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1.1 root 1: .TH EXECL 3 "1 April 1981"
2: .UC 4
3: .SH NAME
4: execl, execv, execle, execlp, execvp, exec, exece, exect, environ \- execute a file
5: .SH SYNOPSIS
6: .nf
7: .B execl(name, arg0, arg1, ..., argn, 0)
8: .B char *name, *arg0, *arg1, ..., *argn;
9: .PP
10: .B execv(name, argv)
11: .B char *name, *argv[];
12: .PP
13: .B "execle(name, arg0, arg1, ..., argn, 0, envp)"
14: .B "char *name, *arg0, *arg1, ..., *argn, *envp[];"
15: .PP
16: .B "exect(name, argv, envp)
17: .B "char *name, *argv[], *envp[];
18: .PP
19: .B extern char **environ;
20: .fi
21: .SH DESCRIPTION
22: These routines provide various interfaces to the
23: .I execve
24: system call. Refer to
25: .IR execve (2)
26: for a description of their properties; only
27: brief descriptions are provided here.
28: .PP
29: .I Exec
30: in all its forms
31: overlays the calling process with the named file, then
32: transfers to the
33: entry point of the core image of the file.
34: There can be no return from a successful exec; the calling
35: core image is lost.
36: .PP
37: The
38: .I name
39: argument
40: is a pointer to the name of the file
41: to be executed.
42: The pointers
43: .IR arg [ 0 ],
44: .IR arg [ 1 "] ..."
45: address null-terminated strings.
46: Conventionally
47: .IR arg [ 0 ]
48: is the name of the
49: file.
50: .PP
51: Two interfaces are available.
52: .I execl
53: is useful when a known file with known arguments is
54: being called;
55: the arguments to
56: .I execl
57: are the character strings
58: constituting the file and the arguments;
59: the first argument is conventionally
60: the same as the file name (or its last component).
61: A 0 argument must end the argument list.
62: .PP
63: The
64: .I execv
65: version is useful when the number of arguments is unknown
66: in advance;
67: the arguments to
68: .I execv
69: are the name of the file to be
70: executed and a vector of strings containing
71: the arguments.
72: The last argument string must be followed
73: by a 0 pointer.
74: .PP
75: The
76: .I exect
77: version is used when the executed file is to be
78: manipulated with
79: .IR ptrace (2).
80: The program is forced to single step a single
81: instruction giving the parent an opportunity to
82: manipulate its state. This is done
83: by setting the trace bit in the process status
84: longword.
85: .PP
86: When a C program is executed,
87: it is called as follows:
88: .PP
89: .nf
90: main(argc, argv, envp)
91: int argc;
92: char **argv, **envp;
93: .fi
94: .PP
95: where
96: .I argc
97: is the argument count
98: and
99: .I argv
100: is an array of character pointers
101: to the arguments themselves.
102: As indicated,
103: .I argc
104: is conventionally at least one
105: and the first member of the array points to a
106: string containing the name of the file.
107: .PP
108: .I Argv
109: is directly usable in another
110: .I execv
111: because
112: .IR argv [ argc ]
113: is 0.
114: .PP
115: .I Envp
116: is a pointer to an array of strings that constitute
117: the
118: .I environment
119: of the process.
120: Each string consists of a name, an \*(lq=\*(rq, and a null-terminated value.
121: The array of pointers is terminated by a null pointer.
122: The shell
123: .IR sh (1)
124: passes an environment entry for each global shell variable
125: defined when the program is called.
126: See
127: .IR environ (7)
128: for some conventionally
129: used names.
130: The C run-time start-off routine places a copy of
131: .I envp
132: in the global cell
133: .IR environ ,
134: which is used
135: by
136: .I execv
137: and
138: .I execl
139: to pass the environment to any subprograms executed by the
140: current program.
141: .PP
142: .I Execlp
143: and
144: .I execvp
145: are called with the same arguments as
146: .I execl
147: and
148: .IR execv ,
149: but duplicate the shell's actions in searching for an executable
150: file in a list of directories.
151: The directory list is obtained from the environment.
152: .SH FILES
153: .ta \w'/bin/sh 'u
154: /bin/sh shell, invoked if command file found
155: by
156: .I execlp
157: or
158: .I execvp
159: .SH "SEE ALSO"
160: execve(2),
161: fork(2),
162: environ(7),
163: csh(1)
164: .SH DIAGNOSTICS
165: If the file cannot be found,
166: if it is not executable,
167: if it does not start with a valid magic number (see
168: .IR a.out (5)),
169: if maximum memory is exceeded,
170: or if the arguments require too much space,
171: a return
172: constitutes the diagnostic;
173: the return value is \-1.
174: Even for the super-user,
175: at least one of the execute-permission bits must be set for
176: a file to be executed.
177: .SH BUGS
178: If
179: .I execvp
180: is called to execute a file that turns out to be a shell
181: command file,
182: and if it is impossible to execute the shell,
183: the values of
184: .I argv[0]
185: and
186: .I argv[\-1]
187: will be modified before return.
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