0x90 0x90 - 2 years ago 168
Linux Question

List all the functions/symbols on the fly in c?


uses symbols from shared libs and functions declared in

Is there a nice and elegant way to print a list of all the function names or symbols at run time ?

It should be possible since the data is in the
segment, but one need to parse it.

Answer Source

Since I had the same need to retrieve all loaded symbol names at runtime, I did some research based upon R..'s answer. So here is a detailed solution for linux shared libraries in ELF format which works with my gcc 4.3.4, but hopefully also with newer versions.

I mostly used the following sources to develop this solution:

And here's my code. I used self explaining variable names and added detailed comments to make it understandable. If something is wrong or missing, please let me know... (Edit: I just realized that the question was for C and my code is for C++. But if you leave out the vector and the string it should work for C as well)

#include <link.h>
#include <string>
#include <vector>

using namespace std;

/* Callback for dl_iterate_phdr.
 * Is called by dl_iterate_phdr for every loaded shared lib until something
 * else than 0 is returned by one call of this function.
int retrieve_symbolnames(struct dl_phdr_info* info, size_t info_size, void* symbol_names_vector) 

    /* ElfW is a macro that creates proper typenames for the used system architecture
     * (e.g. on a 32 bit system, ElfW(Dyn*) becomes "Elf32_Dyn*") */
    ElfW(Dyn*) dyn;
    ElfW(Sym*) sym;
    ElfW(Word*) hash;

    char* strtab;
    char* sym_name;
    ElfW(Word) sym_cnt;

    /* the void pointer (3rd argument) should be a pointer to a vector<string>
     * in this example -> cast it to make it usable */
    vector<string>* symbol_names = reinterpret_cast<vector<string>*>(symbol_names_vector);

    /* Iterate over all headers of the current shared lib
     * (first call is for the executable itself) */
    for (size_t header_index = 0; header_index < info->dlpi_phnum; header_index++)

        /* Further processing is only needed if the dynamic section is reached */
        if (info->dlpi_phdr[header_index].p_type == PT_DYNAMIC)

            /* Get a pointer to the first entry of the dynamic section.
             * It's address is the shared lib's address + the virtual address */
            dyn = (ElfW(Dyn)*)(info->dlpi_addr +  info->dlpi_phdr[header_index].p_vaddr);

            /* Iterate over all entries of the dynamic section until the
             * end of the symbol table is reached. This is indicated by
             * an entry with d_tag == DT_NULL.
             * Only the following entries need to be processed to find the
             * symbol names:
             *  - DT_HASH   -> second word of the hash is the number of symbols
             *  - DT_STRTAB -> pointer to the beginning of a string table that
             *                 contains the symbol names
             *  - DT_SYMTAB -> pointer to the beginning of the symbols table
            while(dyn->d_tag != DT_NULL)
                if (dyn->d_tag == DT_HASH)
                    /* Get a pointer to the hash */
                    hash = (ElfW(Word*))dyn->d_un.d_ptr;

                    /* The 2nd word is the number of symbols */
                    sym_cnt = hash[1];

                else if (dyn->d_tag == DT_STRTAB)
                    /* Get the pointer to the string table */
                    strtab = (char*)dyn->d_un.d_ptr;
                else if (dyn->d_tag == DT_SYMTAB)
                    /* Get the pointer to the first entry of the symbol table */
                    sym = (ElfW(Sym*))dyn->d_un.d_ptr;

                    /* Iterate over the symbol table */
                    for (ElfW(Word) sym_index = 0; sym_index < sym_cnt; sym_index++)
                        /* get the name of the i-th symbol.
                         * This is located at the address of st_name
                         * relative to the beginning of the string table. */
                        sym_name = &strtab[sym[sym_index].st_name];


                /* move pointer to the next entry */

    /* Returning something != 0 stops further iterations,
     * since only the first entry, which is the executable itself, is needed
     * 1 is returned after processing the first entry.
     * If the symbols of all loaded dynamic libs shall be found,
     * the return value has to be changed to 0.
    return 1;


int main()
    vector<string> symbolNames;
    dl_iterate_phdr(retrieve_symbolnames, &symbolNames);

    return 0;
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