1GETENV(P) POSIX Programmer's Manual GETENV(P)
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6 getenv - get value of an environment variable
7
9 #include <stdlib.h>
10 char *getenv(const char *name);
12
15 The getenv() function shall search the environment of the calling
16 process (see the Base Definitions volume of IEEE Std 1003.1-2001, Chap‐
17 ter 8, Environment Variables) for the environment variable name if it
18 exists and return a pointer to the value of the environment variable.
19 If the specified environment variable cannot be found, a null pointer
20 shall be returned. The application shall ensure that it does not modify
21 the string pointed to by the getenv() function.
22 The string pointed to may be overwritten by a subsequent call to
24 getenv(), setenv(), or unsetenv(), but shall not be overwritten by a
25 call to any other function in this volume of IEEE Std 1003.1-2001.
26 If the application modifies environ or the pointers to which it points,
28 the behavior of getenv() is undefined.
29 The getenv() function need not be reentrant. A function that is not
31 required to be reentrant is not required to be thread-safe.
32
34 Upon successful completion, getenv() shall return a pointer to a string
35 containing the value for the specified name. If the specified name can‐
36 not be found in the environment of the calling process, a null pointer
37 shall be returned.
38 The return value from getenv() may point to static data which may be
40 overwritten by subsequent calls to getenv(), setenv(), or unsetenv().
41 On XSI-conformant systems, the return value from getenv() may point to
43 static data which may also be overwritten by subsequent calls to
44 putenv().
45
47 No errors are defined.
48 The following sections are informative.
50
52 Getting the Value of an Environment Variable
53 The following example gets the value of the HOME environment variable.
54 #include <stdlib.h>
57 ...
58 const char *name = "HOME";
59 char *value;
60 value = getenv(name);
63
65 None.
66
68 The clearenv() function was considered but rejected. The putenv() func‐
69 tion has now been included for alignment with the Single UNIX Specifi‐
70 cation.
71 The getenv() function is inherently not reentrant because it returns a
73 value pointing to static data.
74 Conforming applications are required not to modify environ directly,
76 but to use only the functions described here to manipulate the process
77 environment as an abstract object. Thus, the implementation of the
78 environment access functions has complete control over the data struc‐
79 ture used to represent the environment (subject to the requirement that
80 environ be maintained as a list of strings with embedded equal signs
81 for applications that wish to scan the environment). This constraint
82 allows the implementation to properly manage the memory it allocates,
83 either by using allocated storage for all variables (copying them on
84 the first invocation of setenv() or unsetenv()), or keeping track of
85 which strings are currently in allocated space and which are not, via a
86 separate table or some other means. This enables the implementation to
87 free any allocated space used by strings (and perhaps the pointers to
88 them) stored in environ when unsetenv() is called. A C runtime start-up
89 procedure (that which invokes main() and perhaps initializes environ)
90 can also initialize a flag indicating that none of the environment has
91 yet been copied to allocated storage, or that the separate table has
92 not yet been initialized.
93 In fact, for higher performance of getenv(), the implementation could
95 also maintain a separate copy of the environment in a data structure
96 that could be searched much more quickly (such as an indexed hash ta‐
97 ble, or a binary tree), and update both it and the linear list at envi‐
98 ron when setenv() or unsetenv() is invoked.
99 Performance of getenv() can be important for applications which have
101 large numbers of environment variables. Typically, applications like
102 this use the environment as a resource database of user-configurable
103 parameters. The fact that these variables are in the user's shell envi‐
104 ronment usually means that any other program that uses environment
105 variables (such as ls, which attempts to use COLUMNS ), or really
106 almost any utility ( LANG , LC_ALL , and so on) is similarly slowed
107 down by the linear search through the variables.
108 An implementation that maintains separate data structures, or even one
110 that manages the memory it consumes, is not currently required as it
111 was thought it would reduce consensus among implementors who do not
112 want to change their historical implementations.
113 The POSIX Threads Extension states that multi-threaded applications
115 must not modify environ directly, and that IEEE Std 1003.1-2001 is pro‐
116 viding functions which such applications can use in the future to
117 manipulate the environment in a thread-safe manner. Thus, moving away
118 from application use of environ is desirable from that standpoint as
119 well.
120
122 None.
123
125 exec() , putenv() , setenv() , unsetenv() , the Base Definitions volume
126 of IEEE Std 1003.1-2001, Chapter 8, Environment Variables, <stdlib.h>
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129 Portions of this text are reprinted and reproduced in electronic form
130 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
131 -- Portable Operating System Interface (POSIX), The Open Group Base
132 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
133 Electrical and Electronics Engineers, Inc and The Open Group. In the
134 event of any discrepancy between this version and the original IEEE and
135 The Open Group Standard, the original IEEE and The Open Group Standard
136 is the referee document. The original Standard can be obtained online
137 at http://www.opengroup.org/unix/online.html .
138IEEE/The Open Group 2003 GETENV(P)