How to list on-the-fly all the functions/symbols available in C code on a Linux architecture?

Question

Assume main.c uses symbols from shared libs and local functions declared in main.c.

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

It should be possible since the data is loaded to the .code segment.

Answer 1

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:

• ELF Manpage
• Some sample code (found it while searching for "dl_iterate_phdr")

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 = 0;     char* sym_name = 0;     ElfW(Word) sym_cnt = 0;      /* 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];                          symbol_names->push_back(string(sym_name));                     }                 }                  /* move pointer to the next entry */                 dyn++;             }         }     }      /* 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; }