exec(2) exec(2)
NAME
exec: execl, execv, execle, execve, execlp, execvp - execute a file
SYNOPSIS
#include <unistd.h>
int execl (const char *path, const char *arg0, ..., const char *argn,
(char *)0);
int execv (const char *path, char *const *argv);
int execle (const char *path, const char *arg0, ..., const char
*argn, (char *0), const char *envp[]);
int execve (const char *path, char *const *argv, char *const *cnup);
int execlp (const char *file, const char *arg0, ..., const char
*argn, (char *)0);
int execvp (const char *file, char *const *argv);
DESCRIPTION
exec in all its forms overlays a new process image on an old process.
The new process image is constructed from an ordinary, executable
file. This file is either an executable object file, or a file of
data for an interpreter. There can be no return from a successful
exec because the calling process image is overlaid by the new process
image.
An interpreter file begins with a line of the form
#! pathname [arg]
where pathname is the path of the interpreter, and arg is an optional
argument. When an interpreter file is exec'd, the system execs the
specified interpreter. The pathname specified in the interpreter
file is passed as arg0 to the interpreter. If arg was specified in
the interpreter file, it is passed as arg1 to the interpreter. The
remaining arguments to the interpreter are arg0 through argn of the
originally exec'd file.
When a C program is executed, it is called as follows:
int main (int argc, char *argv[], char *envp[]);
where argc is the argument count, argv is an array of character
pointers to the arguments themselves, and envp is an array of
character pointers to the environment strings. As indicated, argc is
at least one, and the first member of the array points to a string
containing the name of the file.
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path points to a path name that identifies the new process file.
file points to the new process file. If file does not contain a
slash character, the path prefix for this file is obtained by a
search of the directories passed in the PATH environment variable
[see environ(5)]. The environment is supplied typically by the shell
[see sh(1)].
If the new process file is not an executable object file, execlp and
execvp use the contents of that file as standard input to sh(1).
The arguments arg0, ..., argn point to null-terminated character
strings. These strings constitute the argument list available to the
new process image. Minimally, arg0 must be present. It will become
the name of the process, as displayed by the ps command.
Conventionally, arg0 points to a string that is the same as path (or
the last component of path). The list of argument strings is
terminated by a (char *)0 argument.
argv is an array of character pointers to null-terminated strings.
These strings constitute the argument list available to the new
process image. By convention, argv must have at least one member,
and it should 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 image.
envp is terminated by a null pointer. For execl, execv, execvp, and
execlp, the C run-time start-off routine places a pointer to the
environment of the calling process in the global object extern char
**environ, and it is used to pass the environment of the calling
process 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 that are being caught by the calling process are set to the
default disposition in the new process image [see signal(2)].
Otherwise, the new process image inherits the signal dispositions of
the calling process.
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) exec(2)
If the effective user-ID is root or super-user, the set-user-ID and
set-group-ID bits will be honored when the process is being
controlled by ptrace.
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)]
scheduler class and priority [see priocntl(2)]
process ID
parent process ID
process group ID
supplementary group IDs
semadj values [see semop(2)]
session 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)]
resource limits [see getrlimit(2)]
utime, stime, cutime, and cstime [see times(2)]
file-locks [see fcntl(2) and lockf(3C)]
controlling terminal
process signal mask [see sigprocmask(2)]
pending signals [see sigpending(2)]
Upon successful completion, exec marks for update the st_atime field
of the file. Should the exec succeed, the process image file is
considered to have been open()-ed. The corresponding close() is
considered to occur at a time after this open, but before process
termination or successful completion of a subsequent call to exec.
exec will fail and return to the calling process if one or more of
the following are true:
EACCES Search permission is denied for a directory
listed in the new process file's path prefix.
E2BIG The number of bytes in the new process's argument
list is greater than the system-imposed limit of
5120 bytes. The argument list limit is sum of
the size of the argument list plus the size of
the environment's exported shell variables.
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exec(2) exec(2)
EACCES The new process file is not an ordinary file.
EACCES The new process file mode denies execution
permission.
EAGAIN Total amount of system memory available when
reading via raw I/O is temporarily insufficient.
EFAULT Required hardware is not present.
EFAULT An a.out that was compiled with the MAU flag is
running on a machine without a MAU.
EFAULT An argument points to an illegal address.
EINTR A signal was caught during the exec system call.
ELIBACC Required shared library does not have execute
permission.
ELIBEXEC Trying to exec(2) a shared library directly.
ELOOP Too many symbolic links were encountered in
translating path or file.
EMULTIHOP Components of path require hopping to multiple
remote machines and the file system type does not
allow it.
ENAMETOOLONG The length of the file or path argument exceeds
{PATH_MAX}, or the length of a file or path
component exceeds {NAME_MAX} while
_POSIX_NO_TRUNC is in effect.
ENOENT One or more components of the new process path
name of the file do not exist or is a null
pathname.
ENOTDIR A component of the new process path of the file
prefix is not a directory.
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.
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exec(2) exec(2)
ENOMEM The new process requires more memory than is
allowed by the system-imposed maximum MAXMEM.
ENOLINK path points to a remote machine and the link to
that machine is no longer active.
SEE ALSO
alarm(2), exit(2), fcntl(2), fork(2), getrlimit(2), nice(2),
priocntl(2), ptrace(2), semop(2), signal(2), sigpending(2),
sigprocmask(2), times(2), umask(2), lockf(3C), system(3S),
environ(5).
sh(1), ps(1) in the User's Reference Manual.
DIAGNOSTICS
If exec returns to the calling process, an error has occurred; the
return value is -1 and errno is set to indicate the error.
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