在C中使用fwrite在输出文件中提供不同的值 [英] fwrite in C giving different values in output files

问题描述

当我在另一个函数中使用fwrite相对于同一函数中的fwrite时，为什么输出文件不同？

why are the output files different when I use fwrite in another function VERSUS fwrite in the same function?

output1.txt包含垃圾值，例如Ê，不是正确

output1.txt contains garbage value like Ê, which is NOT correct

output2.txt包含 b，这是正确的

output2.txt contains "b", which is correct

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

void writeData(char *buf, char *path) {
    FILE *fp1;
    fp1 = fopen(path, "a");
    fwrite(&buf, sizeof(char), strlen(buf), fp1);
}

int main () {


    char buf[2] = "a";
    char buf2[3] = "yb";
    char file1_path[12] = "output1.txt";
    char file2_path[12] = "output2.txt";
    memset(buf, 0, sizeof(buf));
    memcpy(buf, &buf2[1], strlen(buf2));
    printf("%s\n", buf);

    writeData(buf, file1_path);

    FILE *fp2;
    fp2 = fopen(file2_path, "a");
    fwrite(&buf, sizeof(char), strlen(buf), fp2);

   return(0);
}

推荐答案

在 writeData 函数，在调用 fwrite 时：

fwrite(&buf, sizeof(char), strlen(buf), fp1);

变量 buf 是指向字符串中的第一个字符。类型为 char * 。但是，表达式& buf 是指向变量 buf 的指针，其类型为 char ** 。数据不同。

the variable buf is a pointer to the first character in the string to write. It's of typechar *. However the expression &buf is a pointer to the variable buf, its type is char **. It's not the same data.

如果 buf 为一个 array ，因为它们都同时是 buf （实际上是& buf [0] ）和& buf 指向同一位置。但是它们仍然是不同的类型。

It works if buf is an array because both then buf (which is really &buf[0]) and &buf points to the same location. They are still different types though.

例如

char buf[2];

然后 buf 衰变指向数组第一个元素（即<$​​ c $ c>& buf [0] ）的指针，其类型为 char * 。表达式& buf 是指向数组的指针，类型为 char（*）[2] 。

then buf decays to a pointer to the arrays first element (i.e. &buf[0]) and is of type char *. The expression &buf is a pointer to the array and is of type char (*)[2].

图形化

+--------+--------+
| buf[0] | buf[1] |
+--------+--------+
^
|
&buf[0]
|
&buf

两个指向同一位置，但类型和语义不同的指针。

Two pointers to the same location, but different types and different semantic meanings.

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