bufread.c

#define _POSIX_C_SOURCE 200809L

#include <unistd.h>     /* read, write, close, lseek */
#include <fcntl.h>      /* openat */
#include <stdlib.h>     /* exit, strtoll, malloc */
#include <stdio.h>      /* printf */
#include <errno.h>      /* errno */
#include <err.h>        /* NONPOSIX: err, errx */

/* Přečte soubor ‹file› a spočítá xor všech bajtů. Soubor čte skrze
 * buffer, jehož velikost je zadána jako ‹bufsize›. */

int main( int argc, char** argv )
{
    if ( argc < 3 )
        errx( 1, "usage: %s file bufsize\n", argv[ 0 ] );

    const char* filename = argv[ 1 ];
    long long bufsize = strtoll( argv[ 2 ], 0, 0 );

    char* buf = ( char* )malloc( bufsize );
    if ( !buf ) err( 1, "malloc" );

    int fd = openat( AT_FDCWD, filename, O_RDONLY );
    if ( fd == -1 )
        err( 1, "openat %s", filename );

    unsigned char result = 0;

    int bytes;
    while ( ( bytes = read( fd, buf, bufsize ) ) > 0 )
    {
        for ( long long i = 0; i < bufsize; ++i )
        {
            result ^= buf[ i ];
        }
    }
    if ( bytes == -1 )
        err( 1, "read stdin" );

    printf( "0x%02hhx\n", result );

    if ( close( fd ) ) warn( "close %s", filename );
    free( buf );
    return 0;
}