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

NAME

       backtrace, backtrace_symbols, backtrace_symbols_fd - support for application 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, 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.

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 malloc(3)ed by the call; on error, NULL is returned.

VERSIONS

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

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 violated.

       *  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.

EXAMPLE

       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/libc.so.6(__libc_start_main+0xdc) [0xb7e38f9c]
	   ./prog [0x8048711]

   Program source

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

       void
       myfunc3(void)
       {
	   int j, nptrs;
       #define SIZE 100
	   void *buffer[100];
	   char **strings;

	   nptrs = backtrace(buffer, SIZE);
	   printf("backtrace() returned %d addresses
", 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
", 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
", 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 3.25 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)