backtrace_symbols

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 ad-
       dress 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 em-
       ployed 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 at-
       tributes(7).

       +-----------------------+---------------+---------+
       |Interface              | Attribute     | Value   |
       +-----------------------+---------------+---------+
       |backtrace(),           | Thread safety | MT-Safe |
       |backtrace_symbols(),   |               |         |
       |backtrace_symbols_fd() |               |         |
       +-----------------------+---------------+---------+
CONFORMING TO
       These functions are GNU extensions.

NOTES
       These functions make some assumptions about how a function's return ad-
       dress 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.

       *  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 al-
          locate  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);
       }

SEE ALSO
       addr2line(1), gcc(1), gdb(1), ld(1), dlopen(3), malloc(3)

COLOPHON
       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

GNU                               2019-03-06                      BACKTRACE(3)