man2/execve.2

   1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
   2 .\" and Copyright (c) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\"
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  25 .\"
  26 .\" Modified by Michael Haardt <michael@moria.de>
  27 .\" Modified 1993-07-21 by Rik Faith <faith@cs.unc.edu>
  28 .\" Modified 1994-08-21 by Michael Chastain <mec@shell.portal.com>:
  29 .\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>
  30 .\" Modified 1999-11-12 by Urs Thuermann <urs@isnogud.escape.de>
  31 .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
  32 .\" 2006-09-04 Michael Kerrisk <mtk.manpages@gmail.com>
  33 .\"     Added list of process attributes that are not preserved on exec().
  34 .\" 2007-09-14 Ollie Wild <aaw@google.com>, mtk
  35 .\"     Add text describing limits on command-line arguments + environment
  36 .\"
  37 .TH EXECVE 2 2014-10-02 "Linux" "Linux Programmer's Manual"
  38 .SH NAME
  39 execve \- execute program
  40 .SH SYNOPSIS
  41 .B #include <unistd.h>
  42 .sp
  43 .BI "int execve(const char *" filename ", char *const " argv "[], "
  44 .br
  45 .BI "           char *const " envp []);
  46 .SH DESCRIPTION
  47 .BR execve ()
  48 executes the program pointed to by \fIfilename\fP.
  49 \fIfilename\fP must be either a binary executable, or a script
  50 starting with a line of the form:
  51
  52 .in +4n
  53 .nf
  54 \fB#!\fP \fIinterpreter \fP[optional-arg]
  55 .fi
  56 .in
  57
  58 For details of the latter case, see "Interpreter scripts" below.
  59
  60 \fIargv\fP is an array of argument strings passed to the new program.
  61 By convention, the first of these strings should contain the filename
  62 associated with the file being executed.
  63 \fIenvp\fP is an array of strings, conventionally of the form
  64 \fBkey=value\fP, which are passed as environment to the new program.
  65 Both \fIargv\fP and \fIenvp\fP must be terminated by a null pointer.
  66 The argument vector and environment can be accessed by the
  67 called program's main function, when it is defined as:
  68
  69 .in +4n
  70 .nf
  71 int main(int argc, char *argv[], char *envp[])
  72 .fi
  73 .in
  74
  75 .BR execve ()
  76 does not return on success, and the text, data, bss, and
  77 stack of the calling process are overwritten by that of the program
  78 loaded.
  79
  80 If the current program is being ptraced, a \fBSIGTRAP\fP is sent to it
  81 after a successful
  82 .BR execve ().
  83
  84 If the set-user-ID bit is set on the program file pointed to by
  85 \fIfilename\fP,
  86 and the underlying filesystem is not mounted
  87 .I nosuid
  88 (the
  89 .B MS_NOSUID
  90 flag for
  91 .BR mount (2)),
  92 and the calling process is not being ptraced,
  93 then the effective user ID of the calling process is changed
  94 to that of the owner of the program file.
  95 Similarly, when the set-group-ID
  96 bit of the program file is set the effective group ID of the calling
  97 process is set to the group of the program file.
  98
  99 The effective user ID of the process is copied to the saved set-user-ID;
 100 similarly, the effective group ID is copied to the saved set-group-ID.
 101 This copying takes place after any effective ID changes that occur
 102 because of the set-user-ID and set-group-ID permission bits.
 103
 104 If the executable is an a.out dynamically linked
 105 binary executable containing
 106 shared-library stubs, the Linux dynamic linker
 107 .BR ld.so (8)
 108 is called at the start of execution to bring
 109 needed shared libraries into memory
 110 and link the executable with them.
 111
 112 If the executable is a dynamically linked ELF executable, the
 113 interpreter named in the PT_INTERP segment is used to load the needed
 114 shared libraries.
 115 This interpreter is typically
 116 .I /lib/ld-linux.so.2
 117 for binaries linked with glibc.
 118
 119 All process attributes are preserved during an
 120 .BR execve (),
 121 except the following:
 122 .IP * 3
 123 The dispositions of any signals that are being caught are
 124 reset to the default
 125 .RB ( signal (7)).
 126 .IP *
 127 Any alternate signal stack is not preserved
 128 .RB ( sigaltstack (2)).
 129 .IP *
 130 Memory mappings are not preserved
 131 .RB ( mmap (2)).
 132 .IP *
 133 Attached System\ V shared memory segments are detached
 134 .RB ( shmat (2)).
 135 .IP *
 136 POSIX shared memory regions are unmapped
 137 .RB ( shm_open (3)).
 138 .IP *
 139 Open POSIX message queue descriptors are closed
 140 .RB ( mq_overview (7)).
 141 .IP *
 142 Any open POSIX named semaphores are closed
 143 .RB ( sem_overview (7)).
 144 .IP *
 145 POSIX timers are not preserved
 146 .RB ( timer_create (2)).
 147 .IP *
 148 Any open directory streams are closed
 149 .RB ( opendir (3)).
 150 .IP *
 151 Memory locks are not preserved
 152 .RB ( mlock (2),
 153 .BR mlockall (2)).
 154 .IP *
 155 Exit handlers are not preserved
 156 .RB ( atexit (3),
 157 .BR on_exit (3)).
 158 .IP *
 159 The floating-point environment is reset to the default (see
 160 .BR fenv (3)).
 161 .PP
 162 The process attributes in the preceding list are all specified
 163 in POSIX.1-2001.
 164 The following Linux-specific process attributes are also
 165 not preserved during an
 166 .BR execve ():
 167 .IP * 3
 168 The
 169 .BR prctl (2)
 170 .B PR_SET_DUMPABLE
 171 flag is set,
 172 unless a set-user-ID or set-group ID program is being executed,
 173 in which case it is cleared.
 174 .IP *
 175 The
 176 .BR prctl (2)
 177 .B PR_SET_KEEPCAPS
 178 flag is cleared.
 179 .IP *
 180 (Since Linux 2.4.36 / 2.6.23)
 181 If a set-user-ID or set-group-ID program is being executed,
 182 then the parent death signal set by
 183 .BR prctl (2)
 184 .B PR_SET_PDEATHSIG
 185 flag is cleared.
 186 .IP *
 187 The process name, as set by
 188 .BR prctl (2)
 189 .B PR_SET_NAME
 190 (and displayed by
 191 .IR "ps\ \-o comm" ),
 192 is reset to the name of the new executable file.
 193 .IP *
 194 The
 195 .B SECBIT_KEEP_CAPS
 196 .I securebits
 197 flag is cleared.
 198 See
 199 .BR capabilities (7).
 200 .IP *
 201 The termination signal is reset to
 202 .B SIGCHLD
 203 (see
 204 .BR clone (2)).
 205 .PP
 206 Note the following further points:
 207 .IP * 3
 208 All threads other than the calling thread are destroyed during an
 209 .BR execve ().
 210 Mutexes, condition variables, and other pthreads objects are not preserved.
 211 .IP *
 212 The equivalent of \fIsetlocale(LC_ALL, "C")\fP
 213 is executed at program start-up.
 214 .IP *
 215 POSIX.1-2001 specifies that the dispositions of any signals that
 216 are ignored or set to the default are left unchanged.
 217 POSIX.1-2001 specifies one exception: if
 218 .B SIGCHLD
 219 is being ignored,
 220 then an implementation may leave the disposition unchanged or
 221 reset it to the default; Linux does the former.
 222 .IP *
 223 Any outstanding asynchronous I/O operations are canceled
 224 .RB ( aio_read (3),
 225 .BR aio_write (3)).
 226 .IP *
 227 For the handling of capabilities during
 228 .BR execve (),
 229 see
 230 .BR capabilities (7).
 231 .IP *
 232 By default, file descriptors remain open across an
 233 .BR execve ().
 234 File descriptors that are marked close-on-exec are closed;
 235 see the description of
 236 .B FD_CLOEXEC
 237 in
 238 .BR fcntl (2).
 239 (If a file descriptor is closed, this will cause the release
 240 of all record locks obtained on the underlying file by this process.
 241 See
 242 .BR fcntl (2)
 243 for details.)
 244 POSIX.1-2001 says that if file descriptors 0, 1, and 2 would
 245 otherwise be closed after a successful
 246 .BR execve (),
 247 and the process would gain privilege because the set-user_ID or
 248 set-group_ID permission bit was set on the executed file,
 249 then the system may open an unspecified file for each of these
 250 file descriptors.
 251 As a general principle, no portable program, whether privileged or not,
 252 can assume that these three file descriptors will remain
 253 closed across an
 254 .BR execve ().
 255 .\" On Linux it appears that these file descriptors are
 256 .\" always open after an execve(), and it looks like
 257 .\" Solaris 8 and FreeBSD 6.1 are the same. -- mtk, 30 Apr 2007
 258 .SS Interpreter scripts
 259 An interpreter script is a text file that has execute
 260 permission enabled and whose first line is of the form:
 261
 262 .in +4n
 263 .nf
 264 \fB#!\fP \fIinterpreter \fP[optional-arg]
 265 .fi
 266 .in
 267
 268 The
 269 .I interpreter
 270 must be a valid pathname for an
 271 executable which is not itself a script.
 272 If the
 273 .I filename
 274 argument of
 275 .BR execve ()
 276 specifies an interpreter script, then
 277 .I interpreter
 278 will be invoked with the following arguments:
 279
 280 .in +4n
 281 .nf
 282 \fIinterpreter\fP [optional-arg] \fIfilename\fP arg...
 283 .fi
 284 .in
 285
 286 where
 287 .I arg...
 288 is the series of words pointed to by the
 289 .I argv
 290 argument of
 291 .BR execve (),
 292 starting at
 293 .IR argv [1].
 294
 295 For portable use,
 296 .I optional-arg
 297 should either be absent, or be specified as a single word (i.e., it
 298 should not contain white space); see NOTES below.
 299 .SS Limits on size of arguments and environment
 300 Most UNIX implementations impose some limit on the total size
 301 of the command-line argument
 302 .RI ( argv )
 303 and environment
 304 .RI ( envp )
 305 strings that may be passed to a new program.
 306 POSIX.1 allows an implementation to advertise this limit using the
 307 .B ARG_MAX
 308 constant (either defined in
 309 .I <limits.h>
 310 or available at run time using the call
 311 .IR "sysconf(_SC_ARG_MAX)" ).
 312
 313 On Linux prior to kernel 2.6.23, the memory used to store the
 314 environment and argument strings was limited to 32 pages
 315 (defined by the kernel constant
 316 .BR MAX_ARG_PAGES ).
 317 On architectures with a 4-kB page size,
 318 this yields a maximum size of 128 kB.
 319
 320 On kernel 2.6.23 and later, most architectures support a size limit
 321 derived from the soft
 322 .B RLIMIT_STACK
 323 resource limit (see
 324 .BR getrlimit (2))
 325 that is in force at the time of the
 326 .BR execve ()
 327 call.
 328 (Architectures with no memory management unit are excepted:
 329 they maintain the limit that was in effect before kernel 2.6.23.)
 330 This change allows programs to have a much larger
 331 argument and/or environment list.
 332 .\" For some background on the changes to ARG_MAX in kernels 2.6.23 and
 333 .\" 2.6.25, see:
 334 .\"     http://sourceware.org/bugzilla/show_bug.cgi?id=5786
 335 .\"     http://bugzilla.kernel.org/show_bug.cgi?id=10095
 336 .\"     http://thread.gmane.org/gmane.linux.kernel/646709/focus=648101,
 337 .\"     checked into 2.6.25 as commit a64e715fc74b1a7dcc5944f848acc38b2c4d4ee2.
 338 For these architectures, the total size is limited to 1/4 of the allowed
 339 stack size.
 340 (Imposing the 1/4-limit
 341 ensures that the new program always has some stack space.)
 342 .\" Ollie: That doesn't include the lists of pointers, though,
 343 .\" so the actual usage is a bit higher (1 pointer per argument).
 344 Since Linux 2.6.25,
 345 the kernel places a floor of 32 pages on this size limit,
 346 so that, even when
 347 .BR RLIMIT_STACK
 348 is set very low,
 349 applications are guaranteed to have at least as much argument and
 350 environment space as was provided by Linux 2.6.23 and earlier.
 351 (This guarantee was not provided in Linux 2.6.23 and 2.6.24.)
 352 Additionally, the limit per string is 32 pages (the kernel constant
 353 .BR MAX_ARG_STRLEN ),
 354 and the maximum number of strings is 0x7FFFFFFF.
 355 .SH RETURN VALUE
 356 On success,
 357 .BR execve ()
 358 does not return, on error \-1 is returned, and
 359 .I errno
 360 is set appropriately.
 361 .SH ERRORS
 362 .TP
 363 .B E2BIG
 364 The total number of bytes in the environment
 365 .RI ( envp )
 366 and argument list
 367 .RI ( argv )
 368 is too large.
 369 .TP
 370 .B EACCES
 371 Search permission is denied on a component of the path prefix of
 372 .I filename
 373 or the name of a script interpreter.
 374 (See also
 375 .BR path_resolution (7).)
 376 .TP
 377 .B EACCES
 378 The file or a script interpreter is not a regular file.
 379 .TP
 380 .B EACCES
 381 Execute permission is denied for the file or a script or ELF interpreter.
 382 .TP
 383 .B EACCES
 384 The filesystem is mounted
 385 .IR noexec .
 386 .TP
 387 .BR EAGAIN " (since Linux 3.1)"
 388 .\" commit 72fa59970f8698023045ab0713d66f3f4f96945c
 389 Having changed its real UID using one of the
 390 .BR set*uid ()
 391 calls, the caller was\(emand is now still\(emabove its
 392 .BR RLIMIT_NPROC
 393 resource limit (see
 394 .BR setrlimit (2)).
 395 For a more detailed explanation of this error, see NOTES.
 396 .TP
 397 .B EFAULT
 398 .I filename
 399 or one of the pointers in the vectors
 400 .I argv
 401 or
 402 .I envp
 403 points outside your accessible address space.
 404 .TP
 405 .B EINVAL
 406 An ELF executable had more than one PT_INTERP segment (i.e., tried to
 407 name more than one interpreter).
 408 .TP
 409 .B EIO
 410 An I/O error occurred.
 411 .TP
 412 .B EISDIR
 413 An ELF interpreter was a directory.
 414 .TP
 415 .B ELIBBAD
 416 An ELF interpreter was not in a recognized format.
 417 .TP
 418 .B ELOOP
 419 Too many symbolic links were encountered in resolving
 420 .I filename
 421 or the name of a script or ELF interpreter.
 422 .TP
 423 .B EMFILE
 424 The process has the maximum number of files open.
 425 .TP
 426 .B ENAMETOOLONG
 427 .I filename
 428 is too long.
 429 .TP
 430 .B ENFILE
 431 The system limit on the total number of open files has been reached.
 432 .TP
 433 .B ENOENT
 434 The file
 435 .I filename
 436 or a script or ELF interpreter does not exist, or a shared library
 437 needed for file or interpreter cannot be found.
 438 .TP
 439 .B ENOEXEC
 440 An executable is not in a recognized format, is for the wrong
 441 architecture, or has some other format error that means it cannot be
 442 executed.
 443 .TP
 444 .B ENOMEM
 445 Insufficient kernel memory was available.
 446 .TP
 447 .B ENOTDIR
 448 A component of the path prefix of
 449 .I filename
 450 or a script or ELF interpreter is not a directory.
 451 .TP
 452 .B EPERM
 453 The filesystem is mounted
 454 .IR nosuid ,
 455 the user is not the superuser,
 456 and the file has the set-user-ID or set-group-ID bit set.
 457 .TP
 458 .B EPERM
 459 The process is being traced, the user is not the superuser and the
 460 file has the set-user-ID or set-group-ID bit set.
 461 .TP
 462 .B ETXTBSY
 463 Executable was open for writing by one or more processes.
 464 .SH CONFORMING TO
 465 SVr4, 4.3BSD, POSIX.1-2001.
 466 POSIX.1-2001 does not document the #!  behavior
 467 but is otherwise compatible.
 468 .\" SVr4 documents additional error
 469 .\" conditions EAGAIN, EINTR, ELIBACC, ENOLINK, EMULTIHOP; POSIX does not
 470 .\" document ETXTBSY, EPERM, EFAULT, ELOOP, EIO, ENFILE, EMFILE, EINVAL,
 471 .\" EISDIR or ELIBBAD error conditions.
 472 .SH NOTES
 473 Set-user-ID and set-group-ID processes can not be
 474 .BR ptrace (2)d.
 475
 476 The result of mounting a filesystem
 477 .I nosuid
 478 varies across Linux kernel versions:
 479 some will refuse execution of set-user-ID and set-group-ID
 480 executables when this would
 481 give the user powers she did not have already (and return
 482 .BR EPERM ),
 483 some will just ignore the set-user-ID and set-group-ID bits and
 484 .BR exec ()
 485 successfully.
 486 On Linux,
 487 .I argv
 488 and
 489 .I envp
 490 can be specified as NULL.
 491 In both cases, this has the same effect as specifying the argument
 492 as a pointer to a list containing a single null pointer.
 493 .B "Do not take advantage of this misfeature!"
 494 It is nonstandard and nonportable:
 495 on most other UNIX systems doing this will result in an error
 496 .RB ( EFAULT ).
 497 .\" e.g., EFAULT on Solaris 8 and FreeBSD 6.1; but
 498 .\" HP-UX 11 is like Linux -- mtk, Apr 2007
 499 .\" Bug filed 30 Apr 2007: http://bugzilla.kernel.org/show_bug.cgi?id=8408
 500 .\" Bug rejected (because fix would constitute an ABI change).
 501 .\"
 502
 503 POSIX.1-2001 says that values returned by
 504 .BR sysconf (3)
 505 should be invariant over the lifetime of a process.
 506 However, since Linux 2.6.23, if the
 507 .BR RLIMIT_STACK
 508 resource limit changes, then the value reported by
 509 .B _SC_ARG_MAX
 510 will also change,
 511 to reflect the fact that the limit on space for holding
 512 command-line arguments and environment variables has changed.
 513
 514 In most cases where
 515 .BR execve ()
 516 fails, control returns to the original executable image,
 517 and the caller of
 518 .BR execve ()
 519 can then handle the error.
 520 However, in (rare) cases (typically caused by resource exhaustion),
 521 failure may occur past the point of no return:
 522 the original executable image has been torn down,
 523 but the new image could not be completely built.
 524 In such cases, the kernel kills the process with a
 525 .BR SIGKILL
 526 signal.
 527 .\"
 528 .SS Interpreter scripts
 529 A maximum line length of 127 characters is allowed for the first line in
 530 an interpreter scripts.
 531
 532 The semantics of the
 533 .I optional-arg
 534 argument of an interpreter script vary across implementations.
 535 On Linux, the entire string following the
 536 .I interpreter
 537 name is passed as a single argument to the interpreter,
 538 and this string can include white space.
 539 However, behavior differs on some other systems.
 540 Some systems
 541 .\" e.g., Solaris 8
 542 use the first white space to terminate
 543 .IR optional-arg .
 544 On some systems,
 545 .\" e.g., FreeBSD before 6.0, but not FreeBSD 6.0 onward
 546 an interpreter script can have multiple arguments,
 547 and white spaces in
 548 .I optional-arg
 549 are used to delimit the arguments.
 550
 551 Linux ignores the set-user-ID and set-group-ID bits on scripts.
 552 .\"
 553 .\" .SH BUGS
 554 .\" Some Linux versions have failed to check permissions on ELF
 555 .\" interpreters.  This is a security hole, because it allows users to
 556 .\" open any file, such as a rewinding tape device, for reading.  Some
 557 .\" Linux versions have also had other security holes in
 558 .\" .BR execve ()
 559 .\" that could be exploited for denial of service by a suitably crafted
 560 .\" ELF binary. There are no known problems with 2.0.34 or 2.2.15.
 561 .SS execve() and EAGAIN
 562 A more detailed explanation of the
 563 .BR EAGAIN
 564 error that can occur (since Linux 3.1) when calling
 565 .BR execve ()
 566 is as follows.
 567
 568 The
 569 .BR EAGAIN
 570 error can occur when a
 571 .I preceding
 572 call to
 573 .BR setuid (2),
 574 .BR setreuid (2),
 575 or
 576 .BR setresuid (2)
 577 caused the real user ID of the process to change,
 578 and that change caused the process to exceed its
 579 .BR RLIMIT_NPROC
 580 resource limit (i.e., the number of processes belonging
 581 to the new real UID exceeds the resource limit).
 582 From Linux 2.6.0 to 3.0, this caused the
 583 .BR set*uid ()
 584 call to fail.
 585 (Prior to 2.6,
 586 .\" commit 909cc4ae86f3380152a18e2a3c44523893ee11c4
 587 the resource limit was not imposed on processes that
 588 changed their user IDs.)
 589
 590 Since Linux 3.1, the scenario just described no longer causes the
 591 .BR set*uid ()
 592 call to fail,
 593 because it too often led to security holes where buggy applications
 594 didn't check the return status and assumed
 595 that\(emif the caller had root privileges\(emthe call would always succeed.
 596 Instead, the
 597 .BR set*uid ()
 598 calls now successfully change the real UID,
 599 but the kernel sets an internal flag, named
 600 .BR PF_NPROC_EXCEEDED ,
 601 to note that the
 602 .BR RLIMIT_NPROC
 603 resource limit has been exceeded.
 604 If the
 605 .BR PF_NPROC_EXCEEDED
 606 flag is set and the resource limit is still
 607 exceeded at the time of a subsequent
 608 .BR execve ()
 609 call, that call fails with the error
 610 .BR EAGAIN .
 611 This kernel logic ensures that the
 612 .BR RLIMIT_NPROC
 613 resource limit is still enforced for the
 614 common privileged daemon workflow\(emnamely,
 615 .BR fork (2)
 616 +
 617 .BR set*uid ()
 618 +
 619 .BR execve ().
 620
 621 If the resource limit was not still exceeded at the time of the
 622 .BR execve ()
 623 call
 624 (because other processes belonging to this real UID terminated between the
 625 .BR set*uid ()
 626 call and the
 627 .BR execve ()
 628 call), then the
 629 .BR execve ()
 630 call succeeds and the kernel clears the
 631 .BR PF_NPROC_EXCEEDED
 632 process flag.
 633 The flag is also cleared if a subsequent call to
 634 .BR fork (2)
 635 by this process succeeds.
 636 .SS Historical
 637 With UNIX\ V6, the argument list of an
 638 .BR exec ()
 639 call was ended by 0,
 640 while the argument list of
 641 .I main
 642 was ended by \-1.
 643 Thus, this argument list was not directly usable in a further
 644 .BR exec ()
 645 call.
 646 Since UNIX\ V7, both are NULL.
 647 .\"
 648 .\" .SH BUGS
 649 .\" Some Linux versions have failed to check permissions on ELF
 650 .\" interpreters.  This is a security hole, because it allows users to
 651 .\" open any file, such as a rewinding tape device, for reading.  Some
 652 .\" Linux versions have also had other security holes in
 653 .\" .BR execve ()
 654 .\" that could be exploited for denial of service by a suitably crafted
 655 .\" ELF binary. There are no known problems with 2.0.34 or 2.2.15.
 656 .SH EXAMPLE
 657 The following program is designed to be execed by the second program below.
 658 It just echoes its command-line arguments, one per line.
 659
 660 .in +4n
 661 .nf
 662 /* myecho.c */
 663
 664 #include <stdio.h>
 665 #include <stdlib.h>
 666
 667 int
 668 main(int argc, char *argv[])
 669 {
 670     int j;
 671
 672     for (j = 0; j < argc; j++)
 673         printf("argv[%d]: %s\\n", j, argv[j]);
 674
 675     exit(EXIT_SUCCESS);
 676 }
 677 .fi
 678 .in
 679
 680 This program can be used to exec the program named in its command-line
 681 argument:
 682 .in +4n
 683 .nf
 684
 685 /* execve.c */
 686
 687 #include <stdio.h>
 688 #include <stdlib.h>
 689 #include <unistd.h>
 690
 691 int
 692 main(int argc, char *argv[])
 693 {
 694     char *newargv[] = { NULL, "hello", "world", NULL };
 695     char *newenviron[] = { NULL };
 696
 697     if (argc != 2) {
 698         fprintf(stderr, "Usage: %s <file\-to\-exec>\\n", argv[0]);
 699         exit(EXIT_FAILURE);
 700     }
 701
 702     newargv[0] = argv[1];
 703
 704     execve(argv[1], newargv, newenviron);
 705     perror("execve");   /* execve() only returns on error */
 706     exit(EXIT_FAILURE);
 707 }
 708 .fi
 709 .in
 710
 711 We can use the second program to exec the first as follows:
 712
 713 .in +4n
 714 .nf
 715 .RB "$" " cc myecho.c \-o myecho"
 716 .RB "$" " cc execve.c \-o execve"
 717 .RB "$" " ./execve ./myecho"
 718 argv[0]: ./myecho
 719 argv[1]: hello
 720 argv[2]: world
 721 .fi
 722 .in
 723
 724 We can also use these programs to demonstrate the use of a script
 725 interpreter.
 726 To do this we create a script whose "interpreter" is our
 727 .I myecho
 728 program:
 729
 730 .in +4n
 731 .nf
 732 .RB "$" " cat > script"
 733 .B #!./myecho script-arg
 734 .B ^D
 735 .RB "$" " chmod +x script"
 736 .fi
 737 .in
 738
 739 We can then use our program to exec the script:
 740
 741 .in +4n
 742 .nf
 743 .RB "$" " ./execve ./script"
 744 argv[0]: ./myecho
 745 argv[1]: script-arg
 746 argv[2]: ./script
 747 argv[3]: hello
 748 argv[4]: world
 749 .fi
 750 .in
 751 .SH SEE ALSO
 752 .BR chmod (2),
 753 .BR fork (2),
 754 .BR ptrace (2),
 755 .BR execl (3),
 756 .BR fexecve (3),
 757 .BR getopt (3),
 758 .BR credentials (7),
 759 .BR environ (7),
 760 .BR path_resolution (7),
 761 .BR ld.so (8)