man3/dl_iterate_phdr.3

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  21 .TH DL_ITERATE_PHDR 3 "Linux 2.4.21" "Linux Programmer's Manual"
  22 .SH NAME
  23 dl_iterate_phdr \- walk through list of shared objects
  24 .SH SYNOPSIS
  25 .nf
  26 #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 .B 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 .B 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 .nf
  73   struct dl_phdr_info {
  74     ElfW(Addr)        dlpi_addr;  /* Base address of object */
  75     const char       *dlpi_name;  /* (Null-terminated) name of
  76                                      object
  77     const ElfW(Phdr) *dlpi_phdr;  /* Pointer to array of
  78                                      ELF program headers
  79                                      for this object */
  80     ElfW(Half)        dlpi_phnum; /* # of items in 'dlpi_phdr' */
  81   };
  82 .fi
  83
  84 (The
  85 .I ElfW()
  86 macro definition turns its argument into the name of an ELF data
  87 type suitable for the hardware architecture.
  88 For example, on a 32-bit platform,
  89 ElfW(Addr) yields the data type name Elf32_Addr.
  90 Further information on these types can be found in the
  91 .IR <elf.h> " and " <link.h>
  92 header files.)
  93
  94 The
  95 .I dlpi_addr
  96 field indicates the base address of the shared object
  97 (i.e., the difference between the virtual memory address of
  98 the shared object and the offset of that object in the file
  99 from which it was loaded).
 100 The
 101 .I dlpi_name
 102 field is a null-terminated string giving the pathname
 103 from which the shared object was loaded.
 104
 105 To understand the meaning of the
 106 .I dlpi_phdr
 107 and
 108 .I dlpi_phnum
 109 fields, we need to be aware that an ELF shared object consists
 110 of a number of segments, each of which has a corresponding
 111 program header describing the segment.
 112 The
 113 .I dlpi_phdr
 114 field is a pointer to an array of the program headers for this
 115 shared object.
 116 The
 117 .I dlpi_phnum
 118 field indicates the size of this array.
 119
 120 These program headers are structures of the following form:
 121 .nf
 122
 123   typedef struct
 124   {
 125     Elf32_Word  p_type;    /* Segment type */
 126     Elf32_Off   p_offset;  /* Segment file offset */
 127     Elf32_Addr  p_vaddr;   /* Segment virtual address */
 128     Elf32_Addr  p_paddr;   /* Segment physical address */
 129     Elf32_Word  p_filesz;  /* Segment size in file */
 130     Elf32_Word  p_memsz;   /* Segment size in memory */
 131     Elf32_Word  p_flags;   /* Segment flags */
 132     Elf32_Word  p_align;   /* Segment alignment */
 133   } Elf32_Phdr;
 134 .fi
 135
 136 Note that we can calculate the location of a particular program header,
 137 .IR x ,
 138 in virtual memory using the formula:
 139
 140 .nf
 141   addr == info->dlpi_addr + info->dlpi_phdr[x].p_vaddr;
 142 .fi
 143 .SH EXAMPLE PROGRAM
 144 The following program displays a list of pathnames of the
 145 shared objects it has loaded.
 146 For each shared object, the program lists the virtual addresses
 147 at which the object's ELF segments are loaded.
 148
 149 .nf
 150 #define _GNU_SOURCE
 151 #include <link.h>
 152 #include <stdlib.h>
 153 #include <stdio.h>
 154
 155 static int
 156 callback(struct dl_phdr_info *info, size_t size, void *data)
 157 {
 158     int j;
 159
 160     printf("name=%s (%d segments)\\n", info->dlpi_name,
 161         info->dlpi_phnum);
 162
 163     for (j = 0; j < info->dlpi_phnum; j++)
 164          printf("\\t\\t header %2d: address=%10p\\n", j,
 165              (void *) (info->dlpi_addr + info->dlpi_phdr[j].p_vaddr));
 166     return 0;
 167 }
 168
 169 int
 170 main(int argc, char *argv[])
 171 {
 172     dl_iterate_phdr(callback, NULL);
 173
 174     exit(EXIT_SUCCESS);
 175 }
 176 .fi
 177 .SH RETURN VALUE
 178 The
 179 .B dl_iterate_phdr
 180 function returns whatever value was returned by the last call to
 181 .IR callback .
 182 .SH "CONFORMING TO"
 183 The
 184 .B dl_iterate_phdr
 185 function is Linux specific and should be avoided in portable applications.
 186 .SH "SEE ALSO"
 187 .BR ldd (1),
 188 .BR objdump (1),
 189 .BR readelf (1),
 190 .BR dlopen (3),
 191 .BR ld.so (8),
 192 and the
 193 .I "Executable and Linking Format Specification"
 194 available at various locations online.