src/port/unsetenv.c

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