src/port/unsetenv.c

   1 /*-------------------------------------------------------------------------
   2  *
   3  * unsetenv.c
   4  *        unsetenv() emulation for machines without it
   5  *
   6  * Portions Copyright (c) 1996-2020, 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 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. See:
  35          * http://www.greenend.org.uk/rjk/2008/putenv.html
  36          *
  37          * Note that repeatedly setting and unsetting a var using this code will
  38          * leak memory.
  39          */
  40
  41         envstr = (char *) malloc(strlen(name) + 2);
  42         if (!envstr)                            /* not much we can do if no memory */
  43                 return;
  44
  45         /* Override the existing setting by forcibly defining the var */
  46         sprintf(envstr, "%s=", name);
  47         putenv(envstr);
  48
  49         /* Now we can clobber the variable definition this way: */
  50         strcpy(envstr, "=");
  51
  52         /*
  53          * This last putenv cleans up if we have multiple zero-length names as a
  54          * result of unsetting multiple things.
  55          */
  56         putenv(envstr);
  57 }