1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 4. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)execve.2 8.5 (Berkeley) 6/1/94 29.\" $FreeBSD$ 30.\" 31.Dd June 1, 1994 32.Dt EXECVE 2 33.Os 34.Sh NAME 35.Nm execve 36.Nd execute a file 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In unistd.h 41.Ft int 42.Fn execve "const char *path" "char *const argv[]" "char *const envp[]" 43.Sh DESCRIPTION 44The 45.Fn execve 46system call 47transforms the calling process into a new process. 48The new process is constructed from an ordinary file, 49whose name is pointed to by 50.Fa path , 51called the 52.Em new process file . 53This file is either an executable object file, 54or a file of data for an interpreter. 55An executable object file consists of an identifying header, 56followed by pages of data representing the initial program (text) 57and initialized data pages. 58Additional pages may be specified 59by the header to be initialized with zero data; see 60.Xr elf 5 61and 62.Xr a.out 5 . 63.Pp 64An interpreter file begins with a line of the form: 65.Pp 66.Bd -ragged -offset indent -compact 67.Sy \&#! 68.Em interpreter 69.Bq Em arg 70.Ed 71.Pp 72When an interpreter file is 73.Sy execve Ap d , 74the system actually 75.Sy execve Ap s 76the 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.Sy execve Ap d 84file becomes the second argument; 85otherwise, the name of the originally 86.Sy execve Ap d 87file becomes the first argument. 88The original arguments are shifted over to 89become the subsequent arguments. 90The zeroth argument is set to the specified 91.Em interpreter . 92.Pp 93The argument 94.Fa argv 95is a pointer to a null-terminated array of 96character pointers to null-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 100the array; by 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 null-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 . 122If any of the standard descriptors (0, 1, and/or 2) are closed at the 123time 124.Fn execve 125is called, and the process will gain privilege as a result of set-id 126semantics, those descriptors will be re-opened automatically. 127No programs, whether privileged or not, should assume that these descriptors 128will remain closed across a call to 129.Fn execve . 130.Pp 131Signals set to be ignored in the calling process are set to be ignored in 132the 133new process. 134Signals which are set to be caught in the calling process image 135are set to default action in the new process image. 136Blocked signals remain blocked regardless of changes to the signal action. 137The signal stack is reset to be undefined (see 138.Xr sigaction 2 139for more information). 140.Pp 141If the set-user-ID mode bit of the new process image file is set 142(see 143.Xr chmod 2 ) , 144the effective user ID of the new process image is set to the owner ID 145of the new process image file. 146If the set-group-ID mode bit of the new process image file is set, 147the effective group ID of the new process image is set to the group ID 148of the new process image file. 149(The effective group ID is the first element of the group list.) 150The real user ID, real group ID and 151other group IDs of the new process image remain the same as the calling 152process image. 153After any set-user-ID and set-group-ID processing, 154the effective user ID is recorded as the saved set-user-ID, 155and the effective group ID is recorded as the saved set-group-ID. 156These values may be used in changing the effective IDs later (see 157.Xr setuid 2 ) . 158.Pp 159The set-ID bits are not honored if the respective file system has the 160.Cm nosuid 161option enabled or if the new process file is an interpreter file. 162Syscall 163tracing is disabled if effective IDs are changed. 164.Pp 165The new process also inherits the following attributes from 166the calling process: 167.Pp 168.Bl -column parent_process_ID -offset indent -compact 169.It process ID Ta see Xr getpid 2 170.It parent process ID Ta see Xr getppid 2 171.It process group ID Ta see Xr getpgrp 2 172.It access groups Ta see Xr getgroups 2 173.It working directory Ta see Xr chdir 2 174.It root directory Ta see Xr chroot 2 175.It control terminal Ta see Xr termios 4 176.It resource usages Ta see Xr getrusage 2 177.It interval timers Ta see Xr getitimer 2 178.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 sigvec 2 , 181.Xr sigsetmask 2 182.El 183.Pp 184When a program is executed as a result of an 185.Fn execve 186system call, it is entered as follows: 187.Bd -literal -offset indent 188main(argc, argv, envp) 189int argc; 190char **argv, **envp; 191.Ed 192.Pp 193where 194.Fa argc 195is the number of elements in 196.Fa argv 197(the ``arg count'') 198and 199.Fa argv 200points to the array of character pointers 201to the arguments themselves. 202.Sh RETURN VALUES 203As the 204.Fn execve 205system call overlays the current process image 206with a new process image the successful call 207has no process to return to. 208If 209.Fn execve 210does return to the calling process an error has occurred; the 211return value will be -1 and the global variable 212.Va errno 213is set to indicate the error. 214.Sh ERRORS 215The 216.Fn execve 217system call 218will fail and return to the calling process if: 219.Bl -tag -width Er 220.It Bq Er ENOTDIR 221A component of the path prefix is not a directory. 222.It Bq Er ENAMETOOLONG 223A component of a pathname exceeded 255 characters, 224or an entire path name exceeded 1023 characters. 225.It Bq Er ENAMETOOLONG 226When invoking an interpreted script, the interpreter name 227exceeds 228.Dv MAXSHELLCMDLEN 229characters. 230.It Bq Er ENOENT 231The new process file does not exist. 232.It Bq Er ELOOP 233Too many symbolic links were encountered in translating the pathname. 234.It Bq Er EACCES 235Search permission is denied for a component of the path prefix. 236.It Bq Er EACCES 237The new process file is not an ordinary file. 238.It Bq Er EACCES 239The new process file mode denies execute permission. 240.It Bq Er ENOEXEC 241The new process file has the appropriate access 242permission, but has an invalid magic number in its header. 243.It Bq Er ETXTBSY 244The new process file is a pure procedure (shared text) 245file that is currently open for writing or reading by some process. 246.It Bq Er ENOMEM 247The new process requires more virtual memory than 248is allowed by the imposed maximum 249.Pq Xr getrlimit 2 . 250.It Bq Er E2BIG 251The number of bytes in the new process' argument list 252is larger than the system-imposed limit. 253This limit is specified by the 254.Xr sysctl 3 255MIB variable 256.Dv KERN_ARGMAX . 257.It Bq Er EFAULT 258The new process file is not as long as indicated by 259the size values in its header. 260.It Bq Er EFAULT 261The 262.Fa path , 263.Fa argv , 264or 265.Fa envp 266arguments 267point 268to an illegal address. 269.It Bq Er EIO 270An I/O error occurred while reading from the file system. 271.El 272.Sh CAVEAT 273If a program is 274.Em setuid 275to a non-super-user, but is executed when 276the real 277.Em uid 278is ``root'', then the program has some of the powers 279of a super-user as well. 280.Sh SEE ALSO 281.Xr ktrace 1 , 282.Xr _exit 2 , 283.Xr fork 2 , 284.Xr execl 3 , 285.Xr exit 3 , 286.Xr sysctl 3 , 287.Xr a.out 5 , 288.Xr elf 5 , 289.Xr environ 7 , 290.Xr mount 8 291.Sh STANDARDS 292The 293.Fn execve 294system call conforms to 295.St -p1003.1-2001 , 296with the exception of reopening descriptors 0, 1, and/or 2 in certain 297circumstances. 298A future update of the Standard is expected to require this behavior, 299and it may become the default for non-privileged processes as well. 300.\" NB: update this caveat when TC1 is blessed. 301The support for executing interpreted programs is an extension. 302.Sh HISTORY 303The 304.Fn execve 305system call appeared in 306.Bx 4.2 . 307