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 2015-05-07 "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 mode 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.
 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 .IP *
 206 The file descriptor table is unshared, undoing the effect of the
 207 .B CLONE_FILES
 208 flag of
 209 .BR clone (2).
 210 .PP
 211 Note the following further points:
 212 .IP * 3
 213 All threads other than the calling thread are destroyed during an
 214 .BR execve ().
 215 Mutexes, condition variables, and other pthreads objects are not preserved.
 216 .IP *
 217 The equivalent of \fIsetlocale(LC_ALL, "C")\fP
 218 is executed at program start-up.
 219 .IP *
 220 POSIX.1 specifies that the dispositions of any signals that
 221 are ignored or set to the default are left unchanged.
 222 POSIX.1 specifies one exception: if
 223 .B SIGCHLD
 224 is being ignored,
 225 then an implementation may leave the disposition unchanged or
 226 reset it to the default; Linux does the former.
 227 .IP *
 228 Any outstanding asynchronous I/O operations are canceled
 229 .RB ( aio_read (3),
 230 .BR aio_write (3)).
 231 .IP *
 232 For the handling of capabilities during
 233 .BR execve (),
 234 see
 235 .BR capabilities (7).
 236 .IP *
 237 By default, file descriptors remain open across an
 238 .BR execve ().
 239 File descriptors that are marked close-on-exec are closed;
 240 see the description of
 241 .B FD_CLOEXEC
 242 in
 243 .BR fcntl (2).
 244 (If a file descriptor is closed, this will cause the release
 245 of all record locks obtained on the underlying file by this process.
 246 See
 247 .BR fcntl (2)
 248 for details.)
 249 POSIX.1 says that if file descriptors 0, 1, and 2 would
 250 otherwise be closed after a successful
 251 .BR execve (),
 252 and the process would gain privilege because the set-user_ID or
 253 set-group_ID mode bit was set on the executed file,
 254 then the system may open an unspecified file for each of these
 255 file descriptors.
 256 As a general principle, no portable program, whether privileged or not,
 257 can assume that these three file descriptors will remain
 258 closed across an
 259 .BR execve ().
 260 .\" On Linux it appears that these file descriptors are
 261 .\" always open after an execve(), and it looks like
 262 .\" Solaris 8 and FreeBSD 6.1 are the same. -- mtk, 30 Apr 2007
 263 .SS Interpreter scripts
 264 An interpreter script is a text file that has execute
 265 permission enabled and whose first line is of the form:
 266
 267 .in +4n
 268 .nf
 269 \fB#!\fP \fIinterpreter \fP[optional-arg]
 270 .fi
 271 .in
 272
 273 The
 274 .I interpreter
 275 must be a valid pathname for an
 276 executable which is not itself a script.
 277 If the
 278 .I filename
 279 argument of
 280 .BR execve ()
 281 specifies an interpreter script, then
 282 .I interpreter
 283 will be invoked with the following arguments:
 284
 285 .in +4n
 286 .nf
 287 \fIinterpreter\fP [optional-arg] \fIfilename\fP arg...
 288 .fi
 289 .in
 290
 291 where
 292 .I arg...
 293 is the series of words pointed to by the
 294 .I argv
 295 argument of
 296 .BR execve (),
 297 starting at
 298 .IR argv [1].
 299
 300 For portable use,
 301 .I optional-arg
 302 should either be absent, or be specified as a single word (i.e., it
 303 should not contain white space); see NOTES below.
 304 .SS Limits on size of arguments and environment
 305 Most UNIX implementations impose some limit on the total size
 306 of the command-line argument
 307 .RI ( argv )
 308 and environment
 309 .RI ( envp )
 310 strings that may be passed to a new program.
 311 POSIX.1 allows an implementation to advertise this limit using the
 312 .B ARG_MAX
 313 constant (either defined in
 314 .I <limits.h>
 315 or available at run time using the call
 316 .IR "sysconf(_SC_ARG_MAX)" ).
 317
 318 On Linux prior to kernel 2.6.23, the memory used to store the
 319 environment and argument strings was limited to 32 pages
 320 (defined by the kernel constant
 321 .BR MAX_ARG_PAGES ).
 322 On architectures with a 4-kB page size,
 323 this yields a maximum size of 128 kB.
 324
 325 On kernel 2.6.23 and later, most architectures support a size limit
 326 derived from the soft
 327 .B RLIMIT_STACK
 328 resource limit (see
 329 .BR getrlimit (2))
 330 that is in force at the time of the
 331 .BR execve ()
 332 call.
 333 (Architectures with no memory management unit are excepted:
 334 they maintain the limit that was in effect before kernel 2.6.23.)
 335 This change allows programs to have a much larger
 336 argument and/or environment list.
 337 .\" For some background on the changes to ARG_MAX in kernels 2.6.23 and
 338 .\" 2.6.25, see:
 339 .\"     http://sourceware.org/bugzilla/show_bug.cgi?id=5786
 340 .\"     http://bugzilla.kernel.org/show_bug.cgi?id=10095
 341 .\"     http://thread.gmane.org/gmane.linux.kernel/646709/focus=648101,
 342 .\"     checked into 2.6.25 as commit a64e715fc74b1a7dcc5944f848acc38b2c4d4ee2.
 343 For these architectures, the total size is limited to 1/4 of the allowed
 344 stack size.
 345 (Imposing the 1/4-limit
 346 ensures that the new program always has some stack space.)
 347 .\" Ollie: That doesn't include the lists of pointers, though,
 348 .\" so the actual usage is a bit higher (1 pointer per argument).
 349 Since Linux 2.6.25,
 350 the kernel places a floor of 32 pages on this size limit,
 351 so that, even when
 352 .BR RLIMIT_STACK
 353 is set very low,
 354 applications are guaranteed to have at least as much argument and
 355 environment space as was provided by Linux 2.6.23 and earlier.
 356 (This guarantee was not provided in Linux 2.6.23 and 2.6.24.)
 357 Additionally, the limit per string is 32 pages (the kernel constant
 358 .BR MAX_ARG_STRLEN ),
 359 and the maximum number of strings is 0x7FFFFFFF.
 360 .SH RETURN VALUE
 361 On success,
 362 .BR execve ()
 363 does not return, on error \-1 is returned, and
 364 .I errno
 365 is set appropriately.
 366 .SH ERRORS
 367 .TP
 368 .B E2BIG
 369 The total number of bytes in the environment
 370 .RI ( envp )
 371 and argument list
 372 .RI ( argv )
 373 is too large.
 374 .TP
 375 .B EACCES
 376 Search permission is denied on a component of the path prefix of
 377 .I filename
 378 or the name of a script interpreter.
 379 (See also
 380 .BR path_resolution (7).)
 381 .TP
 382 .B EACCES
 383 The file or a script interpreter is not a regular file.
 384 .TP
 385 .B EACCES
 386 Execute permission is denied for the file or a script or ELF interpreter.
 387 .TP
 388 .B EACCES
 389 The filesystem is mounted
 390 .IR noexec .
 391 .TP
 392 .BR EAGAIN " (since Linux 3.1)"
 393 .\" commit 72fa59970f8698023045ab0713d66f3f4f96945c
 394 Having changed its real UID using one of the
 395 .BR set*uid ()
 396 calls, the caller was\(emand is now still\(emabove its
 397 .BR RLIMIT_NPROC
 398 resource limit (see
 399 .BR setrlimit (2)).
 400 For a more detailed explanation of this error, see NOTES.
 401 .TP
 402 .B EFAULT
 403 .I filename
 404 or one of the pointers in the vectors
 405 .I argv
 406 or
 407 .I envp
 408 points outside your accessible address space.
 409 .TP
 410 .B EINVAL
 411 An ELF executable had more than one PT_INTERP segment (i.e., tried to
 412 name more than one interpreter).
 413 .TP
 414 .B EIO
 415 An I/O error occurred.
 416 .TP
 417 .B EISDIR
 418 An ELF interpreter was a directory.
 419 .TP
 420 .B ELIBBAD
 421 An ELF interpreter was not in a recognized format.
 422 .TP
 423 .B ELOOP
 424 Too many symbolic links were encountered in resolving
 425 .I filename
 426 or the name of a script or ELF interpreter.
 427 .TP
 428 .B EMFILE
 429 The process has the maximum number of files open.
 430 .TP
 431 .B ENAMETOOLONG
 432 .I filename
 433 is too long.
 434 .TP
 435 .B ENFILE
 436 The system limit on the total number of open files has been reached.
 437 .TP
 438 .B ENOENT
 439 The file
 440 .I filename
 441 or a script or ELF interpreter does not exist, or a shared library
 442 .\" FIXME but see http://sourceware.org/bugzilla/show_bug.cgi?id=12241
 443 needed for file or interpreter cannot be found.
 444 .TP
 445 .B ENOEXEC
 446 An executable is not in a recognized format, is for the wrong
 447 architecture, or has some other format error that means it cannot be
 448 executed.
 449 .TP
 450 .B ENOMEM
 451 Insufficient kernel memory was available.
 452 .TP
 453 .B ENOTDIR
 454 A component of the path prefix of
 455 .I filename
 456 or a script or ELF interpreter is not a directory.
 457 .TP
 458 .B EPERM
 459 The filesystem is mounted
 460 .IR nosuid ,
 461 the user is not the superuser,
 462 and the file has the set-user-ID or set-group-ID bit set.
 463 .TP
 464 .B EPERM
 465 The process is being traced, the user is not the superuser and the
 466 file has the set-user-ID or set-group-ID bit set.
 467 .TP
 468 .B ETXTBSY
 469 Executable was open for writing by one or more processes.
 470 .SH CONFORMING TO
 471 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 472 POSIX.1-2001 does not document the #!  behavior
 473 but is otherwise compatible.
 474 .\" SVr4 documents additional error
 475 .\" conditions EAGAIN, EINTR, ELIBACC, ENOLINK, EMULTIHOP; POSIX does not
 476 .\" document ETXTBSY, EPERM, EFAULT, ELOOP, EIO, ENFILE, EMFILE, EINVAL,
 477 .\" EISDIR or ELIBBAD error conditions.
 478 .SH NOTES
 479 Set-user-ID and set-group-ID processes can not be
 480 .BR ptrace (2)d.
 481
 482 The result of mounting a filesystem
 483 .I nosuid
 484 varies across Linux kernel versions:
 485 some will refuse execution of set-user-ID and set-group-ID
 486 executables when this would
 487 give the user powers she did not have already (and return
 488 .BR EPERM ),
 489 some will just ignore the set-user-ID and set-group-ID bits and
 490 .BR exec ()
 491 successfully.
 492
 493 On Linux,
 494 .I argv
 495 and
 496 .I envp
 497 can be specified as NULL.
 498 In both cases, this has the same effect as specifying the argument
 499 as a pointer to a list containing a single null pointer.
 500 .B "Do not take advantage of this nonstandard and nonportable misfeature!"
 501 On many other UNIX systems, specifying
 502 .I argv
 503 as NULL will result in an error
 504 .RB ( EFAULT ).
 505 .I Some
 506 other UNIX systems treat the
 507 .I envp==NULL
 508 case the same as Linux.
 509 .\" e.g., EFAULT on Solaris 8 and FreeBSD 6.1; but
 510 .\" HP-UX 11 is like Linux -- mtk, Apr 2007
 511 .\" Bug filed 30 Apr 2007: http://bugzilla.kernel.org/show_bug.cgi?id=8408
 512 .\" Bug rejected (because fix would constitute an ABI change).
 513 .\"
 514
 515 POSIX.1 says that values returned by
 516 .BR sysconf (3)
 517 should be invariant over the lifetime of a process.
 518 However, since Linux 2.6.23, if the
 519 .BR RLIMIT_STACK
 520 resource limit changes, then the value reported by
 521 .B _SC_ARG_MAX
 522 will also change,
 523 to reflect the fact that the limit on space for holding
 524 command-line arguments and environment variables has changed.
 525
 526 In most cases where
 527 .BR execve ()
 528 fails, control returns to the original executable image,
 529 and the caller of
 530 .BR execve ()
 531 can then handle the error.
 532 However, in (rare) cases (typically caused by resource exhaustion),
 533 failure may occur past the point of no return:
 534 the original executable image has been torn down,
 535 but the new image could not be completely built.
 536 In such cases, the kernel kills the process with a
 537 .BR SIGKILL
 538 signal.
 539 .\"
 540 .SS Interpreter scripts
 541 A maximum line length of 127 characters is allowed for the first line in
 542 an interpreter scripts.
 543
 544 The semantics of the
 545 .I optional-arg
 546 argument of an interpreter script vary across implementations.
 547 On Linux, the entire string following the
 548 .I interpreter
 549 name is passed as a single argument to the interpreter,
 550 and this string can include white space.
 551 However, behavior differs on some other systems.
 552 Some systems
 553 .\" e.g., Solaris 8
 554 use the first white space to terminate
 555 .IR optional-arg .
 556 On some systems,
 557 .\" e.g., FreeBSD before 6.0, but not FreeBSD 6.0 onward
 558 an interpreter script can have multiple arguments,
 559 and white spaces in
 560 .I optional-arg
 561 are used to delimit the arguments.
 562
 563 Linux ignores the set-user-ID and set-group-ID bits on scripts.
 564 .\"
 565 .\" .SH BUGS
 566 .\" Some Linux versions have failed to check permissions on ELF
 567 .\" interpreters.  This is a security hole, because it allows users to
 568 .\" open any file, such as a rewinding tape device, for reading.  Some
 569 .\" Linux versions have also had other security holes in
 570 .\" .BR execve ()
 571 .\" that could be exploited for denial of service by a suitably crafted
 572 .\" ELF binary. There are no known problems with 2.0.34 or 2.2.15.
 573 .SS execve() and EAGAIN
 574 A more detailed explanation of the
 575 .BR EAGAIN
 576 error that can occur (since Linux 3.1) when calling
 577 .BR execve ()
 578 is as follows.
 579
 580 The
 581 .BR EAGAIN
 582 error can occur when a
 583 .I preceding
 584 call to
 585 .BR setuid (2),
 586 .BR setreuid (2),
 587 or
 588 .BR setresuid (2)
 589 caused the real user ID of the process to change,
 590 and that change caused the process to exceed its
 591 .BR RLIMIT_NPROC
 592 resource limit (i.e., the number of processes belonging
 593 to the new real UID exceeds the resource limit).
 594 From Linux 2.6.0 to 3.0, this caused the
 595 .BR set*uid ()
 596 call to fail.
 597 (Prior to 2.6,
 598 .\" commit 909cc4ae86f3380152a18e2a3c44523893ee11c4
 599 the resource limit was not imposed on processes that
 600 changed their user IDs.)
 601
 602 Since Linux 3.1, the scenario just described no longer causes the
 603 .BR set*uid ()
 604 call to fail,
 605 because it too often led to security holes where buggy applications
 606 didn't check the return status and assumed
 607 that\(emif the caller had root privileges\(emthe call would always succeed.
 608 Instead, the
 609 .BR set*uid ()
 610 calls now successfully change the real UID,
 611 but the kernel sets an internal flag, named
 612 .BR PF_NPROC_EXCEEDED ,
 613 to note that the
 614 .BR RLIMIT_NPROC
 615 resource limit has been exceeded.
 616 If the
 617 .BR PF_NPROC_EXCEEDED
 618 flag is set and the resource limit is still
 619 exceeded at the time of a subsequent
 620 .BR execve ()
 621 call, that call fails with the error
 622 .BR EAGAIN .
 623 This kernel logic ensures that the
 624 .BR RLIMIT_NPROC
 625 resource limit is still enforced for the
 626 common privileged daemon workflow\(emnamely,
 627 .BR fork (2)
 628 +
 629 .BR set*uid ()
 630 +
 631 .BR execve ().
 632
 633 If the resource limit was not still exceeded at the time of the
 634 .BR execve ()
 635 call
 636 (because other processes belonging to this real UID terminated between the
 637 .BR set*uid ()
 638 call and the
 639 .BR execve ()
 640 call), then the
 641 .BR execve ()
 642 call succeeds and the kernel clears the
 643 .BR PF_NPROC_EXCEEDED
 644 process flag.
 645 The flag is also cleared if a subsequent call to
 646 .BR fork (2)
 647 by this process succeeds.
 648 .SS Historical
 649 With UNIX\ V6, the argument list of an
 650 .BR exec ()
 651 call was ended by 0,
 652 while the argument list of
 653 .I main
 654 was ended by \-1.
 655 Thus, this argument list was not directly usable in a further
 656 .BR exec ()
 657 call.
 658 Since UNIX\ V7, both are NULL.
 659 .\"
 660 .\" .SH BUGS
 661 .\" Some Linux versions have failed to check permissions on ELF
 662 .\" interpreters.  This is a security hole, because it allows users to
 663 .\" open any file, such as a rewinding tape device, for reading.  Some
 664 .\" Linux versions have also had other security holes in
 665 .\" .BR execve ()
 666 .\" that could be exploited for denial of service by a suitably crafted
 667 .\" ELF binary. There are no known problems with 2.0.34 or 2.2.15.
 668 .SH EXAMPLE
 669 The following program is designed to be execed by the second program below.
 670 It just echoes its command-line arguments, one per line.
 671
 672 .in +4n
 673 .nf
 674 /* myecho.c */
 675
 676 #include <stdio.h>
 677 #include <stdlib.h>
 678
 679 int
 680 main(int argc, char *argv[])
 681 {
 682     int j;
 683
 684     for (j = 0; j < argc; j++)
 685         printf("argv[%d]: %s\\n", j, argv[j]);
 686
 687     exit(EXIT_SUCCESS);
 688 }
 689 .fi
 690 .in
 691
 692 This program can be used to exec the program named in its command-line
 693 argument:
 694 .in +4n
 695 .nf
 696
 697 /* execve.c */
 698
 699 #include <stdio.h>
 700 #include <stdlib.h>
 701 #include <unistd.h>
 702
 703 int
 704 main(int argc, char *argv[])
 705 {
 706     char *newargv[] = { NULL, "hello", "world", NULL };
 707     char *newenviron[] = { NULL };
 708
 709     if (argc != 2) {
 710         fprintf(stderr, "Usage: %s <file\-to\-exec>\\n", argv[0]);
 711         exit(EXIT_FAILURE);
 712     }
 713
 714     newargv[0] = argv[1];
 715
 716     execve(argv[1], newargv, newenviron);
 717     perror("execve");   /* execve() returns only on error */
 718     exit(EXIT_FAILURE);
 719 }
 720 .fi
 721 .in
 722
 723 We can use the second program to exec the first as follows:
 724
 725 .in +4n
 726 .nf
 727 .RB "$" " cc myecho.c \-o myecho"
 728 .RB "$" " cc execve.c \-o execve"
 729 .RB "$" " ./execve ./myecho"
 730 argv[0]: ./myecho
 731 argv[1]: hello
 732 argv[2]: world
 733 .fi
 734 .in
 735
 736 We can also use these programs to demonstrate the use of a script
 737 interpreter.
 738 To do this we create a script whose "interpreter" is our
 739 .I myecho
 740 program:
 741
 742 .in +4n
 743 .nf
 744 .RB "$" " cat > script"
 745 .B #!./myecho script-arg
 746 .B ^D
 747 .RB "$" " chmod +x script"
 748 .fi
 749 .in
 750
 751 We can then use our program to exec the script:
 752
 753 .in +4n
 754 .nf
 755 .RB "$" " ./execve ./script"
 756 argv[0]: ./myecho
 757 argv[1]: script-arg
 758 argv[2]: ./script
 759 argv[3]: hello
 760 argv[4]: world
 761 .fi
 762 .in
 763 .SH SEE ALSO
 764 .BR chmod (2),
 765 .BR execveat (2),
 766 .BR fork (2),
 767 .BR ptrace (2),
 768 .BR execl (3),
 769 .BR fexecve (3),
 770 .BR getopt (3),
 771 .BR credentials (7),
 772 .BR environ (7),
 773 .BR path_resolution (7),
 774 .BR ld.so (8)