如何从文件的指定偏移量读取指定数量的字节[关闭]

Question

我想从文件的指定偏移值中读取指定数量的字节。我将偏移量、字节数、文件名作为参数传递给文件。下面的代码不起作用。有人可以帮我吗？谢谢。

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/types.h>
#include<error.h>
#include<errno.h>
#include<fcntl.h>
#define buf_size 512

int main(int argc, char *argv[])
{
    int bytes;
    int offset;
    int fd;
    int i = 0;
    int length = 0;
    ssize_t read_bytes;
    char *file;
    char buf[buf_size];

    if (argc != 4)
        error(1, 0, "Too many or less than the number of arguments");
    file = argv[1];
    offset = atoi(argv[2]);
    bytes = atoi(argv[3]);
    fd = open(file, O_RDONLY);
    if (fd == -1)
        error(1, errno, "Error while opening the file\n");
    while (1) {
        read_bytes = read(fd, buf, bytes);
        if (read_bytes == -1)
            error(1, errno, "Error while reading the file\n");
        length += read_bytes;
        printf("The length is : %d\n", length);
        if (length >= offset) {
            for (i = length ; i < bytes; i++)
                putchar(buf[i]);
            break;
        }
    }
    if (close(fd) == -1)
        error(1, 0, "Error while closing the file\n");

}

Answer 1

由于提问者指出的限制，我决定将一些 POSIX 函数替换为 C 标准库中的函数。这是相当便携的。

#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
#define buf_size 512

int main(int argc, char *argv[])
{
    int bytes;
    int offset;
    FILE *fd;
    int i = 0;
    ssize_t read_bytes;
    char *file;
    char buf[buf_size];

    if (argc != 4)
        printf("Too many or less than the number of arguments\n");
    file = argv[1];
    offset = atoi(argv[2]);
    bytes = atoi(argv[3]);
    fd = fopen(file, "r");
    if (fd == NULL)
        perror("ERROR");
    fseek(fd, offset, SEEK_SET);
    read_bytes = fread(buf, 1, bytes, fd);
    if (read_bytes == 0) {
        if (feof(fd))
            printf("ERROR: End of file.\n");
        else if (ferror(fd))
            printf("ERROR: Unable to read the file.\n");
        return 1;
    }
    for (i = 0; i < bytes; i++)
        putchar(buf[i]);
    if (fclose(fd) == EOF)
        perror("ERROR:");
    return 0;

}

Answer 2

我只对上述问题使用了 open、close、read、write 系统调用。我认为它可以处理部分读取、偏移 > buf_size 和 bytes > buf_size。谢谢。

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/types.h>
#include<error.h>
#include<errno.h>
#include<fcntl.h>
#define buf_size 5

int main(int argc, char *argv[])
{   
    int bytes;
    int offset;
    int fd; 
    char *file;
    char buf[buf_size];
    int rlen = 0;
    int len = 0;
    int i = 0;

    if (argc != 4)
        error(1, 0, "Too many or less than the number of arguments");
    file = argv[1];
    offset = atoi(argv[2]);
    bytes = atoi(argv[3]);
    fd = open(file, O_RDONLY);
    printf("The values of the file descriptor is : %d\n", fd);
    if (fd == -1)
        error(1, errno, "Error while opening the file\n");

    while (1) {
        rlen = read(fd, buf, offset);
        if (rlen == -1)
            error(1, errno, "Error in reading the file\n");
        len = len + rlen;
        if(len == offset) {
            len = 0;
            while (1) {
                rlen = read(fd, buf, bytes);
                if (rlen == -1)
                    error(1, errno, "Error in reading the file\n");
                if (rlen == 0)
                    return 0;
                len = len + rlen;
                for (i = 0; i < rlen; i++) 
                    putchar(buf[i]);
                if (len == bytes) {
                    printf("\nSuccess\n");
                    return 0;
                }
                bytes = bytes - rlen;
            }
        }
    }
}