NAME

dladdr, dladdr1 - translate address to symbolic information

LIBRARY

Dynamic linking library (libdl, -ldl)

SYNOPSIS

#define _GNU_SOURCE
#include <dlfcn.h>

int dladdr(const void *addr, Dl_info *info);
int dladdr1(const void *addr, Dl_info *info, void **extra_info,
            int flags);

DESCRIPTION

The function dladdr() determines whether the address specified in addr is located in one of the shared objects loaded by the calling application. If it is, then dladdr() returns information about the shared object and symbol that overlaps addr. This information is returned in a Dl_info structure:

typedef struct {
    const char *dli_fname;  /* Pathname of shared object that
                               contains address */
    void       *dli_fbase;  /* Base address at which shared
                               object is loaded */
    const char *dli_sname;  /* Name of symbol whose definition
                               overlaps addr */
    void       *dli_saddr;  /* Exact address of symbol named
                               in dli_sname */
} Dl_info;

If no symbol matching addr could be found, then dli_sname and dli_saddr are set to NULL.

The function dladdr1() is like dladdr(), but returns additional information via the argument extra_info. The information returned depends on the value specified in flags, which can have one of the following values:

RTLD_DL_LINKMAP: Obtain a pointer to the link map for the matched file. The extra_info argument points to a pointer to a link_map structure (i.e., struct link_map **), defined in <link.h> as:

struct link_map {
    ElfW(Addr) l_addr;  /* Difference between the
                           address in the ELF file and
                           the address in memory */
    char      *l_name;  /* Absolute pathname where
                           object was found */
    ElfW(Dyn) *l_ld;    /* Dynamic section of the
                           shared object */
    struct link_map *l_next, *l_prev;
                        /* Chain of loaded objects */
    /* Plus additional fields private to the
       implementation */
};

RTLD_DL_SYMENT: Obtain a pointer to the ELF symbol table entry of the matching symbol. The extra_info argument is a pointer to a symbol pointer: const ElfW(Sym) **. The ElfW() macro definition turns its argument into the name of an ELF data type suitable for the hardware architecture. For example, on a 64-bit platform, ElfW(Sym) yields the data type name Elf64_Sym, which is defined in <elf.h> as:

typedef struct  {
    Elf64_Word    st_name;     /* Symbol name */
    unsigned char st_info;     /* Symbol type and binding */
    unsigned char st_other;    /* Symbol visibility */
    Elf64_Section st_shndx;    /* Section index */
    Elf64_Addr    st_value;    /* Symbol value */
    Elf64_Xword   st_size;     /* Symbol size */
} Elf64_Sym;

The st_name field is an index into the string table.

The st_info field encodes the symbol's type and binding. The type can be extracted using the macro ELF64_ST_TYPE(st_info) (or ELF32_ST_TYPE() on 32-bit platforms), which yields one of the following values:

Value	Description
STT_NOTYPE	Symbol type is unspecified
STT_OBJECT	Symbol is a data object
STT_FUNC	Symbol is a code object
STT_SECTION	Symbol associated with a section
STT_FILE	Symbol's name is filename
STT_COMMON	Symbol is a common data object
STT_TLS	Symbol is thread-local data object
STT_GNU_IFUNC	Symbol is indirect code object

The symbol binding can be extracted from the st_info field using the macro ELF64_ST_BIND(st_info) (or ELF32_ST_BIND() on 32-bit platforms), which yields one of the following values:

Value	Description
STB_LOCAL	Local symbol
STB_GLOBAL	Global symbol
STB_WEAK	Weak symbol
STB_GNU_UNIQUE	Unique symbol

The st_other field contains the symbol's visibility, which can be extracted using the macro ELF64_ST_VISIBILITY(st_info) (or ELF32_ST_VISIBILITY() on 32-bit platforms), which yields one of the following values:

Value	Description
STV_DEFAULT	Default symbol visibility rules
STV_INTERNAL	Processor-specific hidden class
STV_HIDDEN	Symbol unavailable in other modules
STV_PROTECTED	Not preemptible, not exported

RETURN VALUE

On success, these functions return a nonzero value. If the address specified in addr could be matched to a shared object, but not to a symbol in the shared object, then the info->dli_sname and info->dli_saddr fields are set to NULL.

If the address specified in addr could not be matched to a shared object, then these functions return 0. In this case, an error message is not available via dlerror(3).

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
dladdr (), dladdr1 ()	Thread safety	MT-Safe

STANDARDS

GNU.

HISTORY

dladdr(): glibc 2.0.
dladdr1(): glibc 2.3.3.

Solaris.

BUGS

Sometimes, the function pointers you pass to dladdr() may surprise you. On some architectures (notably i386 and x86-64), dli_fname and dli_fbase may end up pointing back at the object from which you called dladdr(), even if the function used as an argument should come from a dynamically linked library.

The problem is that the function pointer will still be resolved at compile time, but merely point to the plt (Procedure Linkage Table) section of the original object (which dispatches the call after asking the dynamic linker to resolve the symbol). To work around this, you can try to compile the code to be position-independent: then, the compiler cannot prepare the pointer at compile time any more and gcc(1) will generate code that just loads the final symbol address from the got (Global Offset Table) at run time before passing it to dladdr().