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

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  speci‐
       fies  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 re‐
       cent 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  re‐
       turned  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).

       β”Œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”
       β”‚Interface                                                                                                                             β”‚ Attribute     β”‚ Value   β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚backtrace(), backtrace_symbols(), backtrace_symbols_fd()                                                                              β”‚ Thread safety β”‚ MT-Safe β”‚
       β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”˜

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 op‐
       tion.  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 (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
       addr2line(1), gcc(1), gdb(1), ld(1), dlopen(3), malloc(3)

GNU                                                                            2021-03-22                                                                   BACKTRACE(3)