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
unsetenv(const char * name)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 }
66