unsetenv.c

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/*-------------------------------------------------------------------------
 *
 * unsetenv.c
 *    unsetenv() emulation for machines without it
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/port/unsetenv.c
 *
 *-------------------------------------------------------------------------
 */

#include "c.h"


int
unsetenv(const char *name)
{
    char       *envstr;

    /* Error conditions, per POSIX */
    if (name == NULL || name[0] == '\0' || strchr(name, '=') != NULL)
    {
        errno = EINVAL;
        return -1;
    }

    if (getenv(name) == NULL)
        return 0;               /* no work */

    /*
     * The technique embodied here works if libc follows the Single Unix Spec
     * and actually uses the storage passed to putenv() to hold the environ
     * entry.  When we clobber the entry in the second step we are ensuring
     * that we zap the actual environ member.  However, there are some libc
     * implementations (notably recent BSDs) that do not obey SUS but copy the
     * presented string.  This method fails on such platforms.  Hopefully all
     * such platforms have unsetenv() and thus won't be using this hack. See:
     * http://www.greenend.org.uk/rjk/2008/putenv.html
     *
     * Note that repeatedly setting and unsetting a var using this code will
     * leak memory.
     */

    envstr = (char *) malloc(strlen(name) + 2);
    if (!envstr)                /* not much we can do if no memory */
        return -1;

    /* Override the existing setting by forcibly defining the var */
    sprintf(envstr, "%s=", name);
    if (putenv(envstr))
        return -1;

    /* Now we can clobber the variable definition this way: */
    strcpy(envstr, "=");

    /*
     * This last putenv cleans up if we have multiple zero-length names as a
     * result of unsetting multiple things.
     */
    return putenv(envstr);
}