backtrace(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

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

NAME         top

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

SYNOPSIS         top

       #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         top

       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         top

       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         top

       backtrace(), backtrace_symbols(), and backtrace_symbols_fd() are
       provided in glibc since version 2.1.

ATTRIBUTES         top

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

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

CONFORMING TO         top

       These functions are GNU extensions.

NOTES         top

       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         top

       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 (int 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         top

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

COLOPHON         top

       This page is part of release 5.13 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                            2021-03-22                   BACKTRACE(3)

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