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       backtrace,   backtrace_symbols,   backtrace_symbols_fd  -  support  for
       application self-debugging


       #include <execinfo.h>

       int backtrace(void **buffer, int size);

       char **backtrace_symbols(void *const *buffer, int size);

       void backtrace_symbols_fd(void *const *buffer, int size, int fd);


       backtrace() returns a backtrace for the calling program, in  the  array
       pointed  to  by  buffer.  A backtrace is the series of currently active
       function calls for the program.  Each item in the array pointed  to  by
       buffer  is  of  type  void *,  and  is  the  return  address  from  the
       corresponding stack frame.  The size  argument  specifies  the  maximum
       number  of addresses that can be stored in buffer.  If the backtrace is
       larger than size, then the addresses corresponding  to  the  size  most
       recent  function  calls are returned; to obtain the complete backtrace,
       make sure that buffer and size are large enough.

       Given  the  set  of  addresses  returned  by  backtrace()  in   buffer,
       backtrace_symbols()  translates  the addresses into an array of strings
       that describe the addresses symbolically.  The size argument  specifies
       the number of addresses in buffer.  The symbolic representation of each
       address consists of the function name (if this can  be  determined),  a
       hexadecimal offset into the function, and the actual return address (in
       hexadecimal).  The address of the array of string pointers is  returned
       as   the   function  result  of  backtrace_symbols().   This  array  is
       malloc(3)ed by backtrace_symbols(), and must be freed  by  the  caller.
       (The  strings  pointed  to by the array of pointers need not and should
       not be freed.)

       backtrace_symbols_fd() takes the same  buffer  and  size  arguments  as
       backtrace_symbols(),  but  instead  of returning an array of strings to
       the caller, it writes the strings, one per line, to the file descriptor
       fd.   backtrace_symbols_fd()  does  not  call  malloc(3), and so can be
       employed in situations where the latter function might fail.


       backtrace() returns the number of addresses returned in  buffer,  which
       is  not greater than size.  If the return value is less than size, then
       the full backtrace was stored; if it is equal to size, then it may have
       been  truncated, in which case the addresses of the oldest stack frames
       are not returned.

       On  success,  backtrace_symbols()  returns  a  pointer  to  the   array
       malloc(3)ed by the call; on error, NULL is returned.


       backtrace(),   backtrace_symbols(),   and   backtrace_symbols_fd()  are
       provided in glibc since version 2.1.


       For  an  explanation  of  the  terms  used   in   this   section,   see

       │InterfaceAttributeValue   │
       │backtrace(),           │ Thread safety │ MT-Safe │
       │backtrace_symbols(),   │               │         │
       │backtrace_symbols_fd() │               │         │


       These functions are GNU extensions.


       These  functions  make  some  assumptions about how a function's return
       address is stored on the stack.  Note the following:

       *  Omission of the frame  pointers  (as  implied  by  any  of  gcc(1)'s
          nonzero  optimization  levels)  may  cause  these  assumptions to be

       *  Inlined functions do not have stack frames.

       *  Tail-call optimization causes one stack frame to replace another.

       The symbol names may be unavailable without the use of  special  linker
       options.   For systems using the GNU linker, it is necessary to use the
       -rdynamic linker option.  Note that names of "static" functions are not
       exposed, and won't be available in the backtrace.


       The   program   below   demonstrates   the   use   of  backtrace()  and
       backtrace_symbols().  The following shell session shows what  we  might
       see when running the program:

           $ cc -rdynamic prog.c -o prog
           $ ./prog 3
           backtrace() returned 8 addresses
           ./prog(myfunc3+0x5c) [0x80487f0]
           ./prog [0x8048871]
           ./prog(myfunc+0x21) [0x8048894]
           ./prog(myfunc+0x1a) [0x804888d]
           ./prog(myfunc+0x1a) [0x804888d]
           ./prog(main+0x65) [0x80488fb]
           /lib/ [0xb7e38f9c]
           ./prog [0x8048711]

   Program source

       #include <execinfo.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

           int j, nptrs;
       #define SIZE 100
           void *buffer[100];
           char **strings;

           nptrs = backtrace(buffer, SIZE);
           printf("backtrace() returned %d addresses\n", nptrs);

           /* The call backtrace_symbols_fd(buffer, nptrs, STDOUT_FILENO)
              would produce similar output to the following: */

           strings = backtrace_symbols(buffer, nptrs);
           if (strings == NULL) {

           for (j = 0; j < nptrs; j++)
               printf("%s\n", strings[j]);


       static void   /* "static" means don't export the symbol... */

       myfunc(int ncalls)
           if (ncalls > 1)
               myfunc(ncalls - 1);

       main(int argc, char *argv[])
           if (argc != 2) {
               fprintf(stderr, "%s num-calls\n", argv[0]);



       gcc(1), ld(1), dlopen(3), malloc(3)


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