MicroDD

/*********************
 * MicroDD v. 1.0    *
 * Author: Phracker  *
 * Date: 2018-05-20  *
 * Offered as-is     *
 * No warranty       *
 *********************/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <errno.h>
#include <limits.h>

int main( int argc, char **argv ){
    char infile [FILENAME_MAX];
    char outfile[FILENAME_MAX];
    size_t bc = 0;    // Number of bytes to copy (if zero, copy entire file)
    off_t istart = 0; // starting position in input file
    off_t ostart = 0; // starting position in output file

    /*** BEGIN PARSE COMMAND LINE PARAMETERS ***/
    for( int i = 1; i < argc; i++ ){
        char *name = strtok( argv[i], "=" );
        char *value = strtok( NULL, "\0" );
        if( !strcmp( name, "if" ) ) strncpy( infile, value, strlen( value ) );
        else if( !strcmp( name, "of" ) ) strncpy( outfile, value, strlen( value ) );
        else if( !strcmp( name, "bc" ) ) bc = atoi( value );
        else if( !strcmp( name, "istart" ) ) istart = atoi( value );
        else if( !strcmp( name, "ostart" ) ) ostart = atoi( value );
        else fprintf( stderr, "%s: %s: switch not recognized.\n", argv[0], name );
    }
    /*** END PARSE COMMAND LINE PARAMETERS ***/

    // This section is for determining when the end of a regular file has been reached.
    // Processing for a given file will be skipped if the file was not specified.
    struct stat istat, ostat;
    if( infile[0] ) stat( infile, &istat );
    if( errno == ENOENT ) fprintf( stderr, "%s: %s: File not found.\n", argv[0], infile );
    errno = 0;
    if( outfile[0] ) stat( outfile, &ostat );
    int savederrno = errno;
    if( !bc ){
        if( infile[0] && S_ISREG( istat.st_mode ) ) bc = istat.st_size - istart;
        else if( outfile[0] && savederrno != ENOENT /* Check to see if outfile exists */ && S_ISREG( ostat.st_mode ) ) bc = ostat.st_size - ostart;
    }

    // And now for the actual copying...
    int ip, op;
    if( infile[0] ){
        ip = open( infile, O_RDONLY );
        lseek( ip, istart, SEEK_SET );
    }
    else ip = 0;
    if( outfile[0] ){
        op = open( outfile, O_WRONLY | O_CREAT, 0644 );
        lseek( op, ostart, SEEK_SET );
    }
    else op = 1;
    char c;
    for( int i = 0; i < (bc?bc:INT_MAX); i++ ){
        read( ip, &c, 1 );
        write( op, &c, 1 );
    }
    close( ip );
    close( op );
    return 0;
}