src/port/unsetenv.c

   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 }