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BACKTRACE(3) Linux Programmer's Manual BACKTRACE(3)

NAME

     backtrace, backtrace_symbols, backtrace_symbols_fd - support for appli-
     cation self-debugging

SYNOPSIS

     #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);

DESCRIPTION

     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 corre-
     sponding 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,  back-
     trace_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 mal-
     loc(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,  but  see
     NOTES.

RETURN VALUE

     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 mal-
     loc(3)ed by the call; on error, NULL is returned.

VERSIONS

     backtrace(), backtrace_symbols(), and backtrace_symbols_fd()  are  pro-
     vided in glibc since version 2.1.

ATTRIBUTES

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

     allbox; lbw22 lb lb l l l.  Interface Attribute Value T{ backtrace(),
     backtrace_symbols(),
     backtrace_symbols_fd() T}   Thread safety  MT-Safe

CONFORMING TO

     These functions are GNU extensions.

NOTES

     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 vio-

        lated.

Inlined functions do not have stack frames.

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

backtrace() and backtrace_symbols_fd() don't call malloc() explic-

itly, but they are part of libgcc, which gets loaded dynamically

        when  first  used.   Dynamic loading usually triggers a call to mal-
        loc(3).  If you need certain calls to these  two  functions  to  not
        allocate  memory (in signal handlers, for example), you need to make
        sure libgcc is loaded beforehand.

     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.

EXAMPLE

     The  program  below  demonstrates  the  use  of  backtrace()  and back-
     trace_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/libc.so.6(__libc_start_main+0xdc)      [0xb7e38f9c]     ./prog
         [0x8048711]

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

     #define BT_BUF_SIZE 100

     void myfunc3(void) {
         int j, nptrs;
         void *buffer[BT_BUF_SIZE];
         char **strings;

         nptrs = backtrace(buffer, BT_BUF_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) {
             perror("backtrace_symbols");
             exit(EXIT_FAILURE);
         }

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

         free(strings); }

     static   void    /*  "static"  means  don't  export  the  symbol...  */
     myfunc2(void) {
         myfunc3(); }

     void myfunc(int ncalls) {
         if (ncalls > 1)
             myfunc(ncalls - 1);
         else
             myfunc2(); }

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

         myfunc(atoi(argv[1]));
         exit(EXIT_SUCCESS); }