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
21 The getenv() function shall search the environment of the calling
22 process (see the Base Definitions volume of IEEE Std 1003.1-2001, Chap‐
23 ter 8, Environment Variables) for the environment variable name if it
24 exists and return a pointer to the value of the environment variable.
25 If the specified environment variable cannot be found, a null pointer
26 shall be returned. The application shall ensure that it does not modify
27 the string pointed to by the getenv() function.
28 The string pointed to may be overwritten by a subsequent call to
30 getenv(), setenv(), or unsetenv(), but shall not be overwritten by a
31 call to any other function in this volume of IEEE Std 1003.1-2001.
32 If the application modifies environ or the pointers to which it points,
34 the behavior of getenv() is undefined.
35 The getenv() function need not be reentrant. A function that is not
37 required to be reentrant is not required to be thread-safe.
38
40 Upon successful completion, getenv() shall return a pointer to a string
41 containing the value for the specified name. If the specified name can‐
42 not be found in the environment of the calling process, a null pointer
43 shall be returned.
44 The return value from getenv() may point to static data which may be
46 overwritten by subsequent calls to getenv(), setenv(), or unsetenv().
47 On XSI-conformant systems, the return value from getenv() may point to
49 static data which may also be overwritten by subsequent calls to
50 putenv().
51
53 No errors are defined.
54 The following sections are informative.
56
58 Getting the Value of an Environment Variable
59 The following example gets the value of the HOME environment variable.
60 #include <stdlib.h>
63 ...
64 const char *name = "HOME";
65 char *value;
66 value = getenv(name);
69
71 None.
72
74 The clearenv() function was considered but rejected. The putenv() func‐
75 tion has now been included for alignment with the Single UNIX Specifi‐
76 cation.
77 The getenv() function is inherently not reentrant because it returns a
79 value pointing to static data.
80 Conforming applications are required not to modify environ directly,
82 but to use only the functions described here to manipulate the process
83 environment as an abstract object. Thus, the implementation of the
84 environment access functions has complete control over the data struc‐
85 ture used to represent the environment (subject to the requirement that
86 environ be maintained as a list of strings with embedded equal signs
87 for applications that wish to scan the environment). This constraint
88 allows the implementation to properly manage the memory it allocates,
89 either by using allocated storage for all variables (copying them on
90 the first invocation of setenv() or unsetenv()), or keeping track of
91 which strings are currently in allocated space and which are not, via a
92 separate table or some other means. This enables the implementation to
93 free any allocated space used by strings (and perhaps the pointers to
94 them) stored in environ when unsetenv() is called. A C runtime start-up
95 procedure (that which invokes main() and perhaps initializes environ)
96 can also initialize a flag indicating that none of the environment has
97 yet been copied to allocated storage, or that the separate table has
98 not yet been initialized.
99 In fact, for higher performance of getenv(), the implementation could
101 also maintain a separate copy of the environment in a data structure
102 that could be searched much more quickly (such as an indexed hash ta‐
103 ble, or a binary tree), and update both it and the linear list at envi‐
104 ron when setenv() or unsetenv() is invoked.
105 Performance of getenv() can be important for applications which have
107 large numbers of environment variables. Typically, applications like
108 this use the environment as a resource database of user-configurable
109 parameters. The fact that these variables are in the user's shell envi‐
110 ronment usually means that any other program that uses environment
111 variables (such as ls, which attempts to use COLUMNS ), or really
112 almost any utility ( LANG, LC_ALL, and so on) is similarly slowed down
113 by the linear search through the variables.
114 An implementation that maintains separate data structures, or even one
116 that manages the memory it consumes, is not currently required as it
117 was thought it would reduce consensus among implementors who do not
118 want to change their historical implementations.
119 The POSIX Threads Extension states that multi-threaded applications
121 must not modify environ directly, and that IEEE Std 1003.1-2001 is pro‐
122 viding functions which such applications can use in the future to
123 manipulate the environment in a thread-safe manner. Thus, moving away
124 from application use of environ is desirable from that standpoint as
125 well.
126
128 None.
129
131 exec(), putenv(), setenv(), unsetenv(), the Base Definitions volume of
132 IEEE Std 1003.1-2001, Chapter 8, Environment Variables, <stdlib.h>
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135 Portions of this text are reprinted and reproduced in electronic form
136 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
137 -- Portable Operating System Interface (POSIX), The Open Group Base
138 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
139 Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .
144IEEE/The Open Group 2003 GETENV(3P)