man3/dl_iterate_phdr.3

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  21 .TH DL_ITERATE_PHDR 3 2007-05-18 "GNU" "Linux Programmer's Manual"
  22 .SH NAME
  23 dl_iterate_phdr \- walk through list of shared objects
  24 .SH SYNOPSIS
  25 .nf
  26 .B #define _GNU_SOURCE
  27 .B #include <link.h>
  28
  29 \fBint dl_iterate_phdr(\fP
  30           \fBint (*\fPcallback\fB) \
  31 (struct dl_phdr_info *\fPinfo\fB,\fP
  32                            \fBsize_t\fP size\fB, void *\fPdata\fB),\fP
  33           \fBvoid *\fPdata\fB);\fP
  34 .fi
  35 .SH DESCRIPTION
  36 The
  37 .BR dl_iterate_phdr ()
  38 function allows an application to inquire at run-time to find
  39 out which shared objects it has loaded.
  40
  41 The
  42 .BR dl_iterate_phdr ()
  43 function walks through the list of an
  44 application's shared objects and calls the function
  45 .I callback
  46 once for each object,
  47 until either all shared objects have been processed or
  48 .I callback
  49 returns a non-zero value.
  50
  51 Each call to
  52 .I callback
  53 receives three arguments:
  54 .IR info ,
  55 which is a pointer to a structure containing information
  56 about the shared object;
  57 .IR size ,
  58 which is the size of the structure pointed to by
  59 .IR info ;
  60 and
  61 .IR data ,
  62 which is a copy of whatever value was passed by the calling
  63 program as the second argument (also named
  64 .IR data )
  65 in the call to
  66 .BR dl_iterate_phdr ().
  67
  68 The
  69 .I info
  70 argument is a structure of the following type:
  71
  72 .in +4n
  73 .nf
  74 struct dl_phdr_info {
  75     ElfW(Addr)        dlpi_addr;  /* Base address of object */
  76     const char       *dlpi_name;  /* (Null-terminated) name of
  77                                      object */
  78     const ElfW(Phdr) *dlpi_phdr;  /* Pointer to array of
  79                                      ELF program headers
  80                                      for this object */
  81     ElfW(Half)        dlpi_phnum; /* # of items in 'dlpi_phdr' */
  82 };
  83 .fi
  84 .in
  85
  86 (The
  87 .IR ElfW ()
  88 macro definition turns its argument into the name of an ELF data
  89 type suitable for the hardware architecture.
  90 For example, on a 32-bit platform,
  91 ElfW(Addr) yields the data type name Elf32_Addr.
  92 Further information on these types can be found in the
  93 .IR <elf.h> " and " <link.h>
  94 header files.)
  95
  96 The
  97 .I dlpi_addr
  98 field indicates the base address of the shared object
  99 (i.e., the difference between the virtual memory address of
 100 the shared object and the offset of that object in the file
 101 from which it was loaded).
 102 The
 103 .I dlpi_name
 104 field is a null-terminated string giving the pathname
 105 from which the shared object was loaded.
 106
 107 To understand the meaning of the
 108 .I dlpi_phdr
 109 and
 110 .I dlpi_phnum
 111 fields, we need to be aware that an ELF shared object consists
 112 of a number of segments, each of which has a corresponding
 113 program header describing the segment.
 114 The
 115 .I dlpi_phdr
 116 field is a pointer to an array of the program headers for this
 117 shared object.
 118 The
 119 .I dlpi_phnum
 120 field indicates the size of this array.
 121
 122 These program headers are structures of the following form:
 123 .in +4n
 124 .nf
 125
 126 typedef struct {
 127     Elf32_Word  p_type;    /* Segment type */
 128     Elf32_Off   p_offset;  /* Segment file offset */
 129     Elf32_Addr  p_vaddr;   /* Segment virtual address */
 130     Elf32_Addr  p_paddr;   /* Segment physical address */
 131     Elf32_Word  p_filesz;  /* Segment size in file */
 132     Elf32_Word  p_memsz;   /* Segment size in memory */
 133     Elf32_Word  p_flags;   /* Segment flags */
 134     Elf32_Word  p_align;   /* Segment alignment */
 135 } Elf32_Phdr;
 136 .fi
 137 .in
 138
 139 Note that we can calculate the location of a particular program header,
 140 .IR x ,
 141 in virtual memory using the formula:
 142
 143 .nf
 144   addr == info\->dlpi_addr + info\->dlpi_phdr[x].p_vaddr;
 145 .fi
 146 .SH RETURN VALUE
 147 The
 148 .BR dl_iterate_phdr ()
 149 function returns whatever value was returned by the last call to
 150 .IR callback .
 151 .SH VERSIONS
 152 .BR dl_iterate_phdr ()
 153 has been supported in glibc since version 2.2.4.
 154 .SH "CONFORMING TO"
 155 The
 156 .BR dl_iterate_phdr ()
 157 function is Linux-specific and should be avoided in portable applications.
 158 .SH EXAMPLE
 159 The following program displays a list of pathnames of the
 160 shared objects it has loaded.
 161 For each shared object, the program lists the virtual addresses
 162 at which the object's ELF segments are loaded.
 163
 164 .nf
 165 #define _GNU_SOURCE
 166 #include <link.h>
 167 #include <stdlib.h>
 168 #include <stdio.h>
 169
 170 static int
 171 callback(struct dl_phdr_info *info, size_t size, void *data)
 172 {
 173     int j;
 174
 175     printf("name=%s (%d segments)\\n", info\->dlpi_name,
 176         info\->dlpi_phnum);
 177
 178     for (j = 0; j < info\->dlpi_phnum; j++)
 179          printf("\\t\\t header %2d: address=%10p\\n", j,
 180              (void *) (info\->dlpi_addr + info\->dlpi_phdr[j].p_vaddr));
 181     return 0;
 182 }
 183
 184 int
 185 main(int argc, char *argv[])
 186 {
 187     dl_iterate_phdr(callback, NULL);
 188
 189     exit(EXIT_SUCCESS);
 190 }
 191 .fi
 192 .SH "SEE ALSO"
 193 .BR ldd (1),
 194 .BR objdump (1),
 195 .BR readelf (1),
 196 .BR dlopen (3),
 197 .BR feature_test_macros (7),
 198 .BR ld.so (8),
 199 and the
 200 .I "Executable and Linking Format Specification"
 201 available at various locations online.