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unsetenv.c
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1 /*-------------------------------------------------------------------------
2  *
3  * unsetenv.c
4  * unsetenv() emulation for machines without it
5  *
6  * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
7  * Portions Copyright (c) 1994, Regents of the University of California
8  *
9  *
10  * IDENTIFICATION
11  * src/port/unsetenv.c
12  *
13  *-------------------------------------------------------------------------
14  */
15 
16 #include "c.h"
17 
18 
19 int
20 unsetenv(const char *name)
21 {
22  char *envstr;
23 
24  /* Error conditions, per POSIX */
25  if (name == NULL || name[0] == '\0' || strchr(name, '=') != NULL)
26  {
27  errno = EINVAL;
28  return -1;
29  }
30 
31  if (getenv(name) == NULL)
32  return 0; /* no work */
33 
34  /*
35  * The technique embodied here works if libc follows the Single Unix Spec
36  * and actually uses the storage passed to putenv() to hold the environ
37  * entry. When we clobber the entry in the second step we are ensuring
38  * that we zap the actual environ member. However, there are some libc
39  * implementations (notably recent BSDs) that do not obey SUS but copy the
40  * presented string. This method fails on such platforms. Hopefully all
41  * such platforms have unsetenv() and thus won't be using this hack. See:
42  * http://www.greenend.org.uk/rjk/2008/putenv.html
43  *
44  * Note that repeatedly setting and unsetting a var using this code will
45  * leak memory.
46  */
47 
48  envstr = (char *) malloc(strlen(name) + 2);
49  if (!envstr) /* not much we can do if no memory */
50  return -1;
51 
52  /* Override the existing setting by forcibly defining the var */
53  sprintf(envstr, "%s=", name);
54  if (putenv(envstr))
55  return -1;
56 
57  /* Now we can clobber the variable definition this way: */
58  strcpy(envstr, "=");
59 
60  /*
61  * This last putenv cleans up if we have multiple zero-length names as a
62  * result of unsetting multiple things.
63  */
64  return putenv(envstr);
65 }
const char * name
Definition: encode.c:561
#define malloc(a)
Definition: header.h:50
#define sprintf
Definition: port.h:224
int unsetenv(const char *name)
Definition: unsetenv.c:20
#define putenv(x)
Definition: win32_port.h:506