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 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.

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 avail-
       able 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\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 3.25 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)