BACKTRACE(3)            NetBSD Library Functions Manual           BACKTRACE(3)

     backtrace, backtrace_symbols, backtrace_symbols_fd,
     backtrace_symbols_fmt, backtrace_symbols_fd_fmt -- fill in the backtrace
     of the currently executing thread

          Backtrace Access Library (libexecinfo, -lexecinfo)

     #include <execinfo.h>

     backtrace(void **addrlist, size_t len);

     char **
     backtrace_symbols(void * const *addrlist, size_t len);

     backtrace_symbols_fd(void * const *addrlist, size_t len, int fd);

     char **
     backtrace_symbols_fmt(void * const *addrlist, size_t len,
         const char *fmt);

     backtrace_symbols_fd_fmt(void * const *addrlist, size_t len, int fd,
         const char *fmt);

     The backtrace() function places into the array pointed by addrlist the
     array of the values of the program counter for each frame called up to
     len frames.  The number of frames found (which can be fewer than len) is

     The backtrace_symbols_fmt() function takes an array of previously filled
     addresses from backtrace() in addrlist of len elements, and uses fmt to
     format them.  The formatting characters available are:

           a  The numeric address of each element as would be printed using

           n  The name of the nearest function symbol (smaller than the
              address element) as determined by dladdr(3) if the symbol was
              dynamic, or looked up in the executable if static and the /proc
              filesystem is available to determine the executable path.

           d  The difference of the symbol address and the address element
              printed using 0x%tx.

           D  The difference of the symbol address and the address element
              printed using +0x%tx if non-zero, or nothing if zero.

           f  The filename of the symbol as determined by dladdr(3).

     The array of formatted strings is returned as a contiguous memory address
     which can be freed by a single free(3).

     The backtrace_symbols() function is equivalent of calling
     backtrace_symbols_fmt() with a format argument of ``%a <%n%D> at %f''

     The backtrace_symbols_fd() and backtrace_symbols_fd_fmt() are similar to
     the non _fd named functions, only instead of returning an array of
     strings, they print a new-line separated array of strings in fd, and
     return 0 on success and -1 on failure.

     The backtrace() function returns the number of elements that were filled
     in the backtrace.  The backtrace_symbols() and backtrace_symbols_fmt()
     return a string array on success, and NULL on failure, setting errno.
     Diagnostic output may also be produced by the ELF symbol lookup func-

     dladdr(3), elf(3)

     The backtrace() library of functions first appeared in NetBSD 7.0.

     1.   Errors should not be printed but communicated to the caller differ-

     2.   Because these functions use elf(3) this is a separate library
          instead of being part of libc/libutil so that no library dependen-
          cies are introduced.

     3.   The Linux versions of the functions (there are no _fmt variants) use
          int instead of size_t arguments.

     4.   Since dladdr(3) only deals with dynamic symbols, we need to find the
          symbols from the main portion of the program.  For that we need to
          locate the executable, and we use procfs for finding it, which is
          not portable.

NetBSD 8.0                     November 5, 2015                     NetBSD 8.0

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