c - 如何使用 fread() 将文件内容加载到 C 字符串数组的第二个元素中？

Question

我在理解指针在特定情况下如何工作时遇到一些困难。下面概述了我的困惑的详细信息。

我创建了一个名为 buffer 的 8 字节字符串字符数组，并且尝试使用 fread() 将文件中的 8 个字节加载到该 8 字节字符串数组的元素之一中。

位于/c/file.txt 的文件内容很简单:TESTTEST

我希望程序将 TESTTEST 加载到 buffer[1] 而不是值 ijklmnop，仅此而已。

我知道 buffer 实际上是一个指针数组，在本例中只是三个连续的内存地址，其中包含三个连续字节数组的第一个元素的地址，这些数组不是以 null 结尾的。

我将其想象为一行中的 24 个内存地址，其中包含 ABCDEFGHijklmnopQRSTUVWX 的二进制值，末尾没有\0。

buffer[0]的地址指向A的地址，buffer[1]的地址指向i的地址，buffer[2]的地址指向Q的地址.

我看到 &buffer[1] 的 fread() 将 8 个字节读取到 buffer[1] 中存储的地址位置。

我已经尝试使用 &buffer+1、buffer+1、(buffer+1) 等对此 fread() 进行了各种尝试，但大多数都给了我段错误； &buffer[1] 似乎几乎满足了我的要求。

尝试执行 printf("buffer[1] as %%s is %s\n", buffer[1]);在 fclose() 之前接近尾声时，出现段错误。

我即将达成理解，但有些事情不太正确，而且我没有掌握一些我需要的基本概念。

我不确定终止 (buffer+1)[8] 是否溢出到 buffer[2] 的首地址或产生什么影响。

总的来说，我只是有点困惑，任何帮助将不胜感激。

谢谢!

这是我的整个程序:

#include <stdio.h>
#include <string.h>

int main()
{
    FILE *fp;
    char c[] = "OFFONOFF";
    char *buffer[] = {"ABCDEFGH", "ijklmnop", "QRSTUVWX"};
    char *t;

    printf("the value in file /c/file.txt is TESTTEST, only those 8 bytes\n");
    printf("the ls listing for /c/file.txt shows 9, I am not sure where the other character comes from\n");

    printf("the value as %%s of buffer[0] is %s\n", buffer[0]);
    printf("the value as %%s of buffer[1] is %s\n", buffer[1]);
    printf("the value as %%s of buffer[2] is %s\n", buffer[2]);
    printf("the sizeof(buffer[2]) is %d\n", sizeof(buffer[2]));

    fp = fopen("/c/file.txt", "r");
    fseek(fp, SEEK_SET, 0);

    printf("the value as %%c of buffer[1][3] is %c\n", buffer[1][3]);
    printf("the memory address as %%p for  buffer+1 is %p\n", buffer+1);

    fread(&buffer[1], strlen(c), 1, fp);

    /* I am attempting to terminate the fread into &buffer[1] with a \0, removing this doesn't change much */
    (buffer+1)[8] = "\0";

    printf("the memory addreses below are all in %%p format\n");
    printf("the memory address of buffer is %p\n", buffer);
    printf("memory address of buffer[1] is %p\n", buffer[1]);
    printf("memory address of &buffer is %p\n", &buffer);
    printf("memory address of &buffer+1 is %p\n", &buffer+1);
    printf("memory address of &buffer[1] is %p\n", &buffer[1]);
    printf("memory address of buffer is %p\n", buffer);
    printf("memory address of buffer[15] is %p\n", buffer[15]);
    printf("memory address of buffer+15 is %p\n", buffer+15);
    printf("memory address of *(buffer+1) is %p\n", *(buffer+1));
    printf("the sizeof(buffer[0]) is %d\n", sizeof(buffer[0]));
    printf("the sizeof(*(buffer+1)) is %d\n", sizeof(*(buffer+1)));
    printf("the sizeof(buffer) is %d\n", sizeof(buffer));
    printf("the value of *(buffer+0) as %%s is %s\n", *(buffer+0));
    printf("the value of *buffer, the first element of the array of strings, as %%s is %s\n", *buffer);
    printf("the value of buffer+1, the first element of the array of strings, as %%s is %s\n", buffer+1);
    printf("buffer[0] as %%s is %s\n", buffer[0]);
    printf("buffer+1 as %%s is %s\n", buffer+1);
    printf("buffer[2] as %%s is %s\n", buffer[2]);
    printf("buffer is %s\n", buffer+1);
    printf("sizeof(*buffer) is %d\n", sizeof(*buffer));

    fclose(fp);

    return(0);
}

这是确切的输出:

the value in file /c/file.txt is TESTTEST, only those 8 bytes
the ls listing for /c/file.txt shows 9, I am not sure where the other character comes from
the value as %s of buffer[0] is ABCDEFGH
the value as %s of buffer[1] is ijklmnop
the value as %s of buffer[2] is QRSTUVWX
the sizeof(buffer[2]) is 8
the value as %c of buffer[1][3] is l
the memory address as %p for  buffer+1 is 0x7ffff7b23a28
the memory addreses below are all in %p format
the memory address of buffer is 0x7ffff7b23a20
memory address of buffer[1] is 0x5453455454534554
memory address of &buffer is 0x7ffff7b23a20
memory address of &buffer+1 is 0x7ffff7b23a38
memory address of &buffer[1] is 0x7ffff7b23a28
memory address of buffer is 0x7ffff7b23a20
memory address of buffer[15] is 0x400674
memory address of buffer+15 is 0x7ffff7b23a98
memory address of *(buffer+1) is 0x5453455454534554
the sizeof(buffer[0]) is 8
the sizeof(*(buffer+1)) is 8
the sizeof(buffer) is 24
the value of *(buffer+0) as %s is ABCDEFGH
the value of *buffer, the first element of the array of strings, as %s is ABCDEFGH
the value of buffer+1, the first element of the array of strings, as %s is TESTTEST

@
buffer[0] as %s is ABCDEFGH
buffer+1 as %s is TESTTEST

@
buffer[2] as %s is QRSTUVWX
buffer is TESTTEST

@

Answer 1

行 fread(&buffer[1], strlen(c), 1, fp); 似乎覆盖了指针本身。

看来您想要的是一个二维数组，而不是指向字符串文字的指针数组(顺便说一下，字符串文字在大多数现代平台上都是只读的: char *abc="abc"; abc [0]='d' 通常是段错误):

char buffer[][9] = {"ABCDEFGH", "ijklmnop", "QRSTUVWX"};