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
13 getenv — get value of an environment variable
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16 #include <stdlib.h>
17 char *getenv(const char *name);
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21 The functionality described on this reference page is aligned with the
22 ISO C standard. Any conflict between the requirements described here
23 and the ISO C standard is unintentional. This volume of POSIX.1‐2008
24 defers to the ISO C standard.
25 The getenv() function shall search the environment of the calling
27 process (see the Base Definitions volume of POSIX.1‐2008, Chapter 8,
28 Environment Variables) for the environment variable name if it exists
29 and return a pointer to the value of the environment variable. If the
30 specified environment variable cannot be found, a null pointer shall be
31 returned. The application shall ensure that it does not modify the
32 string pointed to by the getenv() function.
33 The returned string pointer might be invalidated or the string content
35 might be overwritten by a subsequent call to getenv(), setenv(),
36 unsetenv(), or (if supported) putenv() but they shall not be affected
37 by a call to any other function in this volume of POSIX.1‐2008.
38 The getenv() function need not be thread-safe.
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42 Upon successful completion, getenv() shall return a pointer to a string
43 containing the value for the specified name. If the specified name
44 cannot be found in the environment of the calling process, a null
45 pointer shall be returned.
46
48 No errors are defined.
49 The following sections are informative.
51
53 Getting the Value of an Environment Variable
54 The following example gets the value of the HOME environment variable.
55 #include <stdlib.h>
57 ...
58 const char *name = "HOME";
59 char *value;
60 value = getenv(name);
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64 None.
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67 The clearenv() function was considered but rejected. The putenv() func‐
68 tion has now been included for alignment with the Single UNIX Specifi‐
69 cation.
70 The getenv() function is inherently not thread-safe because it returns
72 a value pointing to static data.
73 Conforming applications are required not to directly modify the point‐
75 ers to which environ points, but to use only the setenv(), unsetenv(),
76 and putenv() functions, or assignment to environ itself, to manipulate
77 the process environment. This constraint allows the implementation to
78 properly manage the memory it allocates. This enables the implementa‐
79 tion to free any space it has allocated to strings (and perhaps the
80 pointers to them) stored in environ when unsetenv() is called. A C run‐
81 time start-up procedure (that which invokes main() and perhaps initial‐
82 izes environ) can also initialize a flag indicating that none of the
83 environment has yet been copied to allocated storage, or that the sepa‐
84 rate table has not yet been initialized. If the application switches to
85 a complete new environment by assigning a new value to environ, this
86 can be detected by getenv(), setenv(), unsetenv(), or putenv() and the
87 implementation can at that point reinitialize based on the new environ‐
88 ment. (This may include copying the environment strings into a new
89 array and assigning environ to point to it.)
90 In fact, for higher performance of getenv(), implementations that do
92 not provide putenv() could also maintain a separate copy of the envi‐
93 ronment in a data structure that could be searched much more quickly
94 (such as an indexed hash table, or a binary tree), and update both it
95 and the linear list at environ when setenv() or unsetenv() is invoked.
96 On implementations that do provide putenv(), such a copy might still be
97 worthwhile but would need to allow for the fact that applications can
98 directly modify the content of environment strings added with putenv().
99 For example, if an environment string found by searching the copy is
100 one that was added using putenv(), the implementation would need to
101 check that the string in environ still has the same name (and value, if
102 the copy includes values), and whenever searching the copy produces no
103 match the implementation would then need to search each environment
104 string in environ that was added using putenv() in case any of them
105 have changed their names and now match. Thus, each use of putenv() to
106 add to the environment would reduce the speed advantage of having the
107 copy.
108 Performance of getenv() can be important for applications which have
110 large numbers of environment variables. Typically, applications like
111 this use the environment as a resource database of user-configurable
112 parameters. The fact that these variables are in the user's shell
113 environment usually means that any other program that uses environment
114 variables (such as ls, which attempts to use COLUMNS), or really almost
115 any utility (LANG, LC_ALL, and so on) is similarly slowed down by the
116 linear search through the variables.
117 An implementation that maintains separate data structures, or even one
119 that manages the memory it consumes, is not currently required as it
120 was thought it would reduce consensus among implementors who do not
121 want to change their historical implementations.
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124 A future version may add one or more functions to access and modify the
125 environment in a thread-safe manner.
126
128 exec, putenv(), setenv(), unsetenv()
129 The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
131 Variables, <stdlib.h>
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134 Portions of this text are reprinted and reproduced in electronic form
135 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
136 -- Portable Operating System Interface (POSIX), The Open Group Base
137 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
138 cal and Electronics Engineers, Inc and The Open Group. (This is
139 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
140 event of any discrepancy between this version and the original IEEE and
141 The Open Group Standard, the original IEEE and The Open Group Standard
142 is the referee document. The original Standard can be obtained online
143 at http://www.unix.org/online.html .
144 Any typographical or formatting errors that appear in this page are
146 most likely to have been introduced during the conversion of the source
147 files to man page format. To report such errors, see https://www.ker‐
148 nel.org/doc/man-pages/reporting_bugs.html .
149IEEE/The Open Group 2013 GETENV(3P)