Linux 的 exec 函数之间的区别
Linux 的 exec 函数族中函数的原型如下：
1int execl(const char *pathname, const char *arg, ...);
2int execlp(const char *file, const char *arg, ...);
3int execle(const char *pathname, const char *arg, ...);
4
5int execv(const char *pathname, char *const argv[]);
6int execvp(const char *file, char *const argv[]);
7int execve(const char *file, char *const argv[], char *const envp[]);
8
9int execvpe(const char *file, char *const argv[], char *const envp[]); // GNU 扩展他们的明明遵循以下规则：
l（list）表示通过可变参数列表传递额外参数，要额外传递一个NULL参数表示结尾v（vector）表示通过数组传递额外参数，数组中要要额外包含一个NULL表示结尾p（path）表示搜索PATH环境变量；没有p则不搜索，必须指定路径（绝对路径或相对路径）e（environment）表示指定环境变量；没有e则继承当前进程的环境变量
其中 execle 比较特别，从它的命名中可以看出，它通过可变参数列表传递额外参数，并且可以指定环境变量。
但是从函数原型中看不出环境变量的参数应该怎样传递。
实际上环境变量的参数需要以数组的形式传递，并以 NULL 结尾。
示例
1// `l` 通过可变参数列表传递额外参数
2execl("/usr/bin/ls", "/home", NULL);
3
4// `p` 表示搜索 `PATH` 环境变量
5execlp("ls", "/home", NULL);
6
7// `v` 表示通过数组传递额外参数
8execv("/usr/bin/ls", {"/home", NULL});
9
10// e 表示指定环境变量，以 NULL 结尾
11execvpe("ls", {"/home", NULL}, {"PATH=/usr/bin", NULL});1const char* envp[] = {
2 "PATH=/usr/bin",
3 NULL // 以 NULL 结尾
4};
5
6execle("/usr/bin/ls", "/home", NULL, envp);参考文档
1exec(3) Library Functions Manual exec(3)
2
3NAME
4 execl, execlp, execle, execv, execvp, execvpe - execute a file
5
6LIBRARY
7 Standard C library (libc, -lc)
8
9SYNOPSIS
10 #include <unistd.h>
11
12 extern char **environ;
13
14 int execl(const char *pathname, const char *arg, ...
15 /*, (char *) NULL */);
16 int execlp(const char *file, const char *arg, ...
17 /*, (char *) NULL */);
18 int execle(const char *pathname, const char *arg, ...
19 /*, (char *) NULL, char *const envp[] */);
20 int execv(const char *pathname, char *const argv[]);
21 int execvp(const char *file, char *const argv[]);
22 int execvpe(const char *file, char *const argv[], char *const envp[]);
23
24 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
25
26 execvpe():
27 _GNU_SOURCE
28
29DESCRIPTION
30 The exec() family of functions replaces the current process image with
31 a new process image. The functions described in this manual page are
32 layered on top of execve(2). (See the manual page for execve(2) for
33 further details about the replacement of the current process image.)
34
35 The initial argument for these functions is the name of a file that is
36 to be executed.
37
38 The functions can be grouped based on the letters following the "exec"
39 prefix.
40
41 l - execl(), execlp(), execle()
42 The const char *arg and subsequent ellipses can be thought of as arg0,
43 arg1, ..., argn. Together they describe a list of one or more pointers
44 to null-terminated strings that represent the argument list available
45 to the executed program. The first argument, by convention, should
46 point to the filename associated with the file being executed. The
47 list of arguments must be terminated by a null pointer, and, since
48 these are variadic functions, this pointer must be cast (char *) NULL.
49
50 By contrast with the 'l' functions, the 'v' functions (below) specify
51 the command-line arguments of the executed program as a vector.
52
53 v - execv(), execvp(), execvpe()
54 The char *const argv[] argument is an array of pointers to null-termi‐
55 nated strings that represent the argument list available to the new
56 program. The first argument, by convention, should point to the file‐
57 name associated with the file being executed. The array of pointers
58 must be terminated by a null pointer.
59
60 e - execle(), execvpe()
61 The environment of the new process image is specified via the argument
62 envp. The envp argument is an array of pointers to null-terminated
63 strings and must be terminated by a null pointer.
64
65 All other exec() functions (which do not include 'e' in the suffix)
66 take the environment for the new process image from the external vari‐
67 able environ in the calling process.
68
69 p - execlp(), execvp(), execvpe()
70 These functions duplicate the actions of the shell in searching for an
71 executable file if the specified filename does not contain a slash (/)
72 character. The file is sought in the colon-separated list of directory
73 pathnames specified in the PATH environment variable. If this variable
74 isn't defined, the path list defaults to a list that includes the di‐
75 rectories returned by confstr(_CS_PATH) (which typically returns the
76 value "/bin:/usr/bin") and possibly also the current working directory;
77 see NOTES for further details.
78
79 execvpe() searches for the program using the value of PATH from the
80 caller's environment, not from the envp argument.
81
82 If the specified filename includes a slash character, then PATH is ig‐
83 nored, and the file at the specified pathname is executed.
84
85 In addition, certain errors are treated specially.
86
87 If permission is denied for a file (the attempted execve(2) failed with
88 the error EACCES), these functions will continue searching the rest of
89 the search path. If no other file is found, however, they will return
90 with errno set to EACCES.
91
92 If the header of a file isn't recognized (the attempted execve(2)
93 failed with the error ENOEXEC), these functions will execute the shell
94 (/bin/sh) with the path of the file as its first argument. (If this
95 attempt fails, no further searching is done.)
96
97 All other exec() functions (which do not include 'p' in the suffix)
98 take as their first argument a (relative or absolute) pathname that
99 identifies the program to be executed.
100
101RETURN VALUE
102 The exec() functions return only if an error has occurred. The return
103 value is -1, and errno is set to indicate the error.
104
105ERRORS
106 All of these functions may fail and set errno for any of the errors
107 specified for execve(2).
108
109ATTRIBUTES
110 For an explanation of the terms used in this section, see attrib‐
111 utes(7).
112 ┌───────────────────────────────────────┬───────────────┬─────────────┐
113 │ Interface │ Attribute │ Value │
114 ├───────────────────────────────────────┼───────────────┼─────────────┤
115 │ execl(), execle(), execv() │ Thread safety │ MT-Safe │
116 ├───────────────────────────────────────┼───────────────┼─────────────┤
117 │ execlp(), execvp(), execvpe() │ Thread safety │ MT-Safe env │
118 └───────────────────────────────────────┴───────────────┴─────────────┘
119
120VERSIONS
121 The default search path (used when the environment does not contain the
122 variable PATH) shows some variation across systems. It generally in‐
123 cludes /bin and /usr/bin (in that order) and may also include the cur‐
124 rent working directory. On some other systems, the current working is
125 included after /bin and /usr/bin, as an anti-Trojan-horse measure. The
126 glibc implementation long followed the traditional default where the
127 current working directory is included at the start of the search path.
128 However, some code refactoring during the development of glibc 2.24
129 caused the current working directory to be dropped altogether from the
130 default search path. This accidental behavior change is considered
131 mildly beneficial, and won't be reverted.
132
133 The behavior of execlp() and execvp() when errors occur while attempt‐
134 ing to execute the file is historic practice, but has not traditionally
135 been documented and is not specified by the POSIX standard. BSD (and
136 possibly other systems) do an automatic sleep and retry if ETXTBSY is
137 encountered. Linux treats it as a hard error and returns immediately.
138
139 Traditionally, the functions execlp() and execvp() ignored all errors
140 except for the ones described above and ENOMEM and E2BIG, upon which
141 they returned. They now return if any error other than the ones de‐
142 scribed above occurs.
143
144STANDARDS
145 environ
146 execl()
147 execlp()
148 execle()
149 execv()
150 execvp()
151 POSIX.1-2008.
152
153 execvpe()
154 GNU.
155
156HISTORY
157 environ
158 execl()
159 execlp()
160 execle()
161 execv()
162 execvp()
163 POSIX.1-2001.
164
165 execvpe()
166 glibc 2.11.
167
168BUGS
169 Before glibc 2.24, execl() and execle() employed realloc(3) internally
170 and were consequently not async-signal-safe, in violation of the re‐
171 quirements of POSIX.1. This was fixed in glibc 2.24.
172
173 Architecture-specific details
174 On sparc and sparc64, execv() is provided as a system call by the ker‐
175 nel (with the prototype shown above) for compatibility with SunOS.
176 This function is not employed by the execv() wrapper function on those
177 architectures.
178
179SEE ALSO
180 sh(1), execve(2), execveat(2), fork(2), ptrace(2), fexecve(3), sys‐
181 tem(3), environ(7)
182
183Linux man-pages 6.7 2023-10-31 exec(3)