1.\" $OpenBSD: execve.2,v 1.41 2014/05/30 04:38:43 guenther Exp $ 2.\" $NetBSD: execve.2,v 1.9 1995/02/27 12:32:25 cgd Exp $ 3.\" 4.\" Copyright (c) 1980, 1991, 1993 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of the University nor the names of its contributors 16.\" may be used to endorse or promote products derived from this software 17.\" without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 20.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 21.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 22.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 23.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 24.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 25.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 26.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 27.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 28.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 29.\" SUCH DAMAGE. 30.\" 31.\" @(#)execve.2 8.3 (Berkeley) 1/24/94 32.\" 33.Dd $Mdocdate: May 30 2014 $ 34.Dt EXECVE 2 35.Os 36.Sh NAME 37.Nm execve , 38.Nm exect 39.Nd execute a file 40.Sh SYNOPSIS 41.Fd #include <unistd.h> 42.Ft int 43.Fn execve "const char *path" "char *const argv[]" "char *const envp[]" 44.Ft int 45.Fn exect "const char *path" "char *const argv[]" "char *const envp[]" 46.Sh DESCRIPTION 47.Fn execve 48transforms the calling process into a new process. 49The new process is constructed from an ordinary file, 50whose name is pointed to by 51.Fa path , 52called the 53.Em new process file . 54This file is either an executable object file, 55or a file of data for an interpreter. 56An executable object file consists of an identifying header, 57followed by pages of data representing the initial program (text) 58and initialized data pages. 59Additional pages may be specified by the header to be initialized 60with zero data; see 61.Xr elf 5 . 62.Pp 63An interpreter file begins with a line of the form: 64.Bd -filled -offset indent 65.Sy #!\& 66.Em interpreter 67.Bq Em arg 68.Ed 69.Pp 70When an interpreter file is passed to 71.Fn execve 72the system instead calls 73.Fn execve 74with the specified 75.Em interpreter . 76If the optional 77.Em arg 78is specified, it becomes the first argument to the 79.Em interpreter , 80and the original 81.Ar path 82becomes the second argument; 83otherwise, 84.Ar path 85becomes the first argument. 86The original arguments are shifted over to become the subsequent arguments. 87The zeroth argument, normally the name of the file being executed, is left 88unchanged. 89.Pp 90The argument 91.Fa argv 92is a pointer to a null-terminated array of 93character pointers to NUL-terminated character strings. 94These strings construct the argument list to be made available to the new 95process. 96At least one argument must be present in the array; 97by custom, the first element should be 98the name of the executed program (for example, the last component of 99.Fa path ) . 100.Pp 101The argument 102.Fa envp 103is also a pointer to a null-terminated array of 104character pointers to NUL-terminated strings. 105A pointer to this array is normally stored in the global variable 106.Va environ . 107These strings pass information to the 108new process that is not directly an argument to the command (see 109.Xr environ 7 ) . 110.Pp 111File descriptors open in the calling process image remain open in 112the new process image, except for those for which the close-on-exec 113flag is set (see 114.Xr close 2 115and 116.Xr fcntl 2 ) . 117Descriptors that remain open are unaffected by 118.Fn execve . 119In the case of a new setuid or setgid executable being executed, if 120file descriptors 0, 1, or 2 (representing stdin, stdout, and stderr) 121are currently unallocated, these descriptors will be opened to point to 122some system file like 123.Pa /dev/null . 124The intent is to ensure these descriptors are not unallocated, since 125many libraries make assumptions about the use of these 3 file descriptors. 126.Pp 127Signals set to be ignored in the calling process are set to be ignored in 128the 129new process. 130Signals which are set to be caught in the calling process image 131are set to default action in the new process image. 132Blocked signals remain blocked regardless of changes to the signal action. 133The signal stack is reset to be undefined (see 134.Xr sigaction 2 135for more information). 136.Pp 137If the set-user-ID mode bit of the new process image file is set 138(see 139.Xr chmod 2 ) , 140the effective user ID of the new process image is set to the owner ID 141of the new process image file. 142If the set-group-ID mode bit of the new process image file is set, 143the effective group ID of the new process image is set to the group ID 144of the new process image file. 145(The effective group ID is the first element of the group list.) 146The real user ID, real group ID and 147other group IDs of the new process image remain the same as the calling 148process image. 149After any set-user-ID and set-group-ID processing, 150the effective user ID is recorded as the saved set-user-ID, 151and the effective group ID is recorded as the saved set-group-ID. 152These values may be used in changing the effective IDs later (see 153.Xr setuid 2 ) . 154The set-user-ID and set-group-ID bits have no effect if the 155new process image file is located on a file system mounted with 156the nosuid flag. 157The process will be started without the new permissions. 158.Pp 159The new process also inherits the following attributes from 160the calling process: 161.Pp 162.Bl -tag -width parent_process_ID -offset indent -compact 163.It process ID 164see 165.Xr getpid 2 166.It parent process ID 167see 168.Xr getppid 2 169.It process group ID 170see 171.Xr getpgrp 2 172.It session ID 173see 174.Xr getsid 2 175.It access groups 176see 177.Xr getgroups 2 178.It working directory 179see 180.Xr chdir 2 181.It root directory 182see 183.Xr chroot 2 184.It control terminal 185see 186.Xr termios 4 187.It resource usages 188see 189.Xr getrusage 2 190.It interval timers 191see 192.Xr getitimer 2 193(unless process image file is setuid or setgid, 194in which case all timers are disabled) 195.It resource limits 196see 197.Xr getrlimit 2 198.It file mode mask 199see 200.Xr umask 2 201.It signal mask 202see 203.Xr sigaction 2 , 204.Xr sigsetmask 3 205.El 206.Pp 207When a program is executed as a result of an 208.Fn execve 209call, it is entered as follows: 210.Bd -literal -offset indent 211main(int argc, char **argv, char **envp) 212.Ed 213.Pp 214where 215.Fa argc 216is the number of elements in 217.Fa argv 218(the 219.Dq arg count ) 220and 221.Fa argv 222points to the array of character pointers 223to the arguments themselves. 224.Pp 225The 226.Fn exect 227function is equivalent to 228.Fn execve 229with the additional property that it executes the file with the process 230tracing facilities enabled (see 231.Xr ptrace 2 ) . 232.Sh RETURN VALUES 233As the 234.Fn execve 235function overlays the current process image 236with a new process image the successful call 237has no process to return to. 238If 239.Fn execve 240does return to the calling process an error has occurred; the 241return value will be \-1 and the global variable 242.Va errno 243is set to indicate the error. 244.Sh ERRORS 245.Fn execve 246will fail and return to the calling process if: 247.Bl -tag -width Er 248.It Bq Er ENOTDIR 249A component of the path prefix is not a directory. 250.It Bq Er ENAMETOOLONG 251A component of a pathname exceeded 252.Dv {NAME_MAX} 253characters, or an entire path name exceeded 254.Dv {PATH_MAX} 255characters. 256.It Bq Er ENOENT 257The new process file does not exist. 258.It Bq Er ELOOP 259Too many symbolic links were encountered in translating the pathname. 260.It Bq Er EACCES 261Search permission is denied for a component of the path prefix. 262.It Bq Er EACCES 263The new process file is not an ordinary file. 264.It Bq Er EACCES 265The new process file mode denies execute permission. 266.It Bq Er EACCES 267The new process file is on a filesystem mounted with execution 268disabled 269.Pf ( Dv MNT_NOEXEC 270in 271.In sys/mount.h ) . 272.It Bq Er ENOEXEC 273The new process file has the appropriate access 274permission, but has an invalid magic number in its header. 275.It Bq Er ETXTBSY 276The new process file is a pure procedure (shared text) 277file that is currently open for writing or reading by some process. 278.It Bq Er ENOMEM 279The new process requires more virtual memory than 280is allowed by the imposed maximum 281.Pq Xr getrlimit 2 . 282.It Bq Er E2BIG 283The number of bytes in the new process's argument list 284is larger than the system-imposed limit. 285The limit in the system as released is 262144 bytes 286.Pf ( Dv NCARGS 287in 288.In sys/param.h ) . 289.It Bq Er EFAULT 290The new process file is not as long as indicated by 291the size values in its header. 292.It Bq Er EFAULT 293.Fa path , 294.Fa argv , 295or 296.Fa envp 297point 298to an illegal address. 299.It Bq Er EIO 300An I/O error occurred while reading from the file system. 301.It Bq Er ENFILE 302During startup of an 303.Em interpreter , 304the system file table was found to be full. 305.El 306.Sh SEE ALSO 307.Xr _exit 2 , 308.Xr fork 2 , 309.Xr execl 3 , 310.Xr exit 3 , 311.Xr elf 5 , 312.Xr environ 7 313.Sh STANDARDS 314The 315.Fn execve 316function is expected to conform to 317.St -p1003.1-2008 . 318The 319.Fn exect 320function should not be used in portable applications. 321.Sh HISTORY 322The predecessor of these functions, the former 323.Fn exec 324system call, first appeared in 325.At v1 . 326The 327.Fn execve 328function first appeared in 329.At v7 . 330.Sh CAVEATS 331If a program is 332.Em setuid 333to a non-superuser, but is executed when the real 334.Em uid 335is 336.Dq root , 337then the process has some of the powers of a superuser as well. 338