BACKTRACE(3)		   Linux Programmers Manual		 BACKTRACE(3)



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

SYNOPSIS
       #include 

       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.

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.

CONFORMING TO
       These functions are GNU extensions.

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

       *  Omission  of	the frame pointers (as implied by any of gcc(1)s non-
	  zero 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.

       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 wont 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]

       #include 
       #include 
       #include 
       #include 

       void
       myfunc3(void)
       {
	   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) {
	       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);
       }

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

COLOPHON
       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



GNU				  2008-06-14			  BACKTRACE(3)