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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, 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 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.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌───────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├───────────────────────┼───────────────┼─────────┤
       │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 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.

       *  backtrace() and backtrace_symbols_fd() don't call malloc()  explicitly,  but  they  are
          part of libgcc, which gets loaded dynamically when first used.  Dynamic loading usually
          triggers a call to malloc(3).  If you need certain calls to these two functions to  not
          allocate  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  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>

       #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 5.01 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/.