POSIX/GNU

1. General
   • Wikipedia: POSIX
   • IEEE and The Open Group: The Open Group Base Specifications Issue 7
2. Special Topics
   • Free Software Foundation: The GNU C Library Reference Manual - Generating Unpredictable Bytes (getrandom)
   • Free Software Foundation: The GNU C Library Reference Manual - Erasing Sensitive Data (explicit_bzero)
   • Free Software Foundation: The GNU C Library Reference Manual - Implementing a Job Control Shell
   • Thomas Orozco: A Deep Dive into the SIGTTIN / SIGTTOU Terminal Access Control Mechanism in Linux
3. Inlined (see below)
   • Execution Time & Timestamps (gettimeofday)
   • Name of Current Process (getpid)
   • Change *NIX User Password

#include <sys/time.h>
 
struct timeval t;
uint64_t start;
 
if (gettimeofday(&t, NULL) == 0)
{
    start = 1000000ULL * t.tv_sec + t.tv_usec;
} // if
 
:
:
 
if (gettimeofday(&t, NULL) == 0)
{
    printf("%llu us", (1000000ULL * t.tv_sec + t.tv_usec) - start);
} // if

The following code fragment determines the executable name (possibly incl. path) of current process in variable exe.

#include <unistd.h>
#include <stdio.h>
 
char exe[256]; /* to be adopted, possibly by dynamic alloc */
char path[32]; /* path of 'exe' link in proc filesystem */
ssize_t len;
 
snprintf(path, sizeof(path), "/proc/%d/exe", (int) getpid()); /* not portable */
buf[sizeof(path) - 1] = '\0';
len = readlink(path, exe, sizeof(exe));
 
if (len != -1)
    exe[len] = '\0';
else
    ; /* implement error handling */

int change_passwd ( const char* user, const char* oldpw, const char* newpw )
{
    int fds[2];
    int xstatus = EXIT_FAILURE; /* exit status */
 
    if ( pipe ( fds ) == 0 )
    {
        pid_t pid = fork ( );
 
        if ( pid >= 0 )
        {
            if ( pid == 0 ) /* in child process ? */
            {
                close ( fds[1] ); /* close write end */
 
                if ( dup2 ( fds[0], STDIN_FILENO ) >= 0 )
                {
                    execl ( "/usr/bin/passwd", "passwd", user, (char *) NULL );
                }
 
                exit ( xstatus );
            }
            else
            {
                int status; /* child status */
                ssize_t len = strlen ( newpw );
 
                close ( fds[0] );
                xstatus = EXIT_SUCCESS;
 
                if ( oldpw != NULL ) /* normal user (not root) ? */
                {
                    if ( ( write ( fds[1], oldpw, strlen ( oldpw ) ) < 0 ) ||
                         ( write ( fds[1], "\n", 1 ) != 1 ) )
                    {
                        xstatus = EXIT_FAILURE;
                    }
                }
 
                if ( xstatus == EXIT_SUCCESS )
                {
                    if ( ( write ( fds[1], newpw, len ) != len ) ||
                         ( write ( fds[1], "\n", 1 ) != 1 ) ||
                         ( write ( fds[1], newpw, len ) != len ) ||
                         ( write ( fds[1], "\n", 1 ) != 1 ) )
                    {
                        xstatus = EXIT_FAILURE;
                    }
                }
 
                close ( fds[1] );
 
                if ( waitpid ( pid, &status, 0 ) >= 0 )
                {
                    if ( WIFEXITED ( status ) && ( xstatus == EXIT_SUCCESS ) )
                    {
                        xstatus = WEXITSTATUS ( status );
                    }
                }
                else
                {
                    kill ( pid, SIGKILL ); /* sanity */
                }
            }
        }
    }
 
    return xstatus;
}