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NAME

       backtrace,   backtrace_symbols,  backtrace_symbols_fd  -  support  for  application  self-
       debugging

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <execinfo.h>

       int backtrace(void *buffer[.size], int size);

       char **backtrace_symbols(void *const buffer[.size], int size);
       void backtrace_symbols_fd(void *const buffer[.size], 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 since glibc 2.1.

ATTRIBUTES

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

       ┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │backtrace(), backtrace_symbols(), backtrace_symbols_fd()       │ Thread safety │ MT-Safe │
       └───────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       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.

EXAMPLES

       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 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 (size_t 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)