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



NAME
       backtrace,  backtrace_symbols,  backtrace_symbols_fd - support for application self-debug‐
       ging

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  buf‐
       fer.   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()  trans‐
       lates  the  addresses  into  an array of strings that describe the addresses symbolically.
       The size argument specifies the number of addresses in buffer.  The  symbolic  representa‐
       tion of each address consists of the function name (if this can be determined), a hexadec‐
       imal 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 back‐
       trace_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 lev‐
          els) 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  sys‐
       tems  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  fol‐
       lowing 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 3.74 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 http://www.kernel.org/doc/man-pages/.



GNU                                         2008-06-14                               BACKTRACE(3)