1dl_iterate_phdr(3) Library Functions Manual dl_iterate_phdr(3)
2
6 dl_iterate_phdr - walk through list of shared objects
7
9 Standard C library (libc, -lc)
10
12 #define _GNU_SOURCE /* See feature_test_macros(7) */
13 #include <link.h>
14 int dl_iterate_phdr(
16 int (*callback)(struct dl_phdr_info *info,
17 size_t size, void *data),
18 void *data);
19
21 The dl_iterate_phdr() function allows an application to inquire at run
22 time to find out which shared objects it has loaded, and the order in
23 which they were loaded.
24 The dl_iterate_phdr() function walks through the list of an applica‐
26 tion's shared objects and calls the function callback once for each ob‐
27 ject, until either all shared objects have been processed or callback
28 returns a nonzero value.
29 Each call to callback receives three arguments: info, which is a
31 pointer to a structure containing information about the shared object;
32 size, which is the size of the structure pointed to by info; and data,
33 which is a copy of whatever value was passed by the calling program as
34 the second argument (also named data) in the call to dl_iterate_phdr().
35 The info argument is a structure of the following type:
37 struct dl_phdr_info {
39 ElfW(Addr) dlpi_addr; /* Base address of object */
40 const char *dlpi_name; /* (Null-terminated) name of
41 object */
42 const ElfW(Phdr) *dlpi_phdr; /* Pointer to array of
43 ELF program headers
44 for this object */
45 ElfW(Half) dlpi_phnum; /* # of items in dlpi_phdr */
46 /* The following fields were added in glibc 2.4, after the first
48 version of this structure was available. Check the size
49 argument passed to the dl_iterate_phdr callback to determine
50 whether or not each later member is available. */
51 unsigned long long dlpi_adds;
53 /* Incremented when a new object may
54 have been added */
55 unsigned long long dlpi_subs;
56 /* Incremented when an object may
57 have been removed */
58 size_t dlpi_tls_modid;
59 /* If there is a PT_TLS segment, its module
60 ID as used in TLS relocations, else zero */
61 void *dlpi_tls_data;
62 /* The address of the calling thread's instance
63 of this module's PT_TLS segment, if it has
64 one and it has been allocated in the calling
65 thread, otherwise a null pointer */
66 };
67 (The ElfW() macro definition turns its argument into the name of an ELF
69 data type suitable for the hardware architecture. For example, on a
70 32-bit platform, ElfW(Addr) yields the data type name Elf32_Addr. Fur‐
71 ther information on these types can be found in the <elf.h> and
72 <link.h> header files.)
73 The dlpi_addr field indicates the base address of the shared object
75 (i.e., the difference between the virtual memory address of the shared
76 object and the offset of that object in the file from which it was
77 loaded). The dlpi_name field is a null-terminated string giving the
78 pathname from which the shared object was loaded.
79 To understand the meaning of the dlpi_phdr and dlpi_phnum fields, we
81 need to be aware that an ELF shared object consists of a number of seg‐
82 ments, each of which has a corresponding program header describing the
83 segment. The dlpi_phdr field is a pointer to an array of the program
84 headers for this shared object. The dlpi_phnum field indicates the
85 size of this array.
86 These program headers are structures of the following form:
88 typedef struct {
90 Elf32_Word p_type; /* Segment type */
91 Elf32_Off p_offset; /* Segment file offset */
92 Elf32_Addr p_vaddr; /* Segment virtual address */
93 Elf32_Addr p_paddr; /* Segment physical address */
94 Elf32_Word p_filesz; /* Segment size in file */
95 Elf32_Word p_memsz; /* Segment size in memory */
96 Elf32_Word p_flags; /* Segment flags */
97 Elf32_Word p_align; /* Segment alignment */
98 } Elf32_Phdr;
99 Note that we can calculate the location of a particular program header,
101 x, in virtual memory using the formula:
102 addr == info->dlpi_addr + info->dlpi_phdr[x].p_vaddr;
104 Possible values for p_type include the following (see <elf.h> for fur‐
106 ther details):
107 #define PT_LOAD 1 /* Loadable program segment */
109 #define PT_DYNAMIC 2 /* Dynamic linking information */
110 #define PT_INTERP 3 /* Program interpreter */
111 #define PT_NOTE 4 /* Auxiliary information */
112 #define PT_SHLIB 5 /* Reserved */
113 #define PT_PHDR 6 /* Entry for header table itself */
114 #define PT_TLS 7 /* Thread-local storage segment */
115 #define PT_GNU_EH_FRAME 0x6474e550 /* GCC .eh_frame_hdr segment */
116 #define PT_GNU_STACK 0x6474e551 /* Indicates stack executability */
117 #define PT_GNU_RELRO 0x6474e552 /* Read-only after relocation */
118
120 The dl_iterate_phdr() function returns whatever value was returned by
121 the last call to callback.
122
124 For an explanation of the terms used in this section, see at‐
125 tributes(7).
126 ┌────────────────────────────────────────────┬───────────────┬─────────┐
128 │Interface │ Attribute │ Value │
129 ├────────────────────────────────────────────┼───────────────┼─────────┤
130 │dl_iterate_phdr() │ Thread safety │ MT-Safe │
131 └────────────────────────────────────────────┴───────────────┴─────────┘
132
134 Various other systems provide a version of this function, although de‐
135 tails of the returned dl_phdr_info structure differ. On the BSDs and
136 Solaris, the structure includes the fields dlpi_addr, dlpi_name,
137 dlpi_phdr, and dlpi_phnum in addition to other implementation-specific
138 fields.
139 Future versions of the C library may add further fields to the dl_ph‐
141 dr_info structure; in that event, the size argument provides a mecha‐
142 nism for the callback function to discover whether it is running on a
143 system with added fields.
144
146 None.
147
149 glibc 2.2.4.
150
152 The first object visited by callback is the main program. For the main
153 program, the dlpi_name field will be an empty string.
154
156 The following program displays a list of pathnames of the shared ob‐
157 jects it has loaded. For each shared object, the program lists some
158 information (virtual address, size, flags, and type) for each of the
159 objects ELF segments.
160 The following shell session demonstrates the output produced by the
162 program on an x86-64 system. The first shared object for which output
163 is displayed (where the name is an empty string) is the main program.
164 $ ./a.out
166 Name: "" (9 segments)
167 0: [ 0x400040; memsz: 1f8] flags: 0x5; PT_PHDR
168 1: [ 0x400238; memsz: 1c] flags: 0x4; PT_INTERP
169 2: [ 0x400000; memsz: ac4] flags: 0x5; PT_LOAD
170 3: [ 0x600e10; memsz: 240] flags: 0x6; PT_LOAD
171 4: [ 0x600e28; memsz: 1d0] flags: 0x6; PT_DYNAMIC
172 5: [ 0x400254; memsz: 44] flags: 0x4; PT_NOTE
173 6: [ 0x400970; memsz: 3c] flags: 0x4; PT_GNU_EH_FRAME
174 7: [ (nil); memsz: 0] flags: 0x6; PT_GNU_STACK
175 8: [ 0x600e10; memsz: 1f0] flags: 0x4; PT_GNU_RELRO
176 Name: "linux-vdso.so.1" (4 segments)
177 0: [0x7ffc6edd1000; memsz: e89] flags: 0x5; PT_LOAD
178 1: [0x7ffc6edd1360; memsz: 110] flags: 0x4; PT_DYNAMIC
179 2: [0x7ffc6edd17b0; memsz: 3c] flags: 0x4; PT_NOTE
180 3: [0x7ffc6edd17ec; memsz: 3c] flags: 0x4; PT_GNU_EH_FRAME
181 Name: "/lib64/libc.so.6" (10 segments)
182 0: [0x7f55712ce040; memsz: 230] flags: 0x5; PT_PHDR
183 1: [0x7f557145b980; memsz: 1c] flags: 0x4; PT_INTERP
184 2: [0x7f55712ce000; memsz: 1b6a5c] flags: 0x5; PT_LOAD
185 3: [0x7f55716857a0; memsz: 9240] flags: 0x6; PT_LOAD
186 4: [0x7f5571688b80; memsz: 1f0] flags: 0x6; PT_DYNAMIC
187 5: [0x7f55712ce270; memsz: 44] flags: 0x4; PT_NOTE
188 6: [0x7f55716857a0; memsz: 78] flags: 0x4; PT_TLS
189 7: [0x7f557145b99c; memsz: 544c] flags: 0x4; PT_GNU_EH_FRAME
190 8: [0x7f55712ce000; memsz: 0] flags: 0x6; PT_GNU_STACK
191 9: [0x7f55716857a0; memsz: 3860] flags: 0x4; PT_GNU_RELRO
192 Name: "/lib64/ld-linux-x86-64.so.2" (7 segments)
193 0: [0x7f557168f000; memsz: 20828] flags: 0x5; PT_LOAD
194 1: [0x7f55718afba0; memsz: 15a8] flags: 0x6; PT_LOAD
195 2: [0x7f55718afe10; memsz: 190] flags: 0x6; PT_DYNAMIC
196 3: [0x7f557168f1c8; memsz: 24] flags: 0x4; PT_NOTE
197 4: [0x7f55716acec4; memsz: 604] flags: 0x4; PT_GNU_EH_FRAME
198 5: [0x7f557168f000; memsz: 0] flags: 0x6; PT_GNU_STACK
199 6: [0x7f55718afba0; memsz: 460] flags: 0x4; PT_GNU_RELRO
200 Program source
202 #define _GNU_SOURCE
204 #include <link.h>
205 #include <stdint.h>
206 #include <stdio.h>
207 #include <stdlib.h>
208 static int
210 callback(struct dl_phdr_info *info, size_t size, void *data)
211 {
212 char *type;
213 int p_type;
214 printf("Name: \"%s\" (%d segments)\n", info->dlpi_name,
216 info->dlpi_phnum);
217 for (size_t j = 0; j < info->dlpi_phnum; j++) {
219 p_type = info->dlpi_phdr[j].p_type;
220 type = (p_type == PT_LOAD) ? "PT_LOAD" :
221 (p_type == PT_DYNAMIC) ? "PT_DYNAMIC" :
222 (p_type == PT_INTERP) ? "PT_INTERP" :
223 (p_type == PT_NOTE) ? "PT_NOTE" :
224 (p_type == PT_INTERP) ? "PT_INTERP" :
225 (p_type == PT_PHDR) ? "PT_PHDR" :
226 (p_type == PT_TLS) ? "PT_TLS" :
227 (p_type == PT_GNU_EH_FRAME) ? "PT_GNU_EH_FRAME" :
228 (p_type == PT_GNU_STACK) ? "PT_GNU_STACK" :
229 (p_type == PT_GNU_RELRO) ? "PT_GNU_RELRO" : NULL;
230 printf(" %2zu: [%14p; memsz:%7jx] flags: %#jx; ", j,
232 (void *) (info->dlpi_addr + info->dlpi_phdr[j].p_vaddr),
233 (uintmax_t) info->dlpi_phdr[j].p_memsz,
234 (uintmax_t) info->dlpi_phdr[j].p_flags);
235 if (type != NULL)
236 printf("%s\n", type);
237 else
238 printf("[other (%#x)]\n", p_type);
239 }
240 return 0;
242 }
243 int
245 main(void)
246 {
247 dl_iterate_phdr(callback, NULL);
248 exit(EXIT_SUCCESS);
250 }
251
253 ldd(1), objdump(1), readelf(1), dladdr(3), dlopen(3), elf(5), ld.so(8)
254 Executable and Linking Format Specification, available at various loca‐
256 tions online.
257Linux man-pages 6.04 2023-03-30 dl_iterate_phdr(3)