1.相關ＡＰＩ

通過lseek對檔案進行讀寫

1)open函式

int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);

2)read和write函式

#include <unistd.h>
ssize_t read(int fd, void *buf, size_t count);

ssize_t write(int fd, const void *buf, size_t count);

3)lseek更改檔案偏移位置

#include <sys/types.h>

#include <unistd.h>

off_t lseek(int fd, off_t offset, int whence);

４）把字串轉化成數字型別

標頭檔案：#include <stdlib.h>

strtol() 函式用來將字串轉換為長整型數(long)，其原型為：
long int strtol (const char* str, char** endptr, int base);
【引數說明】str 為要轉換的字串，endstr 為第一個不能轉換的字元的指標，base 為字串 str 所採用的進位制。

實現程式碼：

#include <stdio.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <string.h>
#include <errno.h>          //errno
#include <fcntl.h>
#include <ctype.h>
#include <stdlib.h>         //strtol()
#include <unistd.h>         //ssize_t

int main(int argc, char *argv[])
{
    size_t len;
    off_t offset;
    int fd, ap, j;
    char *buf;
    ssize_t numRead, numWritten;
    if(argc < 3 || strcmp(argv[1], "--help") == 0)
        printf("%s file {r<length>| R<length> | w<string>| s<offset>}...\n", argv[0]);
    fd = open(argv[1], O_RDWR| O_CREAT, S_IRUSR| S_IWUSR| S_IRGRP | S_IWGRP | S_IROTH| S_IWOTH);
    if(fd == -1)
    {
        //開啟檔案出錯
        fprintf(stderr, "Open %s Error: %s\n", argv[1], strerror(errno));
        exit(1);
    }
    for (ap = 2; ap < argc; ++ap) {
        switch (argv[ap][0]){
            case 'r':   //在當前offset偏移處顯示檔案內容
            case 'R':
                len=strtol(&argv[ap][1], NULL, 0);
                buf = malloc(len);
                if(buf == NULL){
                    printf("malloc buf error\n");
                    exit(1);
                }
                numRead = read(fd, buf, len);
                if(numRead == -1){
                    printf("malloc buf error\n");
                    free(buf);
                    exit(1);
                }
                if(numRead == 0){
                    printf("%s: end-of-file\n", argv[ap]);
                } else{
                    printf("%s: ",argv[ap]);
                    for(j = 0; j< numRead; j++){
                        if (argv[ap][0]=='r'){
                            printf("%c", isprint((unsigned char)buf[j]) ? buf[j]: '?');
                        } else{
                            printf("%02x", (unsigned int) buf[j]);
                        }
                    }
                    printf("\n");
                }
                if(buf != NULL)
                    free(buf);
                break;
            case 'w':           //在當前檔案偏移下寫入內容
                numWritten = write(fd, &argv[ap][1], strlen(&argv[ap][1]));
                if(numWritten == -1){
                    printf("write buf error\n");
                    exit(1);
                }
                printf("%s: wrote %ld bytes\n", argv[ap], (long) numWritten);
                break;
            case 's':           //從檔案頭開始更改檔案的偏移位置
                offset = strtol(&argv[ap][1], NULL, 0);
                //offset = getLong(&argv[ap][1], GN_ANY_BASE, argv[ap]);
                if(lseek(fd, offset, SEEK_SET) == -1){
                    printf("lseek offset error\n");
                    exit(1);
                }
                printf("%s: seek successed\n", argv[ap]);
                break;
            default:
                printf("Argument must start with [rRws]\n");
                break;
        }
    }//end for
    printf("file execute success\n");
}