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programming:posix

POSIX/GNU

Execution Time & Timestamps (gettimeofday)

#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 resolution of function gettimeofday() depends on the platform, but mostly all Linux like systems implement it using high resolution timers.

Name of Current Process (getpid)

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 */

The implementation works under Linux and some other *nix derivates, but not for all systems (you might use #ifdef __linux__ if this method is appropriate).

Change *NIX User Password

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;
}
programming/posix.txt · Last modified: 2018/10/29 11:23 by Ralf Hoppe