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NAME
execve, fexecve — execute a file
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int
execve(const char *path, char *const argv[], char *const envp[]);
int
fexecve(int fd, char *const argv[], char *const envp[]);
DESCRIPTION
The execve() 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 new
process file. The fexecve() system call is equivalent to 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:
#! interpreter [arg]
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 execve(). 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 execve().
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 the nosuid option enabled
or if the new process file is an interpreter file. Syscall tracing is disabled if effective
IDs are changed.
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 execve() 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.
The fexecve() 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.
RETURN VALUES
As the 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 error.
ERRORS
The execve() system call will fail and return to the calling process if:
[ENOTDIR] A component of the path prefix is not a directory.
[ENAMETOOLONG] A component of a pathname exceeded 255 characters, or an entire path name
exceeded 1023 characters.
[ENOEXEC] When invoking an interpreted script, the length of the first line,
inclusive of the #! prefix and terminating newline, exceeds
MAXSHELLCMDLEN characters.
[ENOENT] The new process file does not exist.
[ELOOP] Too many symbolic links were encountered in translating the pathname.
[EACCES] Search permission is denied for a component of the path prefix.
[EACCES] The new process file is not an ordinary file.
[EACCES] The new process file mode denies execute permission.
[ENOEXEC] The new process file has the appropriate access permission, but has an
invalid magic number in its header.
[ETXTBSY] The new process file is a pure procedure (shared text) file that is
currently open for writing by some process.
[ENOMEM] The new process requires more virtual memory than is allowed by the
imposed maximum (getrlimit(2)).
[E2BIG] 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 KERN_ARGMAX.
[EFAULT] The new process file is not as long as indicated by the size values in
its header.
[EFAULT] The path, argv, or envp arguments point to an illegal address.
[EIO] An I/O error occurred while reading from the file system.
In addition, the fexecve() will fail and return to the calling process if:
[EBADF] The fd argument is not a valid file descriptor open for executing.
SEE ALSO
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)
STANDARDS
The 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.
HISTORY
The execve() system call appeared in 4.2BSD. The fexecve() system call appeared in
FreeBSD 8.0.
CAVEATS
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 fexecve(). For this construction to work correctly, the fdescfs(5) filesystem shall be
mounted on /dev/fd.