Manpage of DLADDR


Section: Linux Programmer's Manual (3)
Updated: 2015-08-08


dladdr, dladdr1 - translate address to symbolic information  


#define _GNU_SOURCE#include <dlfcn.h>int dladdr(void *addr, Dl_info *info);int dladdr1(void *addr, Dl_info *info, void **extra_info, int flags);Link with -ldl.


The function dladdr() determines whether the address specified in addris 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_infostructure:

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 addrcould be found, then dli_snameand dli_saddrare 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:

Obtain a pointer to the link map for the matched file. The extra_infoargument points to a pointer to a link_mapstructure (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 */
Obtain a pointer to the ELF symbol table entry of the matching symbol. The extra_infoargument 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_namefield is an index into the string table.

The st_infofield 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:

STT_NOTYPESymbol type is unspecified
STT_OBJECTSymbol is a data object
STT_FUNCSymbol is a code object
STT_SECTIONSymbol associated with a section
STT_FILESymbol's name is file name
STT_COMMONSymbol is a common data object
STT_TLSSymbol is thread-local data object
STT_GNU_IFUNCSymbol is indirect code object
The symbol binding can be extracted from the st_infofield using the macro ELF64_ST_BIND(st_info)(or ELF32_ST_BIND()on 32-bit platforms), which yields one of the following values:
STB_LOCALLocal symbol
STB_GLOBALGlobal symbol
STB_WEAKWeak symbol
STB_GNU_UNIQUEUnique symbol
The st_otherfield 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:
STV_DEFAULTDefault symbol visibility rules
STV_INTERNALProcessor-specific hidden class
STV_HIDDENSymbol unavailable in other modules
STV_PROTECTEDNot preemptible, not exported


On success, these functions return a nonzero value. If the address specified in addrcould be matched to a shared object, but not to a symbol in the shared object, then the info->dli_snameand info->dli_saddrfields are set to NULL.

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


dladdr() is present in glibc 2.0 and later. dladdr1() first appeared in glibc 2.3.3.  


For an explanation of the terms used in this section, see attributes(7).
dladdr(), dladdr1() Thread safetyMT-Safe


These functions are nonstandard GNU extensions that are also present on Solaris.  


Sometimes, the function pointers you pass to dladdr() may surprise you. On some architectures (notably i386 and x86_64), dli_fnameand dli_fbasemay 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().  


dl_iterate_phdr(3), dlinfo(3), dlopen(3), dlsym(3),




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