exec(2) INTERACTIVE UNIX System exec(2)
NAME
exec: execl, execv, execle, execve, execlp, execvp - execute
a file
SYNOPSIS
int execl (path, arg0, arg1, ..., argn, (char *)0)
char *path, *arg0, *arg1, ..., *argn;
int execv (path, argv)
char *path, *argv[ ];
int execle (path, arg0, arg1, ..., argn, (char *)0, envp)
char *path, *arg0, *arg1, ..., *argn, *envp[ ];
int execve (path, argv, envp)
char *path, *argv[ ], *envp[ ];
int execlp (file, arg0, arg1, ..., argn, (char *)0)
char *file, *arg0, *arg1, ..., *argn;
int execvp (file, argv)
char *file, *argv[ ];
DESCRIPTION
The exec system call in all its forms transforms the calling
process into a new process. The new process is constructed
from an ordinary, executable file called the new process
file. This file consists of a header (see a.out(4)), a text
segment, and a data segment. The data segment contains an
initialized portion and an uninitialized portion (bss).
There can be no return from a successful exec because the
calling process is overlaid by the new process.
When a C program is executed, it is called as follows:
main (argc, argv, envp)
int argc;
char **argv, **envp;
where argc is the argument count, argv is an array of char-
acter pointers to the arguments themselves, and envp is an
array of character pointers to the environment strings. As
indicated, argc is conventionally at least one, and the
first member of the array points to a string containing the
name of the file.
The path argument points to a path name that identifies the
new process file.
The file argument points to the new process file. The path
prefix for this file is obtained by a search of the direc-
tories passed as the environment line "PATH =" (see
environ(5)). The environment is supplied by the shell (see
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exec(2) INTERACTIVE UNIX System exec(2)
sh(1)).
arg0, arg1, ..., argn are pointers to null-terminated char-
acter strings. These strings constitute the argument list
available to the new process. By convention, at least arg0
must be present and point to a string that is the same as
path (or its last component).
argv is an array of character pointers to null-terminated
strings. These strings constitute the argument list avail-
able to the new process. By convention, argv must have at
least one member, and it must point to a string that is the
same as path (or its last component). argv is terminated by
a null pointer.
envp is an array of character pointers to null-terminated
strings. These strings constitute the environment for the
new process. envp is terminated by a null pointer. For
execl and execv, the C run-time start-off routine places a
pointer to the environment of the calling process in the
global cell:
extern char **environ;
and it is used to pass the environment of the calling pro-
cess to the new process.
File descriptors open in the calling process remain open in
the new process, except for those whose close-on-exec flag
is set (see fcntl(2)). For those file descriptors that
remain open, the file pointer is unchanged.
Signals set to terminate the calling process will be set to
terminate the new process. Signals set to be ignored by the
calling process will be set to be ignored by the new pro-
cess. Signals set to be caught by the calling process will
be set to terminate the new process (see signal(2)).
For signals set by sigset(2), exec will ensure that the new
process has the same system signal action for each signal
type whose action is SIG_DFL, SIG_IGN, or SIG_HOLD as the
calling process. However, if the action is to catch the
signal, then the action will be reset to SIG_DFL, and any
pending signal for this type will be held.
If the set-user-ID mode bit of the new process file is set
(see chmod(2)), exec sets the effective user ID of the new
process to the owner ID of the new process file. Similarly,
if the set-group-ID mode bit of the new process file is set,
the effective group ID of the new process is set to the
group ID of the new process file. The real user ID and real
group ID of the new process remain the same as those of the
calling process.
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exec(2) INTERACTIVE UNIX System exec(2)
The shared memory segments attached to the calling process
will not be attached to the new process (see shmop(2)).
Profiling is disabled for the new process (see profil(2)).
The new process also inherits the following attributes from
the calling process:
nice value (see nice(2))
process ID
parent process ID
process group ID
semadj values (see semop(2))
tty group ID (see exit(2) and signal(2))
trace flag (see ptrace(2) request 0)
time left until an alarm clock signal (see alarm(2))
current working directory
root directory
file mode creation mask (see umask(2))
file size limit (see ulimit(2))
utime, stime, cutime, and cstime (see times(2))
file-locks (see fcntl(2) and lockf(3C))
The exec system call will fail and return to the calling
process if one or more of the following is true:
[ENOENT] One or more components of the new process
path name of the file do not exist.
[ENOTDIR] A component of the new process path of the
file prefix is not a directory.
[EACCES] Search permission is denied for a directory
listed in the new process file's path prefix.
The new process file is not an ordinary file.
The new process file mode denies execution
permission.
[ENOEXEC] The exec is not an execlp or execvp, and the
new process file has the appropriate access
permission but 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 memory than is
allowed by the system-imposed maximum MAXMEM.
[E2BIG] The number of bytes in the new process's
argument list is greater than the system-
imposed limit of 5120 bytes.
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exec(2) INTERACTIVE UNIX System exec(2)
[EFAULT] Required hardware is not present.
path, argv, or envp point to an illegal
address.
[EAGAIN] Not enough memory.
[ELIBACC] Required shared library does not have execute
permission.
[ELIBEXEC] Trying to exec(2) a shared library directly.
[EINTR] A signal was caught during the exec system
call.
[ENOLINK] The path variable points to a remote machine
and the link to that machine is no longer
active.
[EMULTIHOP] Components of path require hopping to multi-
ple remote machines.
[ENAMETOOLONG] (POSIX Only) The length of the path argument
exceeds {PATH_MAX}, or a path name component
is longer than {NAME_MAX} while
{_POSIX_NO_TRUNC} is in effect.
SEE ALSO
alarm(2), exit(2), fcntl(2), fork(2), nice(2), ptrace(2),
semop(2), signal(2), sigset(2), times(2), ulimit(2),
umask(2), lockf(3C), a.out(4), environ(5).
sh(1) in the INTERACTIVE UNIX System User's/System
Administrator's Reference Manual.
DIAGNOSTICS
If exec returns to the calling process, an error has
occurred; the return value will be -1 and errno will be set
to indicate the error.
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