EXECVE(2)

NAME
execve - execute a file

SYNOPSIS
execve(name, argv, envp)
char *name, *argv[], *envp[];

DESCRIPTION
Execve transforms the calling process into a new process. The new process is constructed from an ordinary file called the new process file. 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 a.out(5).

An interpreter file begins with a line of the form ``#! interpreter''. When an interpreter file is execve'd, the system execve's the specified interpreter, giving it the name of the originally exec'd file as an argument and shifting over the rest of the original arguments.

There can be no return from a successful execve because the calling core image is lost. This is the mechanism whereby different process images become active.

The argument argv is a null-terminated array of character pointers to null-terminated character strings. These strings constitute the argument list to be made available to the new process. By convention, at least one argument must be present in this array, and the first element of this array should be the name of the executed program (i.e., the last component of name).

The argument envp is also a null-terminated array of character pointers to null-terminated strings. These strings pass information to the new process that is not directly an argument to the command (see environ(7)).

Descriptors open in the calling process remain open in the new process, except for those for which the close-on-exec flag is set (see close(2)). Descriptors that remain open are unaffected by execve.

Ignored signals remain ignored across an execve, but signals that are caught are reset to their default values. Blocked signals remain blocked regardless of changes to the signal action. The signal stack is reset to be undefined (see sigvec(2) for more information).

Each process has real user and group IDs and an effective user and group IDs. The real ID identifies the person using the system; the effective ID determines his access privileges. Execve changes the effective user and group ID to the owner of the executed file if the file has the “set-user-ID” or “set-group-ID” modes. The real user ID is not affected.

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 tty(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 sigvec(2), sigmask(2)

When the executed program begins, it is called 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 is the array of character pointers to the arguments themselves.

Envp is a pointer to an array of strings that constitute the environment of the process. A pointer to this array is also stored in the global variable ``environ''. Each string consists of a name, an “=”, and a null-terminated value. The array of pointers is terminated by a null pointer. The shell sh(1) passes an environment entry for each global shell variable defined when the program is called. See environ(7) for some conventionally used names.

RETURN VALUE
If execve returns to the calling process an error has occurred; the return value will be -1 and the global variable errno will contain an error code.

ERRORS
Execve will fail and return to the calling process if one or more of the following are true:

[ENOTDIR]
    A component of the path prefix is not a directory.

[EINVAL]
    The pathname contains a character with the high-order bit set.

[ENAMETOOLONG]
    A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 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.

[ENOEXEC]
    The new process file has an invalid entry address (not word-aligned, or with high-order flag bits set).

[ETXTBSY]
    The new process file is a pure procedure (shared text) file that is currently open for writing or reading 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's argument list is larger than the system-imposed limit. The limit in the system as released is 20480 bytes (NCARGS in <sys/param.h>.

[EFAULT]
    The new process file is not as long as indicated by the size values in its header.

[EFAULT]
    Path, argv, or envp point to an illegal address.

[EIO]
    An I/O error occurred while reading from the file system.

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.

SEE ALSO
exit(2), fork(2), execl(3), environ(7)