gdb/linux-fork.c

   1 /* GNU/Linux native-dependent code for debugging multiple forks.
   2
   3    Copyright (C) 2005, 2006 Free Software Foundation, Inc.
   4
   5    This file is part of GDB.
   6
   7    This program is free software; you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 2 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program; if not, write to the Free Software
  19    Foundation, Inc., 51 Franklin Street, Fifth Floor,
  20    Boston, MA 02110-1301, USA.  */
  21
  22 #include "defs.h"
  23 #include "inferior.h"
  24 #include "regcache.h"
  25 #include "gdbcmd.h"
  26 #include "infcall.h"
  27 #include "gdb_assert.h"
  28 #include "gdb_string.h"
  29 #include "linux-fork.h"
  30 #include "linux-nat.h"
  31
  32 #include <sys/ptrace.h>
  33 #include <sys/wait.h>
  34 #include <sys/param.h>
  35 #include <dirent.h>
  36 #include <ctype.h>
  37
  38 struct fork_info *fork_list;
  39 static int highest_fork_num;
  40
  41 /* Prevent warning from -Wmissing-prototypes.  */
  42 extern void _initialize_linux_fork (void);
  43
  44 int detach_fork = 1;            /* Default behavior is to detach
  45                                    newly forked processes (legacy).  */
  46
  47 /* Fork list data structure:  */
  48 struct fork_info
  49 {
  50   struct fork_info *next;
  51   ptid_t ptid;
  52   int num;                      /* Convenient handle (GDB fork id) */
  53   struct regcache *savedregs;   /* Convenient for info fork, saves
  54                                    having to actually switch contexts.  */
  55   int clobber_regs;             /* True if we should restore saved regs.  */
  56   ULONGEST pc;                  /* PC for info fork.  */
  57   off_t *filepos;               /* Set of open file descriptors' offsets.  */
  58   int maxfd;
  59 };
  60
  61 /* Fork list methods:  */
  62
  63 extern int
  64 forks_exist_p (void)
  65 {
  66   return (fork_list != NULL);
  67 }
  68
  69 /* Add a fork to internal fork list.
  70    Called from linux child_follow_fork.  */
  71
  72 extern struct fork_info *
  73 add_fork (pid_t pid)
  74 {
  75   struct fork_info *fp;
  76
  77   if (fork_list == NULL && pid != PIDGET (inferior_ptid))
  78     {
  79       /* Special case -- if this is the first fork in the list
  80          (the list is hitherto empty), and if this new fork is
  81          NOT the current inferior_ptid, then add inferior_ptid
  82          first, as a special zeroeth fork id.  */
  83       highest_fork_num = -1;
  84       add_fork (PIDGET (inferior_ptid));        /* safe recursion */
  85     }
  86
  87   fp = XZALLOC (struct fork_info);
  88   fp->ptid = ptid_build (pid, pid, 0);
  89   fp->num = ++highest_fork_num;
  90   fp->next = fork_list;
  91   fork_list = fp;
  92   return fp;
  93 }
  94
  95 static void
  96 free_fork (struct fork_info *fp)
  97 {
  98   /* Notes on step-resume breakpoints: since this is a concern for
  99      threads, let's convince ourselves that it's not a concern for
 100      forks.  There are two ways for a fork_info to be created.  First,
 101      by the checkpoint command, in which case we're at a gdb prompt
 102      and there can't be any step-resume breakpoint.  Second, by a fork
 103      in the user program, in which case we *may* have stepped into the
 104      fork call, but regardless of whether we follow the parent or the
 105      child, we will return to the same place and the step-resume
 106      breakpoint, if any, will take care of itself as usual.  And
 107      unlike threads, we do not save a private copy of the step-resume
 108      breakpoint -- so we're OK.  */
 109
 110   if (fp)
 111     {
 112       if (fp->savedregs)
 113         regcache_xfree (fp->savedregs);
 114       if (fp->filepos)
 115         xfree (fp->filepos);
 116       xfree (fp);
 117     }
 118 }
 119
 120 static void
 121 delete_fork (ptid_t ptid)
 122 {
 123   struct fork_info *fp, *fpprev;
 124
 125   fpprev = NULL;
 126
 127   for (fp = fork_list; fp; fpprev = fp, fp = fp->next)
 128     if (ptid_equal (fp->ptid, ptid))
 129       break;
 130
 131   if (!fp)
 132     return;
 133
 134   if (fpprev)
 135     fpprev->next = fp->next;
 136   else
 137     fork_list = fp->next;
 138
 139   free_fork (fp);
 140
 141   /* Special case: if there is now only one process in the list,
 142      and if it is (hopefully!) the current inferior_ptid, then
 143      remove it, leaving the list empty -- we're now down to the
 144      default case of debugging a single process.  */
 145   if (fork_list != NULL && fork_list->next == NULL &&
 146       ptid_equal (fork_list->ptid, inferior_ptid))
 147     {
 148       /* Last fork -- delete from list and handle as solo process
 149          (should be a safe recursion).  */
 150       delete_fork (inferior_ptid);
 151     }
 152 }
 153
 154 /* Find a fork_info by matching PTID.  */
 155 static struct fork_info *
 156 find_fork_ptid (ptid_t ptid)
 157 {
 158   struct fork_info *fp;
 159
 160   for (fp = fork_list; fp; fp = fp->next)
 161     if (ptid_equal (fp->ptid, ptid))
 162       return fp;
 163
 164   return NULL;
 165 }
 166
 167 /* Find a fork_info by matching ID.  */
 168 static struct fork_info *
 169 find_fork_id (int num)
 170 {
 171   struct fork_info *fp;
 172
 173   for (fp = fork_list; fp; fp = fp->next)
 174     if (fp->num == num)
 175       return fp;
 176
 177   return NULL;
 178 }
 179
 180 /* Find a fork_info by matching pid.  */
 181 extern struct fork_info *
 182 find_fork_pid (pid_t pid)
 183 {
 184   struct fork_info *fp;
 185
 186   for (fp = fork_list; fp; fp = fp->next)
 187     if (pid == ptid_get_pid (fp->ptid))
 188       return fp;
 189
 190   return NULL;
 191 }
 192
 193 static ptid_t
 194 fork_id_to_ptid (int num)
 195 {
 196   struct fork_info *fork = find_fork_id (num);
 197   if (fork)
 198     return fork->ptid;
 199   else
 200     return pid_to_ptid (-1);
 201 }
 202
 203 static void
 204 init_fork_list (void)
 205 {
 206   struct fork_info *fp, *fpnext;
 207
 208   if (!fork_list)
 209     return;
 210
 211   for (fp = fork_list; fp; fp = fpnext)
 212     {
 213       fpnext = fp->next;
 214       free_fork (fp);
 215     }
 216
 217   fork_list = NULL;
 218 }
 219
 220 /* Fork list <-> gdb interface.  */
 221
 222 /* Utility function for fork_load/fork_save.
 223    Calls lseek in the (current) inferior process.  */
 224
 225 static off_t
 226 call_lseek (int fd, off_t offset, int whence)
 227 {
 228   char exp[80];
 229
 230   snprintf (&exp[0], sizeof (exp), "lseek (%d, %ld, %d)",
 231             fd, (long) offset, whence);
 232   return (off_t) parse_and_eval_long (&exp[0]);
 233 }
 234
 235 /* Load infrun state for the fork PTID.  */
 236
 237 static void
 238 fork_load_infrun_state (struct fork_info *fp)
 239 {
 240   extern void nullify_last_target_wait_ptid ();
 241   int i;
 242
 243   inferior_ptid = fp->ptid;
 244
 245   linux_nat_switch_fork (inferior_ptid);
 246
 247   if (fp->savedregs && fp->clobber_regs)
 248     regcache_cpy (current_regcache, fp->savedregs);
 249
 250   registers_changed ();
 251   reinit_frame_cache ();
 252
 253   /* We must select a new frame before making any inferior calls to
 254      avoid warnings.  */
 255   select_frame (get_current_frame ());
 256
 257   stop_pc = read_pc ();
 258   nullify_last_target_wait_ptid ();
 259
 260   /* Now restore the file positions of open file descriptors.  */
 261   if (fp->filepos)
 262     {
 263       for (i = 0; i <= fp->maxfd; i++)
 264         if (fp->filepos[i] != (off_t) -1)
 265           call_lseek (i, fp->filepos[i], SEEK_SET);
 266       /* NOTE: I can get away with using SEEK_SET and SEEK_CUR because
 267          this is native-only.  If it ever has to be cross, we'll have
 268          to rethink this.  */
 269     }
 270 }
 271
 272 /* Save infrun state for the fork PTID.
 273    Exported for use by linux child_follow_fork.  */
 274
 275 extern void
 276 fork_save_infrun_state (struct fork_info *fp, int clobber_regs)
 277 {
 278   char path[MAXPATHLEN];
 279   struct dirent *de;
 280   DIR *d;
 281
 282   if (fp->savedregs)
 283     regcache_xfree (fp->savedregs);
 284
 285   fp->savedregs = regcache_dup (current_regcache);
 286   fp->clobber_regs = clobber_regs;
 287   fp->pc = read_pc ();
 288
 289   if (clobber_regs)
 290     {
 291       /* Now save the 'state' (file position) of all open file descriptors.
 292          Unfortunately fork does not take care of that for us...  */
 293       snprintf (path, MAXPATHLEN, "/proc/%ld/fd", (long) PIDGET (fp->ptid));
 294       if ((d = opendir (path)) != NULL)
 295         {
 296           long tmp;
 297
 298           fp->maxfd = 0;
 299           while ((de = readdir (d)) != NULL)
 300             {
 301               /* Count open file descriptors (actually find highest
 302                  numbered).  */
 303               tmp = strtol (&de->d_name[0], NULL, 10);
 304               if (fp->maxfd < tmp)
 305                 fp->maxfd = tmp;
 306             }
 307           /* Allocate array of file positions.  */
 308           fp->filepos = xrealloc (fp->filepos,
 309                                   (fp->maxfd + 1) * sizeof (*fp->filepos));
 310
 311           /* Initialize to -1 (invalid).  */
 312           for (tmp = 0; tmp <= fp->maxfd; tmp++)
 313             fp->filepos[tmp] = -1;
 314
 315           /* Now find actual file positions.  */
 316           rewinddir (d);
 317           while ((de = readdir (d)) != NULL)
 318             if (isdigit (de->d_name[0]))
 319               {
 320                 tmp = strtol (&de->d_name[0], NULL, 10);
 321                 fp->filepos[tmp] = call_lseek (tmp, 0, SEEK_CUR);
 322               }
 323           closedir (d);
 324         }
 325     }
 326 }
 327
 328 /* Kill 'em all, let God sort 'em out...  */
 329
 330 extern void
 331 linux_fork_killall (void)
 332 {
 333   /* Walk list and kill every pid.  No need to treat the
 334      current inferior_ptid as special (we do not return a
 335      status for it) -- however any process may be a child
 336      or a parent, so may get a SIGCHLD from a previously
 337      killed child.  Wait them all out.  */
 338   struct fork_info *fp;
 339   pid_t pid, ret;
 340   int status;
 341
 342   for (fp = fork_list; fp; fp = fp->next)
 343     {
 344       pid = PIDGET (fp->ptid);
 345       do {
 346         ptrace (PT_KILL, pid, 0, 0);
 347         ret = waitpid (pid, &status, 0);
 348         /* We might get a SIGCHLD instead of an exit status.  This is
 349          aggravated by the first kill above - a child has just
 350          died.  MVS comment cut-and-pasted from linux-nat.  */
 351       } while (ret == pid && WIFSTOPPED (status));
 352     }
 353   init_fork_list ();    /* Clear list, prepare to start fresh.  */
 354 }
 355
 356 /* The current inferior_ptid has exited, but there are other viable
 357    forks to debug.  Delete the exiting one and context-switch to the
 358    first available.  */
 359
 360 extern void
 361 linux_fork_mourn_inferior (void)
 362 {
 363   /* Wait just one more time to collect the inferior's exit status.
 364      Do not check whether this succeeds though, since we may be
 365      dealing with a process that we attached to.  Such a process will
 366      only report its exit status to its original parent.  */
 367   int status;
 368
 369   waitpid (ptid_get_pid (inferior_ptid), &status, 0);
 370
 371   /* OK, presumably inferior_ptid is the one who has exited.
 372      We need to delete that one from the fork_list, and switch
 373      to the next available fork.  */
 374   delete_fork (inferior_ptid);
 375
 376   /* There should still be a fork - if there's only one left,
 377      delete_fork won't remove it, because we haven't updated
 378      inferior_ptid yet.  */
 379   gdb_assert (fork_list);
 380
 381   fork_load_infrun_state (fork_list);
 382   printf_filtered (_("[Switching to %s]\n"),
 383                    target_pid_to_str (inferior_ptid));
 384
 385   /* If there's only one fork, switch back to non-fork mode.  */
 386   if (fork_list->next == NULL)
 387     delete_fork (inferior_ptid);
 388 }
 389
 390 /* Fork list <-> user interface.  */
 391
 392 static void
 393 delete_fork_command (char *args, int from_tty)
 394 {
 395   ptid_t ptid;
 396
 397   if (!args || !*args)
 398     error (_("Requires argument (fork/checkpoint id to delete)"));
 399
 400   ptid = fork_id_to_ptid (parse_and_eval_long (args));
 401   if (ptid_equal (ptid, minus_one_ptid))
 402     error (_("No such fork/checkpoint id, %s"), args);
 403
 404   if (ptid_equal (ptid, inferior_ptid))
 405     error (_("Please switch to another fork/checkpoint before deleting the current one"));
 406
 407   if (ptrace (PTRACE_KILL, PIDGET (ptid), 0, 0))
 408     error (_("Unable to kill pid %s"), target_tid_to_str (ptid));
 409
 410   if (from_tty)
 411     printf_filtered (_("Killed %s\n"), target_pid_to_str (ptid));
 412
 413   delete_fork (ptid);
 414 }
 415
 416 static void
 417 detach_fork_command (char *args, int from_tty)
 418 {
 419   ptid_t ptid;
 420
 421   if (!args || !*args)
 422     error (_("Requires argument (fork id to detach)"));
 423
 424   ptid = fork_id_to_ptid (parse_and_eval_long (args));
 425   if (ptid_equal (ptid, minus_one_ptid))
 426     error (_("No such fork id, %s"), args);
 427
 428   if (ptid_equal (ptid, inferior_ptid))
 429     error (_("Please switch to another fork before detaching the current one"));
 430
 431   if (ptrace (PTRACE_DETACH, PIDGET (ptid), 0, 0))
 432     error (_("Unable to detach %s"), target_pid_to_str (ptid));
 433
 434   if (from_tty)
 435     printf_filtered (_("Detached %s\n"), target_pid_to_str (ptid));
 436
 437   delete_fork (ptid);
 438 }
 439
 440 /* Print information about currently known forks.  */
 441
 442 static void
 443 info_forks_command (char *arg, int from_tty)
 444 {
 445   struct frame_info *cur_frame;
 446   struct symtab_and_line sal;
 447   struct symtab *cur_symtab;
 448   struct fork_info *fp;
 449   int cur_line;
 450   ULONGEST pc;
 451   int requested = -1;
 452   struct fork_info *printed = NULL;
 453
 454   if (arg && *arg)
 455     requested = (int) parse_and_eval_long (arg);
 456
 457   for (fp = fork_list; fp; fp = fp->next)
 458     {
 459       if (requested > 0 && fp->num != requested)
 460         continue;
 461
 462       printed = fp;
 463       if (ptid_equal (fp->ptid, inferior_ptid))
 464         {
 465           printf_filtered ("* ");
 466           pc = read_pc ();
 467         }
 468       else
 469         {
 470           printf_filtered ("  ");
 471           pc = fp->pc;
 472         }
 473       printf_filtered ("%d %s", fp->num, target_pid_to_str (fp->ptid));
 474       if (fp->num == 0)
 475         printf_filtered (_(" (main process)"));
 476       printf_filtered (_(" at "));
 477       deprecated_print_address_numeric (pc, 1, gdb_stdout);
 478
 479       sal = find_pc_line (pc, 0);
 480       if (sal.symtab)
 481         {
 482           char *tmp = strrchr (sal.symtab->filename, '/');
 483
 484           if (tmp)
 485             printf_filtered (_(", file %s"), tmp + 1);
 486           else
 487             printf_filtered (_(", file %s"), sal.symtab->filename);
 488         }
 489       if (sal.line)
 490         printf_filtered (_(", line %d"), sal.line);
 491       if (!sal.symtab && !sal.line)
 492         {
 493           struct minimal_symbol *msym;
 494
 495           msym = lookup_minimal_symbol_by_pc (pc);
 496           if (msym)
 497             printf_filtered (", <%s>", SYMBOL_LINKAGE_NAME (msym));
 498         }
 499
 500       putchar_filtered ('\n');
 501     }
 502   if (printed == NULL)
 503     {
 504       if (requested > 0)
 505         printf_filtered (_("No fork number %d.\n"), requested);
 506       else
 507         printf_filtered (_("No forks.\n"));
 508     }
 509 }
 510
 511 /* Save/restore mode variable 'detach_fork':
 512    We need to temporarily take over this mode variable, while
 513    preserving the user-specified state, and make sure that it
 514    gets restored in case of error.
 515
 516    The int pointer that we use comes from the caller, so we can
 517    be called more than once (even though currently we don't need to).  */
 518
 519 static void
 520 restore_detach_fork (void *arg)
 521 {
 522   detach_fork = *(int *) arg;
 523 }
 524
 525 static struct cleanup *
 526 save_detach_fork (int *saved_val)
 527 {
 528   *saved_val = detach_fork;
 529   return make_cleanup (restore_detach_fork, (void *) saved_val);
 530 }
 531
 532 static void
 533 checkpoint_command (char *args, int from_tty)
 534 {
 535   struct target_waitstatus last_target_waitstatus;
 536   ptid_t last_target_ptid;
 537   struct value *fork_fn = NULL, *ret;
 538   struct fork_info *fp;
 539   pid_t retpid;
 540   struct cleanup *old_chain;
 541   long i;
 542   /* Make this temp var static, 'cause it's used in the error context.  */
 543   static int temp_detach_fork;
 544
 545   /* Make the inferior fork, record its (and gdb's) state.  */
 546
 547   if (lookup_minimal_symbol ("fork", NULL, NULL) != NULL)
 548     fork_fn = find_function_in_inferior ("fork");
 549   if (!fork_fn)
 550     if (lookup_minimal_symbol ("_fork", NULL, NULL) != NULL)
 551       fork_fn = find_function_in_inferior ("fork");
 552   if (!fork_fn)
 553     error (_("checkpoint: can't find fork function in inferior."));
 554
 555   ret = value_from_longest (builtin_type_int, 0);
 556   old_chain = save_detach_fork (&temp_detach_fork);
 557   detach_fork = 0;
 558   ret = call_function_by_hand (fork_fn, 0, &ret);
 559   do_cleanups (old_chain);
 560   if (!ret)     /* Probably can't happen.  */
 561     error (_("checkpoint: call_function_by_hand returned null."));
 562
 563   retpid = value_as_long (ret);
 564   get_last_target_status (&last_target_ptid, &last_target_waitstatus);
 565   if (from_tty)
 566     {
 567       int parent_pid;
 568
 569       printf_filtered (_("checkpoint: fork returned pid %ld.\n"),
 570                        (long) retpid);
 571       if (info_verbose)
 572         {
 573           parent_pid = ptid_get_lwp (last_target_ptid);
 574           if (parent_pid == 0)
 575             parent_pid = ptid_get_pid (last_target_ptid);
 576           printf_filtered (_("   gdb says parent = %ld.\n"),
 577                            (long) parent_pid);
 578         }
 579     }
 580
 581   fp = find_fork_pid (retpid);
 582   if (!fp)
 583     error (_("Failed to find new fork"));
 584   fork_save_infrun_state (fp, 1);
 585 }
 586
 587 static void
 588 linux_fork_context (struct fork_info *newfp, int from_tty)
 589 {
 590   /* Now we attempt to switch processes.  */
 591   struct fork_info *oldfp = find_fork_ptid (inferior_ptid);
 592   ptid_t ptid;
 593   int id, i;
 594
 595   if (!newfp)
 596     error (_("No such fork/process"));
 597
 598   if (!oldfp)
 599     oldfp = add_fork (ptid_get_pid (inferior_ptid));
 600
 601   fork_save_infrun_state (oldfp, 1);
 602   fork_load_infrun_state (newfp);
 603
 604   printf_filtered (_("Switching to %s\n"),
 605                    target_pid_to_str (inferior_ptid));
 606
 607   print_stack_frame (get_selected_frame (NULL), 1, SRC_AND_LOC);
 608 }
 609
 610 /* Switch inferior process (fork) context, by process id.  */
 611 static void
 612 process_command (char *args, int from_tty)
 613 {
 614   struct fork_info *fp;
 615
 616   if (!args || !*args)
 617     error (_("Requires argument (process id to switch to)"));
 618
 619   if ((fp = find_fork_pid (parse_and_eval_long (args))) == NULL)
 620     error (_("Not found: process id %s"), args);
 621
 622   linux_fork_context (fp, from_tty);
 623 }
 624
 625 /* Switch inferior process (fork) context, by fork id.  */
 626 static void
 627 fork_command (char *args, int from_tty)
 628 {
 629   struct fork_info *fp;
 630
 631   if (!args || !*args)
 632     error (_("Requires argument (fork id to switch to)"));
 633
 634   if ((fp = find_fork_id (parse_and_eval_long (args))) == NULL)
 635     error (_("Not found: fork id %s"), args);
 636
 637   linux_fork_context (fp, from_tty);
 638 }
 639
 640 /* Switch inferior process (fork) context, by checkpoint id.  */
 641 static void
 642 restart_command (char *args, int from_tty)
 643 {
 644   struct fork_info *fp;
 645
 646   if (!args || !*args)
 647     error (_("Requires argument (checkpoint id to restart)"));
 648
 649   if ((fp = find_fork_id (parse_and_eval_long (args))) == NULL)
 650     error (_("Not found: checkpoint id %s"), args);
 651
 652   linux_fork_context (fp, from_tty);
 653 }
 654
 655 void
 656 _initialize_linux_fork (void)
 657 {
 658   init_fork_list ();
 659
 660   /* Set/show detach-on-fork: user-settable mode.  */
 661
 662   add_setshow_boolean_cmd ("detach-on-fork", class_obscure, &detach_fork, _("\
 663 Set whether gdb will detach the child of a fork."), _("\
 664 Show whether gdb will detach the child of a fork."), _("\
 665 Tells gdb whether to detach the child of a fork."),
 666                            NULL, NULL, &setlist, &showlist);
 667
 668   /* Set/show restart-auto-finish: user-settable count.  Causes the
 669      first "restart" of a fork to do some number of "finish" commands
 670      before returning to user.
 671
 672      Useful because otherwise the virgin fork process will be stopped
 673      somewhere in the un-interesting fork system call.  */
 674
 675   /* Checkpoint command: create a fork of the inferior process
 676      and set it aside for later debugging.  */
 677
 678   add_com ("checkpoint", class_obscure, checkpoint_command, _("\
 679 Fork a duplicate process (experimental)."));
 680
 681   /* Restart command: restore the context of a specified fork
 682      process.  May be used for "program forks" as well as for
 683      "debugger forks" (checkpoints).  */
 684
 685   add_com ("restart", class_obscure, restart_command, _("\
 686 restart <n>: restore program context from a checkpoint.\n\
 687 Argument 'n' is checkpoint ID, as displayed by 'info checkpoints'."));
 688
 689   /* Delete checkpoint command: kill the process and remove it from
 690      fork list.  */
 691
 692   add_cmd ("checkpoint", class_obscure, delete_fork_command, _("\
 693 Delete a fork/checkpoint (experimental)."),
 694            &deletelist);
 695
 696   /* Detach-checkpoint command: release the process to run independantly,
 697      and remove it from the fork list.  */
 698
 699   add_com ("detach-checkpoint", class_obscure, detach_fork_command, _("\
 700 Detach from a fork/checkpoint (experimental)."));
 701
 702   /* Info checkpoints command: list all forks/checkpoints
 703      currently under gdb's control.  */
 704
 705   add_info ("checkpoints", info_forks_command,
 706             _("IDs of currently known forks/checkpoints."));
 707
 708   /* Command aliases (let "fork" and "checkpoint" be used
 709      interchangeably).  */
 710
 711   add_alias_cmd ("fork", "checkpoint", class_obscure, 1, &deletelist);
 712   add_com_alias ("detach-fork", "detach-checkpoint", class_obscure, 1);
 713   add_info_alias ("forks", "checkpoints", 0);
 714
 715   /* "fork <n>" (by analogy to "thread <n>").  */
 716   add_com ("fork", class_obscure, fork_command, _("\
 717 fork <n>: Switch between forked processes.\n\
 718 Argument 'n' is fork ID, as displayed by 'info forks'."));
 719
 720   /* "process <proc id>" as opposed to "fork <fork id>".  */
 721   add_com ("process", class_obscure, process_command, _("\
 722 process <pid>: Switch between forked processes.\n\
 723 Argument 'pid' is process ID, as displayed by 'info forks' or 'shell ps'."));
 724 }