1.\" $OpenBSD: execve.2,v 1.33 2007/05/31 19:19:32 jmc 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 31 2007 $ 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 a.out 5 62and 63.Xr elf 5 . 64.Pp 65An interpreter file begins with a line of the form: 66.Bd -filled -offset indent 67.Sy #!\& 68.Em interpreter 69.Bq Em arg 70.Ed 71.Pp 72When an interpreter file is 73.Fn execve Ap d , 74the system 75.Fn execve Ap s 76runs the specified 77.Em interpreter . 78If the optional 79.Em arg 80is specified, it becomes the first argument to the 81.Em interpreter , 82and the name of the originally 83.Fn execve Ap d 84file becomes the second argument; 85otherwise, the name of the originally 86.Fn execve Ap d 87file becomes the first argument. 88The original arguments are shifted over to become the subsequent arguments. 89The zeroth argument, normally the name of the 90.Fn execve Ap d 91file, is left unchanged. 92.Pp 93The argument 94.Fa argv 95is a pointer to a null-terminated array of 96character pointers to NUL-terminated character strings. 97These strings construct the argument list to be made available to the new 98process. 99At least one argument must be present in the array; 100by custom, the first element should be 101the name of the executed program (for example, the last component of 102.Fa path ) . 103.Pp 104The argument 105.Fa envp 106is also a pointer to a null-terminated array of 107character pointers to NUL-terminated strings. 108A pointer to this array is normally stored in the global variable 109.Va environ . 110These strings pass information to the 111new process that is not directly an argument to the command (see 112.Xr environ 7 ) . 113.Pp 114File descriptors open in the calling process image remain open in 115the new process image, except for those for which the close-on-exec 116flag is set (see 117.Xr close 2 118and 119.Xr fcntl 2 ) . 120Descriptors that remain open are unaffected by 121.Fn execve . 122In the case of a new setuid or setgid executable being executed, if 123file descriptors 0, 1, or 2 (representing stdin, stdout, and stderr) 124are currently unallocated, these descriptors will be opened to point to 125some system file like 126.Pa /dev/null . 127The intent is to ensure these descriptors are not unallocated, since 128many libraries make assumptions about the use of these 3 file descriptors. 129.Pp 130Signals set to be ignored in the calling process are set to be ignored in 131the 132new process. 133Signals which are set to be caught in the calling process image 134are set to default action in the new process image. 135Blocked signals remain blocked regardless of changes to the signal action. 136The signal stack is reset to be undefined (see 137.Xr sigaction 2 138for more information). 139.Pp 140If the set-user-ID mode bit of the new process image file is set 141(see 142.Xr chmod 2 ) , 143the effective user ID of the new process image is set to the owner ID 144of the new process image file. 145If the set-group-ID mode bit of the new process image file is set, 146the effective group ID of the new process image is set to the group ID 147of the new process image file. 148(The effective group ID is the first element of the group list.) 149The real user ID, real group ID and 150other group IDs of the new process image remain the same as the calling 151process image. 152After any set-user-ID and set-group-ID processing, 153the effective user ID is recorded as the saved set-user-ID, 154and the effective group ID is recorded as the saved set-group-ID. 155These values may be used in changing the effective IDs later (see 156.Xr setuid 2 ) . 157The set-user-ID and set-group-ID bits have no effect if the 158new process image file is located on a file system mounted with 159the nosuid flag. 160The process will be started without the new permissions. 161.Pp 162The new process also inherits the following attributes from 163the calling process: 164.Pp 165.Bl -column parent_process_ID -offset indent -compact 166.It process ID Ta see Xr getpid 2 167.It parent process ID Ta see Xr getppid 2 168.It process group ID Ta see Xr getpgrp 2 169.It session ID Ta see Xr getsid 2 170.It access groups Ta see Xr getgroups 2 171.It working directory Ta see Xr chdir 2 172.It root directory Ta see Xr chroot 2 173.It control terminal Ta see Xr termios 4 174.It resource usages Ta see Xr getrusage 2 175.It interval timers Ta see Xr getitimer 2 176(unless process image file is setuid or setgid, 177in which case all timers are disabled) 178.It resource limits Ta see Xr getrlimit 2 179.It file mode mask Ta see Xr umask 2 180.It signal mask Ta see Xr sigaction 2 , 181.Xr sigsetmask 3 182.El 183.Pp 184When a program is executed as a result of an 185.Fn execve 186call, it is entered as follows: 187.Bd -literal -offset indent 188main(int argc, char **argv, char **envp) 189.Ed 190.Pp 191where 192.Fa argc 193is the number of elements in 194.Fa argv 195(the 196.Dq arg count ) 197and 198.Fa argv 199points to the array of character pointers 200to the arguments themselves. 201.Pp 202The 203.Fn exect 204function is equivalent to 205.Fn execve 206with the additional property that it executes the file with the program 207tracing facilities enabled (see 208.Xr ptrace 2 ) . 209.Sh RETURN VALUES 210As the 211.Fn execve 212function overlays the current process image 213with a new process image the successful call 214has no process to return to. 215If 216.Fn execve 217does return to the calling process an error has occurred; the 218return value will be \-1 and the global variable 219.Va errno 220is set to indicate the error. 221.Sh ERRORS 222.Fn execve 223will fail and return to the calling process if: 224.Bl -tag -width Er 225.It Bq Er ENOTDIR 226A component of the path prefix is not a directory. 227.It Bq Er ENAMETOOLONG 228A component of a pathname exceeded 229.Dv {NAME_MAX} 230characters, or an entire path name exceeded 231.Dv {PATH_MAX} 232characters. 233.It Bq Er ENOENT 234The new process file does not exist. 235.It Bq Er ELOOP 236Too many symbolic links were encountered in translating the pathname. 237.It Bq Er EACCES 238Search permission is denied for a component of the path prefix. 239.It Bq Er EACCES 240The new process file is not an ordinary file. 241.It Bq Er EACCES 242The new process file mode denies execute permission. 243.It Bq Er EACCES 244The new process file is on a filesystem mounted with execution 245disabled 246.Pf ( Dv MNT_NOEXEC 247in 248.Ao Pa sys/mount.h Ac ) . 249.It Bq Er ENOEXEC 250The new process file has the appropriate access 251permission, but has an invalid magic number in its header. 252.It Bq Er ETXTBSY 253The new process file is a pure procedure (shared text) 254file that is currently open for writing or reading by some process. 255.It Bq Er ENOMEM 256The new process requires more virtual memory than 257is allowed by the imposed maximum 258.Pq Xr getrlimit 2 . 259.It Bq Er E2BIG 260The number of bytes in the new process's argument list 261is larger than the system-imposed limit. 262The limit in the system as released is 262144 bytes 263.Pf ( Dv NCARGS 264in 265.Ao Pa sys/param.h Ac ) . 266.It Bq Er EFAULT 267The new process file is not as long as indicated by 268the size values in its header. 269.It Bq Er EFAULT 270.Fa path , 271.Fa argv , 272or 273.Fa envp 274point 275to an illegal address. 276.It Bq Er EIO 277An I/O error occurred while reading from the file system. 278.It Bq Er ENFILE 279During startup of an 280.Em interpreter , 281the system file table was found to be full. 282.El 283.Sh SEE ALSO 284.Xr _exit 2 , 285.Xr fork 2 , 286.Xr execl 3 , 287.Xr exit 3 , 288.Xr a.out 5 , 289.Xr elf 5 , 290.Xr environ 7 291.Sh STANDARDS 292The 293.Fn exect 294function should not be used in portable applications. 295.Sh HISTORY 296The 297.Fn execve 298function call first appeared in 299.At v7 300and 301.Bx 3 . 302.Sh CAVEATS 303If a program is 304.Em setuid 305to a non-superuser, but is executed when the real 306.Em uid 307is 308.Dq root , 309then the program has some of the powers of a superuser as well. 310