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.\" 3. 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.Dd April 29, 2026 29.Dt EXECVE 2 30.Os 31.Sh NAME 32.Nm execve , 33.Nm fexecve 34.Nd execute a file 35.Sh LIBRARY 36.Lb libc 37.Sh SYNOPSIS 38.In unistd.h 39.Ft int 40.Fn execve "const char *path" "char *const argv[]" "char *const envp[]" 41.Ft int 42.Fn fexecve "int fd" "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 . 53The 54.Fn fexecve 55system call is equivalent to 56.Fn execve 57except that the file to be executed is determined by the file 58descriptor 59.Fa fd 60instead of a 61.Fa path . 62This file is either an executable object file, 63or a file of data for an interpreter. 64An executable object file consists of an identifying header, 65followed by pages of data representing the initial program (text) 66and initialized data pages. 67Additional pages may be specified 68by the header to be initialized with zero data; see 69.Xr elf 5 70and 71.Xr a.out 5 . 72.Pp 73An interpreter file begins with a line of the form: 74.Pp 75.Bd -ragged -offset indent -compact 76.Sy \&#! 77.Em interpreter 78.Bq Em arg 79.Ed 80.Pp 81When an interpreter file is 82.Sy execve Ap d , 83the system actually 84.Sy execve Ap s 85the specified 86.Em interpreter . 87If the optional 88.Em arg 89is specified, it becomes the first argument to the 90.Em interpreter , 91and the name of the originally 92.Sy execve Ap d 93file becomes the second argument; 94otherwise, the name of the originally 95.Sy execve Ap d 96file becomes the first argument. 97The original arguments are shifted over to 98become the subsequent arguments. 99The zeroth argument is set to the specified 100.Em interpreter . 101.Pp 102The argument 103.Fa argv 104is a pointer to a null-terminated array of 105character pointers to null-terminated character strings. 106These strings construct the argument list to be made available to the new 107process. 108At least one argument must be present in 109the array; by custom, the first element should be 110the name of the executed program (for example, the last component of 111.Fa path ) . 112.Pp 113The argument 114.Fa envp 115is also a pointer to a null-terminated array of 116character pointers to null-terminated strings. 117A pointer to this array is normally stored in the global variable 118.Va environ . 119These strings pass information to the 120new process that is not directly an argument to the command (see 121.Xr environ 7 ) . 122.Pp 123File descriptors open in the calling process image remain open in 124the new process image, except for those for which the close-on-exec 125flag is set (see 126.Xr close 2 127and 128.Xr fcntl 2 ) . 129Descriptors that remain open are unaffected by 130.Fn execve , 131except those with the close-on-fork flag 132.Dv FD_CLOFORK 133which is cleared from all file descriptors. 134If any of the standard descriptors (0, 1, and/or 2) are closed at the 135time 136.Fn execve 137is called, and the process will gain privilege as a result of set-id 138semantics, those descriptors will be re-opened automatically. 139No programs, whether privileged or not, should assume that these descriptors 140will remain closed across a call to 141.Fn execve . 142.Pp 143Signals set to be ignored in the calling process are set to be ignored in 144the 145new process. 146Signals which are set to be caught in the calling process image 147are set to default action in the new process image. 148Blocked signals remain blocked regardless of changes to the signal action. 149The signal stack is reset to be undefined (see 150.Xr sigaction 2 151for more information). 152.Pp 153If the set-user-ID mode bit of the new process image file is set 154(see 155.Xr chmod 2 ) , 156the effective user ID of the new process image is set to the owner ID 157of the new process image file. 158If the set-group-ID mode bit of the new process image file is set, 159the effective group ID of the new process image is set to the group ID 160of the new process image file. 161(The effective group ID is the first element of the group list.) 162The real user ID, real group ID and 163other group IDs of the new process image remain the same as the calling 164process image. 165After any set-user-ID and set-group-ID processing, 166the effective user ID is recorded as the saved set-user-ID, 167and the effective group ID is recorded as the saved set-group-ID. 168These values may be used in changing the effective IDs later (see 169.Xr setuid 2 ) . 170.Pp 171The set-ID bits are not honored if the respective file system has the 172.Cm nosuid 173option enabled or if the new process file is an interpreter file. 174Syscall 175tracing is disabled if effective IDs are changed. 176.Pp 177The new process also inherits the following attributes from 178the calling process: 179.Pp 180.Bl -column parent_process_ID -offset indent -compact 181.It process ID Ta see Xr getpid 2 182.It parent process ID Ta see Xr getppid 2 183.It process group ID Ta see Xr getpgrp 2 184.It access groups Ta see Xr getgroups 2 185.It working directory Ta see Xr chdir 2 186.It root directory Ta see Xr chroot 2 187.It control terminal Ta see Xr termios 4 188.It resource usages Ta see Xr getrusage 2 189.It interval timers Ta see Xr getitimer 2 190.It resource limits Ta see Xr getrlimit 2 191.It file mode mask Ta see Xr umask 2 192.It signal mask Ta see Xr sigaction 2 , 193.Xr sigprocmask 2 194.El 195.Pp 196When a program is executed as a result of an 197.Fn execve 198system call, it is entered as follows: 199.Bd -literal -offset indent 200main(argc, argv, envp) 201int argc; 202char **argv, **envp; 203.Ed 204.Pp 205where 206.Fa argc 207is the number of elements in 208.Fa argv 209(the ``arg count'') 210and 211.Fa argv 212points to the array of character pointers 213to the arguments themselves. 214.Pp 215The 216.Fn fexecve 217ignores the file offset of 218.Fa fd . 219Since execute permission is checked by 220.Fn fexecve , 221the file descriptor 222.Fa fd 223need not have been opened with the 224.Dv O_EXEC 225flag. 226However, if the file to be executed denies read permission for the process 227preparing to do the exec, the only way to provide the 228.Fa fd 229to 230.Fn fexecve 231is to use the 232.Dv O_EXEC 233flag when opening 234.Fa fd . 235Opening without 236.Dv O_EXEC 237may be necessary in the case of executing an interpreted program, as the 238interpreter will not be able to acquire a descriptor to the script for reading 239without mounting 240.Xr fdescfs 4 241on 242.Pa /dev/fd 243with the 244.Cm nodup 245option. 246Note that the file to be executed can not be open for writing. 247.Sh RETURN VALUES 248As the 249.Fn execve 250system call overlays the current process image 251with a new process image the successful call 252has no process to return to. 253If 254.Fn execve 255does return to the calling process an error has occurred; the 256return value will be -1 and the global variable 257.Va errno 258is set to indicate the error. 259.Sh ERRORS 260The 261.Fn execve 262system call 263will fail and return to the calling process if: 264.Bl -tag -width Er 265.It Bq Er ENOTDIR 266A component of the path prefix is not a directory. 267.It Bq Er ENAMETOOLONG 268A component of a pathname exceeded 255 characters, 269or an entire path name exceeded 1023 characters. 270.It Bq Er ENOEXEC 271When invoking an interpreted script, the length of the first line, 272inclusive of the 273.Sy \&#! 274prefix and terminating newline, exceeds 275.Dv MAXSHELLCMDLEN 276characters. 277.It Bq Er ENOENT 278The new process file does not exist. 279.It Bq Er ELOOP 280Too many symbolic links were encountered in translating the pathname. 281.It Bq Er EACCES 282Search permission is denied for a component of the path prefix. 283.It Bq Er EACCES 284The new process file is not an ordinary file. 285.It Bq Er EACCES 286The new process file mode denies execute permission. 287.It Bq Er EINVAL 288.Fa argv 289did not contain at least one element. 290.It Bq Er ENOEXEC 291The new process file has the appropriate access 292permission, but has an invalid magic number in its header. 293.It Bq Er ETXTBSY 294The new process file is a pure procedure (shared text) 295file that is currently open for writing by some process. 296.It Bq Er ENOMEM 297The new process requires more virtual memory than 298is allowed by the imposed maximum 299.Pq Xr getrlimit 2 . 300.It Bq Er E2BIG 301The number of bytes in the new process' argument list 302is larger than the system-imposed limit. 303This limit is specified by the 304.Xr sysctl 3 305MIB variable 306.Dv KERN_ARGMAX . 307.It Bq Er EFAULT 308The new process file is not as long as indicated by 309the size values in its header. 310.It Bq Er EFAULT 311The 312.Fa path , 313.Fa argv , 314or 315.Fa envp 316arguments 317point 318to an illegal address. 319.It Bq Er EIO 320An I/O error occurred while reading from the file system. 321.It Bq Er EINTEGRITY 322Corrupted data was detected while reading from the file system. 323.El 324.Pp 325In addition, the 326.Fn fexecve 327will fail and return to the calling process if: 328.Bl -tag -width Er 329.It Bq Er EBADF 330The 331.Fa fd 332argument is not a valid file descriptor open for executing. 333.El 334.Sh SEE ALSO 335.Xr ktrace 1 , 336.Xr _exit 2 , 337.Xr fork 2 , 338.Xr open 2 , 339.Xr execl 3 , 340.Xr exit 3 , 341.Xr sysctl 3 , 342.Xr fdescfs 4 , 343.Xr a.out 5 , 344.Xr elf 5 , 345.Xr environ 7 , 346.Xr mount 8 347.Sh STANDARDS 348The 349.Fn execve 350system call conforms to 351.St -p1003.1-2001 , 352with the exception of reopening descriptors 0, 1, and/or 2 in certain 353circumstances. 354A future update of the Standard is expected to require this behavior, 355and it may become the default for non-privileged processes as well. 356.\" NB: update this caveat when TC1 is blessed. 357The support for executing interpreted programs is an extension. 358The 359.Fn fexecve 360system call conforms to The Open Group Extended API Set 2 specification. 361.Sh HISTORY 362The 363.Fn execve 364system call appeared in 365.At v7 . 366The 367.Fn fexecve 368system call appeared in 369.Fx 8.0 . 370.Sh CAVEATS 371If a program is 372.Em setuid 373to a non-super-user, but is executed when 374the real 375.Em uid 376is ``root'', then the program has some of the powers 377of a super-user as well. 378.Pp 379When executing an interpreted program through 380.Fn fexecve , 381kernel supplies 382.Pa /dev/fd/n 383as a second argument to the interpreter, 384where 385.Ar n 386is the file descriptor passed in the 387.Fa fd 388argument to 389.Fn fexecve . 390For this construction to work correctly, the 391.Xr fdescfs 4 392filesystem shall be mounted on 393.Pa /dev/fd . 394