This is proc/modify_env.c (Listing 6-4, page 131), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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Function list (Bold in this list means a function is not static)

/* modify_env.c

   Demonstrate modification of the process environment list.

   Usage: modify_env name=value...

   Note: some UNIX implementations do not provide clearenv(), setenv(),
   and unsetenv().
#define _GNU_SOURCE     /* Get various declarations from <stdlib.h> */
#include <stdlib.h>
#include "tlpi_hdr.h"

extern char **environ;
main(int argc, char *argv[])
    int j;
    char **ep;

    clearenv();         /* Erase entire environment */

    /* Add any definitions specified on command line to environment */

    for (j = 1; j < argc; j++)
        if (putenv(argv[j]) != 0)
            errExit("putenv: %s", argv[j]);

    /* Add a definition for GREET if one does not already exist */

    if (setenv("GREET", "Hello world", 0) == -1)

    /* Remove any existing definition of BYE */


    /* Display current environment */

    for (ep = environ; *ep != NULL; ep++)



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