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 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\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 3.54 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/.