您需要修改：

 而（与fgets（内容，strlen的（内容），FP1）！= NULL）
 

您需要的的sizeof 阵列内容，而不是长度。

 而（与fgets（内容的sizeof（内容），FP1）！= NULL）
 

就算你初始化内容使用前的strlen（）将返回0，你会读出什么从文件

另外，如果你希望写你需要新的文件时重新读取输入文件或者 FCLOSE（）输入文件和 fopen（）函数，或退（）了。

I'm writing a basic program that copies a string from an existing text file and copies the text into a new text file. I'm almost there but I'm having a few small issues. First, I output the line of text to the screen after copying and it's giving me 3 random characters after the string. I want to know why this is happening. Also, the program is creating the new text file but not putting the string into the file.

Here's my code:

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

int main(void)
{
char content[80];
char newcontent[80];

//Step 1: Open text files and check that they open//
FILE *fp1, *fp2;
fp1 = fopen("details.txt","r");
fp2 = fopen("copydetails.txt","w");

    if(fp1 == NULL || fp2 == NULL)
    {
    printf("Error reading file\n");
    exit(0);
    }
    printf("Files open correctly\n");
//Step 2: Get text from original file//
while(fgets(content, strlen(content), fp1) !=NULL)
    {
    fputs (content, stdout);
    strcpy (content, newcontent);
    }
    printf("%s", newcontent);
printf("Text retrieved from original file\n");

//Step 3: Copy text to new file//
    while(fgets(content, strlen(content), fp1) !=NULL)
        {
            fprintf(fp2, newcontent);
        }
        printf("file created and text copied to it");
//Step 4: Close both files and end program//
        fclose(fp1);
        fclose(fp2);
return 0;
}

You need to change:

while(fgets(content, strlen(content), fp1) !=NULL)

you need the sizeof the array content, not the length.

while(fgets(content, sizeof(content), fp1) !=NULL)

Even if you had initialised content before using it strlen() would return 0 and you would have read nothing from the file.

Also, if you want to re-read the input file when writing the new file you need to either fclose() the input file and fopen() it or rewind() it.