Safe
 170 .TE
 171
 172 .SH CONFORMING TO
 173 The
 174 .BR dl_iterate_phdr ()
 175 function is not specified in any standard.
 176 Various other systems provide a version of this function,
 177 although details of the returned
 178 .I dl_phdr_info
 179 structure differ.
 180 On the BSDs and Solaris, the structure includes the fields
 181 .IR dlpi_addr ,
 182 .IR dlpi_name ,
 183 .IR dlpi_phdr ,
 184 and
 185 .IR dlpi_phnum
 186 in addition to other implementation-specific fields.
 187 .SH NOTES
 188 Future versions of the C library may add further fields to the
 189 .IR dl_phdr_info
 190 structure; in that event, the
 191 .I size
 192 argument provides a mechanism for the callback function to discover
 193 whether it is running on a system with added fields.
 194 .SH EXAMPLE
 195 The following program displays a list of pathnames of the
 196 shared objects it has loaded.
 197 For each shared object, the program lists the virtual addresses
 198 at which the object's ELF segments are loaded.
 199
 200 .nf
 201 #define _GNU_SOURCE
 202 #include <link.h>
 203 #include <stdlib.h>
 204 #include <stdio.h>
 205
 206 static int
 207 callback(struct dl_phdr_info *info, size_t size, void *data)
 208 {
 209     int j;
 210
 211     printf("name=%s (%d segments)\\n", info\->dlpi_name,
 212         info\->dlpi_phnum);
 213
 214     for (j = 0; j < info\->dlpi_phnum; j++)
 215          printf("\\t\\t header %2d: address=%10p\\n", j,
 216              (void *) (info\->dlpi_addr + info\->dlpi_phdr[j].p_vaddr));
 217     return 0;
 218 }
 219
 220 int
 221 main(int argc, char *argv[])
 222 {
 223     dl_iterate_phdr(callback, NULL);
 224
 225     exit(EXIT_SUCCESS);
 226 }
 227 .fi
 228 .SH SEE ALSO
 229 .BR ldd (1),
 230 .BR objdump (1),
 231 .BR readelf (1),
 232 .BR dladdr (3),
 233 .BR dlopen (3),
 234 .BR elf (5),
 235 .BR ld.so (8)
 236
 237 .IR "Executable and Linking Format Specification" ,
 238 available at various locations online.