execute a file
Standard C Library (libc, -lc)
, 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
, called the new
() system call
is equivalent to
() except that the
file to be executed is determined by the file descriptor
instead of a
. 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
An interpreter file begins with a line of the form:
When an interpreter file is execve
'd, the system
's the specified
. If the optional
is specified, it becomes the first argument
to the interpreter
, and the name of the
'd file becomes the second
argument; otherwise, the name of the originally
'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
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
. These strings pass information to
the new process that is not directly an argument to the command (see
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
Descriptors that remain open are unaffected by
(). If any of the standard
descriptors (0, 1, and/or 2) are closed at the time
() 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
for more information).
If the set-user-ID mode bit of the new process image file is set (see
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
The set-ID bits are not honored if the respective file system has the
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:
When a program is executed as a result of an
() system call, it is entered as
main(argc, argv, envp)
char **argv, **envp;
is the number of elements in
(the ``arg count'') and
points to the array of character
pointers to the arguments themselves.
() ignores the file offset of
. Since execute permission is checked by
(), the file descriptor
need not have been opened with the
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
() is to use the
flag when opening
. Note that the file to be executed can not
be open for writing.
() system call overlays the
current process image with a new process image the successful call has no
process to return to. If
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.
() 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
- 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
- The number of bytes in the new process' argument list is larger than the
system-imposed limit. This limit is specified by the
- The new process file is not as long as indicated by the size values in its
- The path,
envp arguments point to an illegal
- An I/O error occurred while reading from the file system.
In addition, the
() will fail and
return to the calling process if:
- The fd argument is not a valid file
descriptor open for executing.
() system call conforms to
IEEE Std 1003.1-2001
, 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
system call conforms to The Open Group Extended API Set 2 specification.
() system call appeared in
() system call appeared in
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
(), kernel supplies
as a second argument to the
interpreter, where n
is the file descriptor
passed in the fd
(). For this construction to work
filesystem shall be mounted on