gdb/linux-fork.c

   1 /* GNU/Linux native-dependent code for debugging multiple forks.
   2
   3    Copyright (C) 2005, 2006, 2007 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   stop_pc = read_pc ();
 254   nullify_last_target_wait_ptid ();
 255
 256   /* Now restore the file positions of open file descriptors.  */
 257   if (fp->filepos)
 258     {
 259       for (i = 0; i <= fp->maxfd; i++)
 260         if (fp->filepos[i] != (off_t) -1)
 261           call_lseek (i, fp->filepos[i], SEEK_SET);
 262       /* NOTE: I can get away with using SEEK_SET and SEEK_CUR because
 263          this is native-only.  If it ever has to be cross, we'll have
 264          to rethink this.  */
 265     }
 266 }
 267
 268 /* Save infrun state for the fork PTID.
 269    Exported for use by linux child_follow_fork.  */
 270
 271 extern void
 272 fork_save_infrun_state (struct fork_info *fp, int clobber_regs)
 273 {
 274   char path[MAXPATHLEN];
 275   struct dirent *de;
 276   DIR *d;
 277
 278   if (fp->savedregs)
 279     regcache_xfree (fp->savedregs);
 280
 281   fp->savedregs = regcache_dup (current_regcache);
 282   fp->clobber_regs = clobber_regs;
 283   fp->pc = read_pc ();
 284
 285   if (clobber_regs)
 286     {
 287       /* Now save the 'state' (file position) of all open file descriptors.
 288          Unfortunately fork does not take care of that for us...  */
 289       snprintf (path, MAXPATHLEN, "/proc/%ld/fd", (long) PIDGET (fp->ptid));
 290       if ((d = opendir (path)) != NULL)
 291         {
 292           long tmp;
 293
 294           fp->maxfd = 0;
 295           while ((de = readdir (d)) != NULL)
 296             {
 297               /* Count open file descriptors (actually find highest
 298                  numbered).  */
 299               tmp = strtol (&de->d_name[0], NULL, 10);
 300               if (fp->maxfd < tmp)
 301                 fp->maxfd = tmp;
 302             }
 303           /* Allocate array of file positions.  */
 304           fp->filepos = xrealloc (fp->filepos,
 305                                   (fp->maxfd + 1) * sizeof (*fp->filepos));
 306
 307           /* Initialize to -1 (invalid).  */
 308           for (tmp = 0; tmp <= fp->maxfd; tmp++)
 309             fp->filepos[tmp] = -1;
 310
 311           /* Now find actual file positions.  */
 312           rewinddir (d);
 313           while ((de = readdir (d)) != NULL)
 314             if (isdigit (de->d_name[0]))
 315               {
 316                 tmp = strtol (&de->d_name[0], NULL, 10);
 317                 fp->filepos[tmp] = call_lseek (tmp, 0, SEEK_CUR);
 318               }
 319           closedir (d);
 320         }
 321     }
 322 }
 323
 324 /* Kill 'em all, let God sort 'em out...  */
 325
 326 extern void
 327 linux_fork_killall (void)
 328 {
 329   /* Walk list and kill every pid.  No need to treat the
 330      current inferior_ptid as special (we do not return a
 331      status for it) -- however any process may be a child
 332      or a parent, so may get a SIGCHLD from a previously
 333      killed child.  Wait them all out.  */
 334   struct fork_info *fp;
 335   pid_t pid, ret;
 336   int status;
 337
 338   for (fp = fork_list; fp; fp = fp->next)
 339     {
 340       pid = PIDGET (fp->ptid);
 341       do {
 342         ptrace (PT_KILL, pid, 0, 0);
 343         ret = waitpid (pid, &status, 0);
 344         /* We might get a SIGCHLD instead of an exit status.  This is
 345          aggravated by the first kill above - a child has just
 346          died.  MVS comment cut-and-pasted from linux-nat.  */
 347       } while (ret == pid && WIFSTOPPED (status));
 348     }
 349   init_fork_list ();    /* Clear list, prepare to start fresh.  */
 350 }
 351
 352 /* The current inferior_ptid has exited, but there are other viable
 353    forks to debug.  Delete the exiting one and context-switch to the
 354    first available.  */
 355
 356 extern void
 357 linux_fork_mourn_inferior (void)
 358 {
 359   /* Wait just one more time to collect the inferior's exit status.
 360      Do not check whether this succeeds though, since we may be
 361      dealing with a process that we attached to.  Such a process will
 362      only report its exit status to its original parent.  */
 363   int status;
 364
 365   waitpid (ptid_get_pid (inferior_ptid), &status, 0);
 366
 367   /* OK, presumably inferior_ptid is the one who has exited.
 368      We need to delete that one from the fork_list, and switch
 369      to the next available fork.  */
 370   delete_fork (inferior_ptid);
 371
 372   /* There should still be a fork - if there's only one left,
 373      delete_fork won't remove it, because we haven't updated
 374      inferior_ptid yet.  */
 375   gdb_assert (fork_list);
 376
 377   fork_load_infrun_state (fork_list);
 378   printf_filtered (_("[Switching to %s]\n"),
 379                    target_pid_to_str (inferior_ptid));
 380
 381   /* If there's only one fork, switch back to non-fork mode.  */
 382   if (fork_list->next == NULL)
 383     delete_fork (inferior_ptid);
 384 }
 385
 386 /* Fork list <-> user interface.  */
 387
 388 static void
 389 delete_fork_command (char *args, int from_tty)
 390 {
 391   ptid_t ptid;
 392
 393   if (!args || !*args)
 394     error (_("Requires argument (fork/checkpoint id to delete)"));
 395
 396   ptid = fork_id_to_ptid (parse_and_eval_long (args));
 397   if (ptid_equal (ptid, minus_one_ptid))
 398     error (_("No such fork/checkpoint id, %s"), args);
 399
 400   if (ptid_equal (ptid, inferior_ptid))
 401     error (_("Please switch to another fork/checkpoint before deleting the current one"));
 402
 403   if (ptrace (PTRACE_KILL, PIDGET (ptid), 0, 0))
 404     error (_("Unable to kill pid %s"), target_tid_to_str (ptid));
 405
 406   if (from_tty)
 407     printf_filtered (_("Killed %s\n"), target_pid_to_str (ptid));
 408
 409   delete_fork (ptid);
 410 }
 411
 412 static void
 413 detach_fork_command (char *args, int from_tty)
 414 {
 415   ptid_t ptid;
 416
 417   if (!args || !*args)
 418     error (_("Requires argument (fork id to detach)"));
 419
 420   ptid = fork_id_to_ptid (parse_and_eval_long (args));
 421   if (ptid_equal (ptid, minus_one_ptid))
 422     error (_("No such fork id, %s"), args);
 423
 424   if (ptid_equal (ptid, inferior_ptid))
 425     error (_("Please switch to another fork before detaching the current one"));
 426
 427   if (ptrace (PTRACE_DETACH, PIDGET (ptid), 0, 0))
 428     error (_("Unable to detach %s"), target_pid_to_str (ptid));
 429
 430   if (from_tty)
 431     printf_filtered (_("Detached %s\n"), target_pid_to_str (ptid));
 432
 433   delete_fork (ptid);
 434 }
 435
 436 /* Print information about currently known forks.  */
 437
 438 static void
 439 info_forks_command (char *arg, int from_tty)
 440 {
 441   struct frame_info *cur_frame;
 442   struct symtab_and_line sal;
 443   struct symtab *cur_symtab;
 444   struct fork_info *fp;
 445   int cur_line;
 446   ULONGEST pc;
 447   int requested = -1;
 448   struct fork_info *printed = NULL;
 449
 450   if (arg && *arg)
 451     requested = (int) parse_and_eval_long (arg);
 452
 453   for (fp = fork_list; fp; fp = fp->next)
 454     {
 455       if (requested > 0 && fp->num != requested)
 456         continue;
 457
 458       printed = fp;
 459       if (ptid_equal (fp->ptid, inferior_ptid))
 460         {
 461           printf_filtered ("* ");
 462           pc = read_pc ();
 463         }
 464       else
 465         {
 466           printf_filtered ("  ");
 467           pc = fp->pc;
 468         }
 469       printf_filtered ("%d %s", fp->num, target_pid_to_str (fp->ptid));
 470       if (fp->num == 0)
 471         printf_filtered (_(" (main process)"));
 472       printf_filtered (_(" at "));
 473       deprecated_print_address_numeric (pc, 1, gdb_stdout);
 474
 475       sal = find_pc_line (pc, 0);
 476       if (sal.symtab)
 477         {
 478           char *tmp = strrchr (sal.symtab->filename, '/');
 479
 480           if (tmp)
 481             printf_filtered (_(", file %s"), tmp + 1);
 482           else
 483             printf_filtered (_(", file %s"), sal.symtab->filename);
 484         }
 485       if (sal.line)
 486         printf_filtered (_(", line %d"), sal.line);
 487       if (!sal.symtab && !sal.line)
 488         {
 489           struct minimal_symbol *msym;
 490
 491           msym = lookup_minimal_symbol_by_pc (pc);
 492           if (msym)
 493             printf_filtered (", <%s>", SYMBOL_LINKAGE_NAME (msym));
 494         }
 495
 496       putchar_filtered ('\n');
 497     }
 498   if (printed == NULL)
 499     {
 500       if (requested > 0)
 501         printf_filtered (_("No fork number %d.\n"), requested);
 502       else
 503         printf_filtered (_("No forks.\n"));
 504     }
 505 }
 506
 507 /* Save/restore mode variable 'detach_fork':
 508    We need to temporarily take over this mode variable, while
 509    preserving the user-specified state, and make sure that it
 510    gets restored in case of error.
 511
 512    The int pointer that we use comes from the caller, so we can
 513    be called more than once (even though currently we don't need to).  */
 514
 515 static void
 516 restore_detach_fork (void *arg)
 517 {
 518   detach_fork = *(int *) arg;
 519 }
 520
 521 static struct cleanup *
 522 save_detach_fork (int *saved_val)
 523 {
 524   *saved_val = detach_fork;
 525   return make_cleanup (restore_detach_fork, (void *) saved_val);
 526 }
 527
 528 static void
 529 checkpoint_command (char *args, int from_tty)
 530 {
 531   struct target_waitstatus last_target_waitstatus;
 532   ptid_t last_target_ptid;
 533   struct value *fork_fn = NULL, *ret;
 534   struct fork_info *fp;
 535   pid_t retpid;
 536   struct cleanup *old_chain;
 537   long i;
 538   /* Make this temp var static, 'cause it's used in the error context.  */
 539   static int temp_detach_fork;
 540
 541   /* Make the inferior fork, record its (and gdb's) state.  */
 542
 543   if (lookup_minimal_symbol ("fork", NULL, NULL) != NULL)
 544     fork_fn = find_function_in_inferior ("fork");
 545   if (!fork_fn)
 546     if (lookup_minimal_symbol ("_fork", NULL, NULL) != NULL)
 547       fork_fn = find_function_in_inferior ("fork");
 548   if (!fork_fn)
 549     error (_("checkpoint: can't find fork function in inferior."));
 550
 551   ret = value_from_longest (builtin_type_int, 0);
 552   old_chain = save_detach_fork (&temp_detach_fork);
 553   detach_fork = 0;
 554   ret = call_function_by_hand (fork_fn, 0, &ret);
 555   do_cleanups (old_chain);
 556   if (!ret)     /* Probably can't happen.  */
 557     error (_("checkpoint: call_function_by_hand returned null."));
 558
 559   retpid = value_as_long (ret);
 560   get_last_target_status (&last_target_ptid, &last_target_waitstatus);
 561   if (from_tty)
 562     {
 563       int parent_pid;
 564
 565       printf_filtered (_("checkpoint: fork returned pid %ld.\n"),
 566                        (long) retpid);
 567       if (info_verbose)
 568         {
 569           parent_pid = ptid_get_lwp (last_target_ptid);
 570           if (parent_pid == 0)
 571             parent_pid = ptid_get_pid (last_target_ptid);
 572           printf_filtered (_("   gdb says parent = %ld.\n"),
 573                            (long) parent_pid);
 574         }
 575     }
 576
 577   fp = find_fork_pid (retpid);
 578   if (!fp)
 579     error (_("Failed to find new fork"));
 580   fork_save_infrun_state (fp, 1);
 581 }
 582
 583 static void
 584 linux_fork_context (struct fork_info *newfp, int from_tty)
 585 {
 586   /* Now we attempt to switch processes.  */
 587   struct fork_info *oldfp = find_fork_ptid (inferior_ptid);
 588   ptid_t ptid;
 589   int id, i;
 590
 591   if (!newfp)
 592     error (_("No such fork/process"));
 593
 594   if (!oldfp)
 595     oldfp = add_fork (ptid_get_pid (inferior_ptid));
 596
 597   fork_save_infrun_state (oldfp, 1);
 598   fork_load_infrun_state (newfp);
 599
 600   printf_filtered (_("Switching to %s\n"),
 601                    target_pid_to_str (inferior_ptid));
 602
 603   print_stack_frame (get_selected_frame (NULL), 1, SRC_AND_LOC);
 604 }
 605
 606 /* Switch inferior process (fork) context, by process id.  */
 607 static void
 608 process_command (char *args, int from_tty)
 609 {
 610   struct fork_info *fp;
 611
 612   if (!args || !*args)
 613     error (_("Requires argument (process id to switch to)"));
 614
 615   if ((fp = find_fork_pid (parse_and_eval_long (args))) == NULL)
 616     error (_("Not found: process id %s"), args);
 617
 618   linux_fork_context (fp, from_tty);
 619 }
 620
 621 /* Switch inferior process (fork) context, by fork id.  */
 622 static void
 623 fork_command (char *args, int from_tty)
 624 {
 625   struct fork_info *fp;
 626
 627   if (!args || !*args)
 628     error (_("Requires argument (fork id to switch to)"));
 629
 630   if ((fp = find_fork_id (parse_and_eval_long (args))) == NULL)
 631     error (_("Not found: fork id %s"), args);
 632
 633   linux_fork_context (fp, from_tty);
 634 }
 635
 636 /* Switch inferior process (fork) context, by checkpoint id.  */
 637 static void
 638 restart_command (char *args, int from_tty)
 639 {
 640   struct fork_info *fp;
 641
 642   if (!args || !*args)
 643     error (_("Requires argument (checkpoint id to restart)"));
 644
 645   if ((fp = find_fork_id (parse_and_eval_long (args))) == NULL)
 646     error (_("Not found: checkpoint id %s"), args);
 647
 648   linux_fork_context (fp, from_tty);
 649 }
 650
 651 void
 652 _initialize_linux_fork (void)
 653 {
 654   init_fork_list ();
 655
 656   /* Set/show detach-on-fork: user-settable mode.  */
 657
 658   add_setshow_boolean_cmd ("detach-on-fork", class_obscure, &detach_fork, _("\
 659 Set whether gdb will detach the child of a fork."), _("\
 660 Show whether gdb will detach the child of a fork."), _("\
 661 Tells gdb whether to detach the child of a fork."),
 662                            NULL, NULL, &setlist, &showlist);
 663
 664   /* Set/show restart-auto-finish: user-settable count.  Causes the
 665      first "restart" of a fork to do some number of "finish" commands
 666      before returning to user.
 667
 668      Useful because otherwise the virgin fork process will be stopped
 669      somewhere in the un-interesting fork system call.  */
 670
 671   /* Checkpoint command: create a fork of the inferior process
 672      and set it aside for later debugging.  */
 673
 674   add_com ("checkpoint", class_obscure, checkpoint_command, _("\
 675 Fork a duplicate process (experimental)."));
 676
 677   /* Restart command: restore the context of a specified fork
 678      process.  May be used for "program forks" as well as for
 679      "debugger forks" (checkpoints).  */
 680
 681   add_com ("restart", class_obscure, restart_command, _("\
 682 restart <n>: restore program context from a checkpoint.\n\
 683 Argument 'n' is checkpoint ID, as displayed by 'info checkpoints'."));
 684
 685   /* Delete checkpoint command: kill the process and remove it from
 686      fork list.  */
 687
 688   add_cmd ("checkpoint", class_obscure, delete_fork_command, _("\
 689 Delete a fork/checkpoint (experimental)."),
 690            &deletelist);
 691
 692   /* Detach checkpoint command: release the process to run independently,
 693      and remove it from the fork list.  */
 694
 695   add_cmd ("checkpoint", class_obscure, detach_fork_command, _("\
 696 Detach from a fork/checkpoint (experimental)."),
 697            &detachlist);
 698
 699   /* Info checkpoints command: list all forks/checkpoints
 700      currently under gdb's control.  */
 701
 702   add_info ("checkpoints", info_forks_command,
 703             _("IDs of currently known forks/checkpoints."));
 704
 705   /* Command aliases (let "fork" and "checkpoint" be used
 706      interchangeably).  */
 707
 708   add_alias_cmd ("fork", "checkpoint", class_obscure, 1, &deletelist);
 709   add_alias_cmd ("fork", "checkpoint", class_obscure, 1, &detachlist);
 710   add_info_alias ("forks", "checkpoints", 0);
 711
 712   /* "fork <n>" (by analogy to "thread <n>").  */
 713   add_com ("fork", class_obscure, fork_command, _("\
 714 fork <n>: Switch between forked processes.\n\
 715 Argument 'n' is fork ID, as displayed by 'info forks'."));
 716
 717   /* "process <proc id>" as opposed to "fork <fork id>".  */
 718   add_com ("process", class_obscure, process_command, _("\
 719 process <pid>: Switch between forked processes.\n\
 720 Argument 'pid' is process ID, as displayed by 'info forks' or 'shell ps'."));
 721 }