1GETENV(3P) POSIX Programmer's Manual GETENV(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 getenv — get value of an environment variable
13
15 #include <stdlib.h>
16 char *getenv(const char *name);
18
20 The functionality described on this reference page is aligned with the
21 ISO C standard. Any conflict between the requirements described here
22 and the ISO C standard is unintentional. This volume of POSIX.1‐2017
23 defers to the ISO C standard.
24 The getenv() function shall search the environment of the calling
26 process (see the Base Definitions volume of POSIX.1‐2017, Chapter 8,
27 Environment Variables) for the environment variable name if it exists
28 and return a pointer to the value of the environment variable. If the
29 specified environment variable cannot be found, a null pointer shall be
30 returned. The application shall ensure that it does not modify the
31 string pointed to by the getenv() function.
32 The returned string pointer might be invalidated or the string content
34 might be overwritten by a subsequent call to getenv(), setenv(),
35 unsetenv(),
36 or (if supported) putenv() but they shall not be affected by a call to
37 any other function in this volume of POSIX.1‐2017.
38 The returned string pointer might also be invalidated if the calling
40 thread is terminated.
41 The getenv() function need not be thread-safe.
43
45 Upon successful completion, getenv() shall return a pointer to a string
46 containing the value for the specified name. If the specified name
47 cannot be found in the environment of the calling process, a null
48 pointer shall be returned.
49
51 No errors are defined.
52 The following sections are informative.
54
56 Getting the Value of an Environment Variable
57 The following example gets the value of the HOME environment variable.
58 #include <stdlib.h>
61 ...
62 const char *name = "HOME";
63 char *value;
64 value = getenv(name);
66
68 None.
69
71 The clearenv() function was considered but rejected. The putenv() func‐
72 tion has now been included for alignment with the Single UNIX Specifi‐
73 cation.
74 The getenv() function is inherently not thread-safe because it returns
76 a value pointing to static data.
77 Conforming applications are required not to directly modify the point‐
79 ers to which environ points, but to use only the setenv(), unsetenv(),
80 and putenv() functions, or assignment to environ itself, to manipulate
81 the process environment. This constraint allows the implementation to
82 properly manage the memory it allocates. This enables the implementa‐
83 tion to free any space it has allocated to strings (and perhaps the
84 pointers to them) stored in environ when unsetenv() is called. A C run‐
85 time start-up procedure (that which invokes main() and perhaps initial‐
86 izes environ) can also initialize a flag indicating that none of the
87 environment has yet been copied to allocated storage, or that the sepa‐
88 rate table has not yet been initialized. If the application switches to
89 a complete new environment by assigning a new value to environ, this
90 can be detected by getenv(), setenv(), unsetenv(), or putenv() and the
91 implementation can at that point reinitialize based on the new environ‐
92 ment. (This may include copying the environment strings into a new
93 array and assigning environ to point to it.)
94 In fact, for higher performance of getenv(), implementations that do
96 not provide putenv() could also maintain a separate copy of the envi‐
97 ronment in a data structure that could be searched much more quickly
98 (such as an indexed hash table, or a binary tree), and update both it
99 and the linear list at environ when setenv() or unsetenv() is invoked.
100 On implementations that do provide putenv(), such a copy might still be
101 worthwhile but would need to allow for the fact that applications can
102 directly modify the content of environment strings added with putenv().
103 For example, if an environment string found by searching the copy is
104 one that was added using putenv(), the implementation would need to
105 check that the string in environ still has the same name (and value, if
106 the copy includes values), and whenever searching the copy produces no
107 match the implementation would then need to search each environment
108 string in environ that was added using putenv() in case any of them
109 have changed their names and now match. Thus, each use of putenv() to
110 add to the environment would reduce the speed advantage of having the
111 copy.
112 Performance of getenv() can be important for applications which have
114 large numbers of environment variables. Typically, applications like
115 this use the environment as a resource database of user-configurable
116 parameters. The fact that these variables are in the user's shell
117 environment usually means that any other program that uses environment
118 variables (such as ls, which attempts to use COLUMNS), or really almost
119 any utility (LANG, LC_ALL, and so on) is similarly slowed down by the
120 linear search through the variables.
121 An implementation that maintains separate data structures, or even one
123 that manages the memory it consumes, is not currently required as it
124 was thought it would reduce consensus among implementors who do not
125 want to change their historical implementations.
126
128 A future version may add one or more functions to access and modify the
129 environment in a thread-safe manner.
130
132 exec, putenv(), setenv(), unsetenv()
133 The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment
135 Variables, <stdlib.h>
136
138 Portions of this text are reprinted and reproduced in electronic form
139 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
140 table Operating System Interface (POSIX), The Open Group Base Specifi‐
141 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
142 Electrical and Electronics Engineers, Inc and The Open Group. In the
143 event of any discrepancy between this version and the original IEEE and
144 The Open Group Standard, the original IEEE and The Open Group Standard
145 is the referee document. The original Standard can be obtained online
146 at http://www.opengroup.org/unix/online.html .
147 Any typographical or formatting errors that appear in this page are
149 most likely to have been introduced during the conversion of the source
150 files to man page format. To report such errors, see https://www.ker‐
151 nel.org/doc/man-pages/reporting_bugs.html .
152IEEE/The Open Group 2017 GETENV(3P)