| 1 | .\" Copyright (c) 1980 Regents of the University of California. |
| 2 | .\" All rights reserved. The Berkeley software License Agreement |
| 3 | .\" specifies the terms and conditions for redistribution. |
| 4 | .\" |
| 5 | .\" @(#)execve.2 6.7 (Berkeley) %G% |
| 6 | .\" |
| 7 | .TH EXECVE 2 "" |
| 8 | .UC 4 |
| 9 | .SH NAME |
| 10 | execve \- execute a file |
| 11 | .SH SYNOPSIS |
| 12 | .ft B |
| 13 | execve(name, argv, envp) |
| 14 | .br |
| 15 | char *name, *argv[], *envp[]; |
| 16 | .fi |
| 17 | .SH DESCRIPTION |
| 18 | .I Execve |
| 19 | transforms the calling process into a new process. |
| 20 | The new process is constructed from an ordinary file |
| 21 | called the \fInew process file\fP. |
| 22 | This file is either an executable object file, |
| 23 | or a file of data for an interpreter. |
| 24 | An executable object file consists of an identifying header, |
| 25 | followed by pages of data representing the initial program (text) |
| 26 | and initialized data pages. Additional pages may be specified |
| 27 | by the header to be initialized with zero data. See |
| 28 | .IR a.out (5). |
| 29 | .PP |
| 30 | An interpreter file begins with a line of the form ``#! \fIinterpreter\fP''. |
| 31 | When an interpreter file is |
| 32 | .IR execve\| 'd, |
| 33 | the system \fIexecve\fP\|'s the specified \fIinterpreter\fP, giving |
| 34 | it the name of the originally exec'd file as an argument and |
| 35 | shifting over the rest of the original arguments. |
| 36 | .PP |
| 37 | There can be no return from a successful \fIexecve\fP because the calling |
| 38 | core image is lost. |
| 39 | This is the mechanism whereby different process images become active. |
| 40 | .PP |
| 41 | The argument \fIargv\fP is a null-terminated array of character pointers |
| 42 | to null-terminated character strings. These strings constitute |
| 43 | the argument list to be made available to the new |
| 44 | process. By convention, at least one argument must be present in |
| 45 | this array, and the first element of this array should be |
| 46 | the name of the executed program (i.e., the last component of \fIname\fP). |
| 47 | .PP |
| 48 | The argument \fIenvp\fP is also a null-terminated array of character pointers |
| 49 | to null-terminated strings. These strings pass information to the |
| 50 | new process that is not directly an argument to the command (see |
| 51 | .IR environ (7)). |
| 52 | .PP |
| 53 | Descriptors open in the calling process remain open in |
| 54 | the new process, except for those for which the close-on-exec |
| 55 | flag is set (see |
| 56 | .IR close (2)). |
| 57 | Descriptors that remain open are unaffected by |
| 58 | .IR execve . |
| 59 | .PP |
| 60 | Ignored signals remain ignored across an |
| 61 | .IR execve , |
| 62 | but signals that are caught are reset to their default values. |
| 63 | Blocked signals remain blocked regardless of changes to the signal action. |
| 64 | The signal stack is reset to be undefined (see |
| 65 | .IR sigvec (2) |
| 66 | for more information). |
| 67 | .PP |
| 68 | Each process has |
| 69 | .I real |
| 70 | user and group IDs and an |
| 71 | .I effective |
| 72 | user and group IDs. The |
| 73 | .I real |
| 74 | ID identifies the person using the system; the |
| 75 | .I effective |
| 76 | ID determines his access privileges. |
| 77 | .I Execve |
| 78 | changes the effective user and group ID to |
| 79 | the owner of the executed file if the file has the \*(lqset-user-ID\*(rq |
| 80 | or \*(lqset-group-ID\*(rq modes. The |
| 81 | .I real |
| 82 | user ID is not affected. |
| 83 | .PP |
| 84 | The new process also inherits the following attributes from |
| 85 | the calling process: |
| 86 | .PP |
| 87 | .in +5n |
| 88 | .nf |
| 89 | .ta +2i |
| 90 | process ID see \fIgetpid\fP\|(2) |
| 91 | parent process ID see \fIgetppid\fP\|(2) |
| 92 | process group ID see \fIgetpgrp\fP\|(2) |
| 93 | access groups see \fIgetgroups\fP\|(2) |
| 94 | working directory see \fIchdir\fP\|(2) |
| 95 | root directory see \fIchroot\fP\|(2) |
| 96 | control terminal see \fItty\fP\|(4) |
| 97 | resource usages see \fIgetrusage\fP\|(2) |
| 98 | interval timers see \fIgetitimer\fP\|(2) |
| 99 | resource limits see \fIgetrlimit\fP\|(2) |
| 100 | file mode mask see \fIumask\fP\|(2) |
| 101 | signal mask see \fIsigvec\fP\|(2), \fIsigmask\fP\|(2) |
| 102 | .in -5n |
| 103 | .fi |
| 104 | .PP |
| 105 | When the executed program begins, it is called as follows: |
| 106 | .PP |
| 107 | .DT |
| 108 | .nf |
| 109 | main(argc, argv, envp) |
| 110 | int argc; |
| 111 | char **argv, **envp; |
| 112 | .fi |
| 113 | .PP |
| 114 | where |
| 115 | .I argc |
| 116 | is the number of elements in \fIargv\fP |
| 117 | (the ``arg count'') |
| 118 | and |
| 119 | .I argv |
| 120 | is the array of character pointers |
| 121 | to the arguments themselves. |
| 122 | .PP |
| 123 | .I Envp |
| 124 | is a pointer to an array of strings that constitute |
| 125 | the |
| 126 | .I environment |
| 127 | of the process. |
| 128 | A pointer to this array is also stored in the global variable ``environ''. |
| 129 | Each string consists of a name, an \*(lq=\*(rq, and a null-terminated value. |
| 130 | The array of pointers is terminated by a null pointer. |
| 131 | The shell |
| 132 | .IR sh (1) |
| 133 | passes an environment entry for each global shell variable |
| 134 | defined when the program is called. |
| 135 | See |
| 136 | .IR environ (7) |
| 137 | for some conventionally |
| 138 | used names. |
| 139 | .SH "RETURN VALUE |
| 140 | If |
| 141 | .I execve |
| 142 | returns to the calling process an error has occurred; the |
| 143 | return value will be \-1 and the global variable |
| 144 | .I errno |
| 145 | will contain an error code. |
| 146 | .SH ERRORS |
| 147 | .I Execve |
| 148 | will fail and return to the calling process if one or more |
| 149 | of the following are true: |
| 150 | .TP 15 |
| 151 | [ENOTDIR] |
| 152 | A component of the path prefix is not a directory. |
| 153 | .TP 15 |
| 154 | [EINVAL] |
| 155 | The pathname contains a character with the high-order bit set. |
| 156 | .TP 15 |
| 157 | [ENAMETOOLONG] |
| 158 | A component of a pathname exceeded 255 characters, |
| 159 | or an entire path name exceeded 1023 characters. |
| 160 | .TP 15 |
| 161 | [ENOENT] |
| 162 | The new process file does not exist. |
| 163 | .TP 15 |
| 164 | [ELOOP] |
| 165 | Too many symbolic links were encountered in translating the pathname. |
| 166 | .TP 15 |
| 167 | [EACCES] |
| 168 | Search permission is denied for a component of the path prefix. |
| 169 | .TP 15 |
| 170 | [EACCES] |
| 171 | The new process file is not an ordinary file. |
| 172 | .TP 15 |
| 173 | [EACCES] |
| 174 | The new process file mode denies execute permission. |
| 175 | .TP 15 |
| 176 | [ENOEXEC] |
| 177 | The new process file has the appropriate access |
| 178 | permission, but has an invalid magic number in its header. |
| 179 | .TP 15 |
| 180 | [ETXTBSY] |
| 181 | The new process file is a pure procedure (shared text) |
| 182 | file that is currently open for writing or reading by some process. |
| 183 | .TP 15 |
| 184 | [ENOMEM] |
| 185 | The new process requires more virtual memory than |
| 186 | is allowed by the imposed maximum |
| 187 | .RI ( getrlimit (2)). |
| 188 | .TP 15 |
| 189 | [E2BIG] |
| 190 | The number of bytes in the new process's argument list |
| 191 | is larger than the system-imposed limit. |
| 192 | The limit in the system as released is 20480 bytes |
| 193 | (NCARGS in |
| 194 | .IR <sys/param.h> . |
| 195 | .TP 15 |
| 196 | [EFAULT] |
| 197 | The new process file is not as long as indicated by |
| 198 | the size values in its header. |
| 199 | .TP 15 |
| 200 | [EFAULT] |
| 201 | \fIPath\fP\|, \fIargv\fP\|, or \fIenvp\fP point |
| 202 | to an illegal address. |
| 203 | .TP 15 |
| 204 | [EIO] |
| 205 | An I/O error occurred while reading from the file system. |
| 206 | .SH CAVEATS |
| 207 | If a program is |
| 208 | .I setuid |
| 209 | to a non-super-user, but is executed when |
| 210 | the real \fIuid\fP is ``root'', then the program has some of the powers |
| 211 | of a super-user as well. |
| 212 | .SH "SEE ALSO" |
| 213 | exit(2), fork(2), execl(3), environ(7) |