EXEC(2)
NAME
execl, execv, execle, execve, execlp, execvp, exec, exece, environ − execute a file
SYNOPSIS
execl(name, arg0, arg1, ..., argn, 0)
char *name, *arg0, *arg1, ..., *argn;
execv(name, argv)
char *name, *argv[ ];
execle(name, arg0, arg1, ..., argn, 0, envp)
char *name, *arg0, *arg1, ..., *argn, *envp[ ];
execve(name, argv, envp);
char *name, *argv[ ], *envp[ ];
extern char **environ;
DESCRIPTION
Exec in all its forms overlays the calling process with the named file, then transfers to the entry point of the core image of the file. There can be no return from a successful exec; the calling core image is lost.
Files remain open across exec unless explicit arrangement has been made; see ioctl(2). Ignored signals remain ignored across these calls, but signals that are caught (see signal(2)) are reset to their default values.
Each user has a real user ID and group ID and an effective user ID and group ID. The real ID identifies the person using the system; the effective ID determines his access privileges. Exec 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 name argument is a pointer to the name of the file to be executed. The pointers arg[0], arg[1] ... address null-terminated strings. Conventionally arg[0] is the name of the file.
From C, two interfaces are available. Execl is useful when a known file with known arguments is being called; the arguments to execl are the character strings constituting the file and the arguments; the first argument is conventionally the same as the file name (or its last component). A 0 argument must end the argument list.
The execv version is useful when the number of arguments is unknown in advance; the arguments to execv are the name of the file to be executed and a vector of strings containing the arguments. The last argument string must be followed by a 0 pointer.
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 and argv is an array of character pointers to the arguments themselves. 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.
Argv is directly usable in another execv because argv[argc] is 0.
Envp is a pointer to an array of strings that constitute the environment of the process. 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(5) for some conventionally used names. The C run-time start-off routine places a copy of envp in the global cell environ, which is used by execv and execl to pass the environment to any subprograms executed by the current program. The exec routines use lower-level routines as follows to pass an environment explicitly:
execle(file, arg0, arg1, . . . , argn, 0, environ);
execve(file, argv, environ);
Execlp and execvp are called with the same arguments as execl and execv, but duplicate the shell’s actions in searching for an executable file in a list of directories. The directory list is obtained from the environment.
FILES
/bin/shshell, invoked if command file found by execlp or execvp
RETURN VALUE
If exec 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.
DIAGNOSTICS
Exec will fail and return to the calling process if one or more of the following is true:
[EACCES] Search permission is denied for a directory listed in the new process file’s path prefix.
[EACCES] The new process file mode denies execute permission.
[EACCES] The new process file is not a regular file.
[EFAULT] Name points outside the process’s allocated address space.
[EFAULT] Name, argv, or envp point to an illegal address (outside of the user’s image).
[ENFILE] Insufficient system space to contain i-node.
[ENOENT] One or more components of the new process file’s path name does not exist.
[ENOENT] The target file is a directory.
[ENOEXEC] The new process file has the appropriate access permission but the file header contains an invalid magic number.
[ENOEXEC] The text or data segment is null.
[ENOEXEC] An overlay is larger than the overlay max size.
[ENOEXEC] Unable to read file header.
[ENOEXEC] Found a bad magic number in the file header.
[ENOMEM] The new process requires more memory than is allowed.
[ENOTDIR] A component of the new process path name is not a directory.
[ETXTBSY] The new process file is a pure procedure (shared text) file that is currently open for reading or writing by some process.
RESTRICTIONS
If execvp is called to execute a file that turns out to be a shell command file, and if it is impossible to execute the shell, the values of argv[0] and argv[−1] will be modified before return.
SEE ALSO
ASSEMBLER
(exec = 11.)
sys exec; name; argv
(exece = 59.)
sys exece; name; argv; envp
Plain exec is obsoleted by exece, but remains for historical reasons.
When the called file starts execution on the PDP11, the stack pointer points to a word containing the number of arguments. Just above this number is a list of pointers to the argument strings, followed by a null pointer, followed by the pointers to the environment strings and then another null pointer. The strings themselves follow; a 0 word is left at the very top of memory.
sp→nargs
arg0
...
argn
0
env0
...
envm
0
arg0:<arg0\0>
...
env0:<env0\0>
0
On the Interdata 8/32, the stack begins at a conventional place (currently 0xD0000) and grows upwards. After exec, the layout of data on the stack is as follows.
int0
arg0:byte...
...
argp0:intarg0
...
int0
envp0:intenv0
...
int0
%2→space40
intnargs
intargp0
intenvp0
%3→
This arrangement happens to conform well to C calling conventions.