table of contents
|EXECVE(2)||System Calls Manual||EXECVE(2)|
— execute a file
Standard C Library (libc, -lc)
char *path, char *const
argv, char *const
fd, char *const
argv, char *const
system call transforms the calling process into a new process. The new
process is constructed from an ordinary file, whose name is pointed to by
path, called the
fexecve() system call is
execve() except that the file to be
executed is determined by the file descriptor fd
instead of a path. This file is either an executable
object file, or a file of data for an interpreter. An executable object file
consists of an identifying header, followed by pages of data representing
the initial program (text) and initialized data pages. Additional pages may
be specified by the header to be initialized with zero data; see
elf(5) and a.out(5).
An interpreter file begins with a line of the form:
When an interpreter file is execve'd, the system actually execve's the specified interpreter. If the optional arg is specified, it becomes the first argument to the interpreter, and the name of the originally execve'd file becomes the second argument; otherwise, the name of the originally execve'd file becomes the first argument. The original arguments are shifted over to become the subsequent arguments. The zeroth argument is set to the specified interpreter.
The argument argv is a pointer to a null-terminated array of character pointers to null-terminated character strings. These strings construct the argument list to be made available to the new process. At least one argument must be present in the array; by custom, the first element should be the name of the executed program (for example, the last component of path).
The argument envp is also a pointer to a null-terminated array of character pointers to null-terminated strings. A pointer to this array is normally stored in the global variable environ. These strings pass information to the new process that is not directly an argument to the command (see environ(7)).
File descriptors open in the calling process image
remain open in the new process image, except for those for which the
close-on-exec flag is set (see close(2) and
fcntl(2)). Descriptors that remain open are unaffected by
If any of the standard descriptors (0, 1, and/or 2) are closed at the time
execve() is called, and the process will gain
privilege as a result of set-id semantics, those descriptors will be
re-opened automatically. No programs, whether privileged or not, should
assume that these descriptors will remain closed across a call to
Signals set to be ignored in the calling process are set to be ignored in the new process. Signals which are set to be caught in the calling process image are set to default action in the new process image. Blocked signals remain blocked regardless of changes to the signal action. The signal stack is reset to be undefined (see sigaction(2) for more information).
If the set-user-ID mode bit of the new process image file is set (see chmod(2)), the effective user ID of the new process image is set to the owner ID of the new process image file. If the set-group-ID mode bit of the new process image file is set, the effective group ID of the new process image is set to the group ID of the new process image file. (The effective group ID is the first element of the group list.) The real user ID, real group ID and other group IDs of the new process image remain the same as the calling process image. After any set-user-ID and set-group-ID processing, the effective user ID is recorded as the saved set-user-ID, and the effective group ID is recorded as the saved set-group-ID. These values may be used in changing the effective IDs later (see setuid(2)).
The set-ID bits are not honored if the respective file system has
nosuid option enabled or if the new process file
is an interpreter file. Syscall tracing is disabled if effective IDs are
The new process also inherits the following attributes from the calling process:
|process ID||see getpid(2)|
|parent process ID||see getppid(2)|
|process group ID||see getpgrp(2)|
|access groups||see getgroups(2)|
|working directory||see chdir(2)|
|root directory||see chroot(2)|
|control terminal||see termios(4)|
|resource usages||see getrusage(2)|
|interval timers||see getitimer(2)|
|resource limits||see getrlimit(2)|
|file mode mask||see umask(2)|
|signal mask||see sigaction(2), sigprocmask(2)|
When a program is executed as a result of an
system call, it is entered as follows:
main(argc, argv, envp) int argc; char **argv, **envp;
where argc is the number of elements in argv (the ``arg count'') and argv points to the array of character pointers to the arguments themselves.
ignores the file offset of fd. Since execute
permission is checked by
fexecve(), the file
descriptor fd need not have been opened with the
O_EXEC flag. However, if the file to be executed
denies read permission for the process preparing to do the exec, the only
way to provide the fd to
fexecve() is to use the
O_EXEC flag when opening fd.
Note that the file to be executed can not be open for writing.
execve() system call overlays the
current process image with a new process image the successful call has no
process to return to. If
execve() does return to the
calling process an error has occurred; the return value will be -1 and the
global variable errno is set to indicate the
execve() system call will fail and
return to the calling process if:
- A component of the path prefix is not a directory.
- A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
- When invoking an interpreted script, the length of the first line,
inclusive of the #! prefix and terminating newline,
- The new process file does not exist.
- Too many symbolic links were encountered in translating the pathname.
- Search permission is denied for a component of the path prefix.
- The new process file is not an ordinary file.
- The new process file mode denies execute permission.
- The new process file has the appropriate access permission, but has an invalid magic number in its header.
- The new process file is a pure procedure (shared text) file that is currently open for writing by some process.
- The new process requires more virtual memory than is allowed by the imposed maximum (getrlimit(2)).
- The number of bytes in the new process' argument list is larger than the
system-imposed limit. This limit is specified by the
sysctl(3) MIB variable
- The new process file is not as long as indicated by the size values in its header.
- The path, argv, or envp arguments point to an illegal address.
- An I/O error occurred while reading from the file system.
In addition, the
fexecve() will fail and
return to the calling process if:
- The fd argument is not a valid file descriptor open for executing.
ktrace(1), _exit(2), fork(2), open(2), execl(3), exit(3), sysctl(3), a.out(5), elf(5), fdescfs(5), environ(7), mount(8)
execve() system call conforms to
IEEE Std 1003.1-2001 (“POSIX.1”), with
the exception of reopening descriptors 0, 1, and/or 2 in certain
circumstances. A future update of the Standard is expected to require this
behavior, and it may become the default for non-privileged processes as
well. The support for executing interpreted programs is an extension. The
fexecve() system call conforms to The Open Group
Extended API Set 2 specification.
execve() system call appeared in
call appeared in FreeBSD 8.0.
If a program is setuid to a non-super-user, but is executed when the real uid is ``root'', then the program has some of the powers of a super-user as well.
When executing an interpreted program through
fexecve(), kernel supplies
/dev/fd/n as a second argument to the interpreter,
where n is the file descriptor passed in the
fd argument to
this construction to work correctly, the fdescfs(5)
filesystem shall be mounted on /dev/fd.
|September 21, 2010||Debian|