man3/dl_iterate_phdr.3

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  25 .TH DL_ITERATE_PHDR 3 2017-09-15 "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 .PP
  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 and the order in which they were loaded.
  44 .PP
  45 The
  46 .BR dl_iterate_phdr ()
  47 function walks through the list of an
  48 application's shared objects and calls the function
  49 .I callback
  50 once for each object,
  51 until either all shared objects have been processed or
  52 .I callback
  53 returns a nonzero value.
  54 .PP
  55 Each call to
  56 .I callback
  57 receives three arguments:
  58 .IR info ,
  59 which is a pointer to a structure containing information
  60 about the shared object;
  61 .IR size ,
  62 which is the size of the structure pointed to by
  63 .IR info ;
  64 and
  65 .IR data ,
  66 which is a copy of whatever value was passed by the calling
  67 program as the second argument (also named
  68 .IR data )
  69 in the call to
  70 .BR dl_iterate_phdr ().
  71 .PP
  72 The
  73 .I info
  74 argument is a structure of the following type:
  75 .PP
  76 .in +4n
  77 .EX
  78 struct dl_phdr_info {
  79     ElfW(Addr)        dlpi_addr;  /* Base address of object */
  80     const char       *dlpi_name;  /* (Null-terminated) name of
  81                                      object */
  82     const ElfW(Phdr) *dlpi_phdr;  /* Pointer to array of
  83                                      ELF program headers
  84                                      for this object */
  85     ElfW(Half)        dlpi_phnum; /* # of items in \fIdlpi_phdr\fP */
  86
  87     /* The following fields were added in glibc 2.4, after the first
  88        version of this structure was available.  Check the \fIsize\fP
  89        argument passed to the dl_iterate_phdr callback to determine
  90        whether or not each later member is available.  */
  91
  92     unsigned long long int dlpi_adds;
  93                     /* Incremented when a new object may
  94                        have been added */
  95     unsigned long long int dlpi_subs;
  96                     /* Incremented when an object may
  97                        have been removed */
  98     size_t dlpi_tls_modid;
  99                     /* If there is a PT_TLS segment, its module
 100                        ID as used in TLS relocations, else zero */
 101     void  *dlpi_tls_data;
 102                     /* The address of the calling thread's instance
 103                        of this module's PT_TLS segment, if it has
 104                        one and it has been allocated in the calling
 105                        thread, otherwise a null pointer */
 106 };
 107 .EE
 108 .in
 109 .PP
 110 (The
 111 .IR ElfW ()
 112 macro definition turns its argument into the name of an ELF data
 113 type suitable for the hardware architecture.
 114 For example, on a 32-bit platform,
 115 .I ElfW(Addr)
 116 yields the data type name
 117 .IR Elf32_Addr .
 118 Further information on these types can be found in the
 119 .IR <elf.h> " and " <link.h>
 120 header files.)
 121 .PP
 122 The
 123 .I dlpi_addr
 124 field indicates the base address of the shared object
 125 (i.e., the difference between the virtual memory address of
 126 the shared object and the offset of that object in the file
 127 from which it was loaded).
 128 The
 129 .I dlpi_name
 130 field is a null-terminated string giving the pathname
 131 from which the shared object was loaded.
 132 .PP
 133 To understand the meaning of the
 134 .I dlpi_phdr
 135 and
 136 .I dlpi_phnum
 137 fields, we need to be aware that an ELF shared object consists
 138 of a number of segments, each of which has a corresponding
 139 program header describing the segment.
 140 The
 141 .I dlpi_phdr
 142 field is a pointer to an array of the program headers for this
 143 shared object.
 144 The
 145 .I dlpi_phnum
 146 field indicates the size of this array.
 147 .PP
 148 These program headers are structures of the following form:
 149 .PP
 150 .in +4n
 151 .EX
 152 typedef struct {
 153     Elf32_Word  p_type;    /* Segment type */
 154     Elf32_Off   p_offset;  /* Segment file offset */
 155     Elf32_Addr  p_vaddr;   /* Segment virtual address */
 156     Elf32_Addr  p_paddr;   /* Segment physical address */
 157     Elf32_Word  p_filesz;  /* Segment size in file */
 158     Elf32_Word  p_memsz;   /* Segment size in memory */
 159     Elf32_Word  p_flags;   /* Segment flags */
 160     Elf32_Word  p_align;   /* Segment alignment */
 161 } Elf32_Phdr;
 162 .EE
 163 .in
 164 .PP
 165 Note that we can calculate the location of a particular program header,
 166 .IR x ,
 167 in virtual memory using the formula:
 168 .PP
 169 .in +4n
 170 .EX
 171 addr == info\->dlpi_addr + info\->dlpi_phdr[x].p_vaddr;
 172 .EE
 173 .in
 174 .PP
 175 Possible values for
 176 .I p_type
 177 include the following (see
 178 .IR <elf.h>
 179 for further details):
 180 .PP
 181 .in +4n
 182 .EX
 183 #define PT_LOAD         1    /* Loadable program segment */
 184 #define PT_DYNAMIC      2    /* Dynamic linking information */
 185 #define PT_INTERP       3    /* Program interpreter */
 186 #define PT_NOTE         4    /* Auxiliary information */
 187 #define PT_SHLIB        5    /* Reserved */
 188 #define PT_PHDR         6    /* Entry for header table itself */
 189 #define PT_TLS          7    /* Thread-local storage segment */
 190 #define PT_GNU_EH_FRAME 0x6474e550 /* GCC .eh_frame_hdr segment */
 191 #define PT_GNU_STACK  0x6474e551 /* Indicates stack executability */
 192 .\" For PT_GNU_STACK, see http://www.airs.com/blog/archives/518
 193 #define PT_GNU_RELRO  0x6474e552 /* Read-only after relocation */
 194 .EE
 195 .in
 196 .SH RETURN VALUE
 197 The
 198 .BR dl_iterate_phdr ()
 199 function returns whatever value was returned by the last call to
 200 .IR callback .
 201 .SH VERSIONS
 202 .BR dl_iterate_phdr ()
 203 has been supported in glibc since version 2.2.4.
 204 .SH ATTRIBUTES
 205 For an explanation of the terms used in this section, see
 206 .BR attributes (7).
 207 .TS
 208 allbox;
 209 lb lb lb
 210 l l l.
 211 Interface       Attribute       Value
 212 T{
 213 .BR dl_iterate_phdr ()
 214 T}      Thread safety   MT-Safe
 215 .TE
 216 .sp 1
 217 .SH CONFORMING TO
 218 The
 219 .BR dl_iterate_phdr ()
 220 function is not specified in any standard.
 221 Various other systems provide a version of this function,
 222 although details of the returned
 223 .I dl_phdr_info
 224 structure differ.
 225 On the BSDs and Solaris, the structure includes the fields
 226 .IR dlpi_addr ,
 227 .IR dlpi_name ,
 228 .IR dlpi_phdr ,
 229 and
 230 .IR dlpi_phnum
 231 in addition to other implementation-specific fields.
 232 .SH NOTES
 233 Future versions of the C library may add further fields to the
 234 .IR dl_phdr_info
 235 structure; in that event, the
 236 .I size
 237 argument provides a mechanism for the callback function to discover
 238 whether it is running on a system with added fields.
 239 .PP
 240 The first object visited by
 241 .IR callback
 242 is the main program.
 243 For the main program, the
 244 .I dlpi_name
 245 field will be an empty string.
 246 .SH EXAMPLE
 247 The following program displays a list of pathnames of the
 248 shared objects it has loaded.
 249 For each shared object, the program lists some information
 250 (virtual address, size, flags, and type)
 251 for each of the objects ELF segments.
 252 .PP
 253 The following shell session demonstrates the output
 254 produced by the program on an x86-64 system.
 255 The first shared object for which output is displayed
 256 (where the name is an empty string)
 257 is the main program.
 258 .PP
 259 .in +4n
 260 .EX
 261 $ \fB./a.out\fP
 262 Name: "" (9 segments)
 263      0: [      0x400040; memsz:    1f8] flags: 0x5; PT_PHDR
 264      1: [      0x400238; memsz:     1c] flags: 0x4; PT_INTERP
 265      2: [      0x400000; memsz:    ac4] flags: 0x5; PT_LOAD
 266      3: [      0x600e10; memsz:    240] flags: 0x6; PT_LOAD
 267      4: [      0x600e28; memsz:    1d0] flags: 0x6; PT_DYNAMIC
 268      5: [      0x400254; memsz:     44] flags: 0x4; PT_NOTE
 269      6: [      0x400970; memsz:     3c] flags: 0x4; PT_GNU_EH_FRAME
 270      7: [         (nil); memsz:      0] flags: 0x6; PT_GNU_STACK
 271      8: [      0x600e10; memsz:    1f0] flags: 0x4; PT_GNU_RELRO
 272 Name: "linux-vdso.so.1" (4 segments)
 273      0: [0x7ffc6edd1000; memsz:    e89] flags: 0x5; PT_LOAD
 274      1: [0x7ffc6edd1360; memsz:    110] flags: 0x4; PT_DYNAMIC
 275      2: [0x7ffc6edd17b0; memsz:     3c] flags: 0x4; PT_NOTE
 276      3: [0x7ffc6edd17ec; memsz:     3c] flags: 0x4; PT_GNU_EH_FRAME
 277 Name: "/lib64/libc.so.6" (10 segments)
 278      0: [0x7f55712ce040; memsz:    230] flags: 0x5; PT_PHDR
 279      1: [0x7f557145b980; memsz:     1c] flags: 0x4; PT_INTERP
 280      2: [0x7f55712ce000; memsz: 1b6a5c] flags: 0x5; PT_LOAD
 281      3: [0x7f55716857a0; memsz:   9240] flags: 0x6; PT_LOAD
 282      4: [0x7f5571688b80; memsz:    1f0] flags: 0x6; PT_DYNAMIC
 283      5: [0x7f55712ce270; memsz:     44] flags: 0x4; PT_NOTE
 284      6: [0x7f55716857a0; memsz:     78] flags: 0x4; PT_TLS
 285      7: [0x7f557145b99c; memsz:   544c] flags: 0x4; PT_GNU_EH_FRAME
 286      8: [0x7f55712ce000; memsz:      0] flags: 0x6; PT_GNU_STACK
 287      9: [0x7f55716857a0; memsz:   3860] flags: 0x4; PT_GNU_RELRO
 288 Name: "/lib64/ld-linux-x86-64.so.2" (7 segments)
 289      0: [0x7f557168f000; memsz:  20828] flags: 0x5; PT_LOAD
 290      1: [0x7f55718afba0; memsz:   15a8] flags: 0x6; PT_LOAD
 291      2: [0x7f55718afe10; memsz:    190] flags: 0x6; PT_DYNAMIC
 292      3: [0x7f557168f1c8; memsz:     24] flags: 0x4; PT_NOTE
 293      4: [0x7f55716acec4; memsz:    604] flags: 0x4; PT_GNU_EH_FRAME
 294      5: [0x7f557168f000; memsz:      0] flags: 0x6; PT_GNU_STACK
 295      6: [0x7f55718afba0; memsz:    460] flags: 0x4; PT_GNU_RELRO
 296 .EE
 297 .in
 298 .PP
 299 .SS Program source
 300 \&
 301 .EX
 302 #define _GNU_SOURCE
 303 #include <link.h>
 304 #include <stdlib.h>
 305 #include <stdio.h>
 306
 307 static int
 308 callback(struct dl_phdr_info *info, size_t size, void *data)
 309 {
 310     char *type;
 311     int p_type, j;
 312
 313     printf("Name: \\"%s\\" (%d segments)\\n", info\->dlpi_name,
 314                info\->dlpi_phnum);
 315
 316     for (j = 0; j < info\->dlpi_phnum; j++) {
 317         p_type = info\->dlpi_phdr[j].p_type;
 318         type =  (p_type == PT_LOAD) ? "PT_LOAD" :
 319                 (p_type == PT_DYNAMIC) ? "PT_DYNAMIC" :
 320                 (p_type == PT_INTERP) ? "PT_INTERP" :
 321                 (p_type == PT_NOTE) ? "PT_NOTE" :
 322                 (p_type == PT_INTERP) ? "PT_INTERP" :
 323                 (p_type == PT_PHDR) ? "PT_PHDR" :
 324                 (p_type == PT_TLS) ? "PT_TLS" :
 325                 (p_type == PT_GNU_EH_FRAME) ? "PT_GNU_EH_FRAME" :
 326                 (p_type == PT_GNU_STACK) ? "PT_GNU_STACK" :
 327                 (p_type == PT_GNU_RELRO) ? "PT_GNU_RELRO" : NULL;
 328
 329         printf("    %2d: [%14p; memsz:%7lx] flags: 0x%x; ", j,
 330                 (void *) (info\->dlpi_addr + info\->dlpi_phdr[j].p_vaddr),
 331                 info\->dlpi_phdr[j].p_memsz,
 332                 info\->dlpi_phdr[j].p_flags);
 333         if (type != NULL)
 334             printf("%s\\n", type);
 335         else
 336             printf("[other (0x%x)]\\n", p_type);
 337     }
 338
 339     return 0;
 340 }
 341
 342 int
 343 main(int argc, char *argv[])
 344 {
 345     dl_iterate_phdr(callback, NULL);
 346
 347     exit(EXIT_SUCCESS);
 348 }
 349 .EE
 350 .SH SEE ALSO
 351 .BR ldd (1),
 352 .BR objdump (1),
 353 .BR readelf (1),
 354 .BR dladdr (3),
 355 .BR dlopen (3),
 356 .BR elf (5),
 357 .BR ld.so (8)
 358 .PP
 359 .IR "Executable and Linking Format Specification" ,
 360 available at various locations online.