seek's stream or file before outputting to stdout

cdr.c

/* cdr.c
 * David Weinstein (2011)
 * offset bytes into a file and write to stdout
 */
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <assert.h>

#define BLK_SIZE_4K   4096
#define BLK_SIZE_8K   8192
#define BLK_SIZE_16K  16384

#define DEFAULT_BLOCK_SIZE BLK_SIZE_4K

void usage(int argc, char** argv)
{
    printf("usage: \n" \
           "\tcdr <file> <offset>\n\n");
    exit(0);
}
int 
main(int argc, char **argv)
{
    if( argc < 3 )
        usage(argc, argv);

    char* filename = argv[1];

    char buff[DEFAULT_BLOCK_SIZE];
    off_t offset = atol(argv[2]);
    assert(offset >= 0);

    int ifd;
    struct stat sb;
    
    if(0 == strncmp(filename, "-", 1)) {
        ifd = STDIN_FILENO;
        ssize_t ret = read(ifd, buff, offset);
        if (-1 == ret) {
            perror ("read");
            goto cleanup;
        }
        if (0 == ret) {
            // eof
            goto cleanup;
        }
    } else {
        if ((stat (filename, &sb)) == -1) {
            perror ("stat");
            return (-1);
        }
        if (-1 == (ifd = open (filename, O_RDONLY, 0))) {
            perror ("open");
            return (-2);
        }
        if (-1 == lseek(ifd, offset, SEEK_SET)) {
            perror ("lseek");
            goto cleanup;
        }

    }

    while(1)
    {
        ssize_t ret = read(ifd, buff, DEFAULT_BLOCK_SIZE);

        if (-1 == ret) {
            perror ("read");
            goto cleanup;
        }
        if (0 == ret) {
            // eof
            goto cleanup;
        }

        if( -1 == write(STDOUT_FILENO, buff, ret) ) {
            perror("write");
            goto cleanup;
        }
    }

cleanup:
    close(ifd);

    return 0;
}