程序崩潰時，試圖從文件中獲取字符串並訪問字符串中的單個字母

Question

我正在創建一個C程序，用於將所有小寫字母轉換爲大寫字母，反之亦然。在C程序中，我試圖從文本文件中獲取字符串並訪問每個字符。但是我的程序崩潰了。我在這裏做的錯誤是什麼？下面是我的代碼：

//Program to read contents from a source file and convert the lower case characters to a upper case and vice versa and print it on a target file 

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

void merge(FILE *fp,int n,char *fdata,FILE *fpr) 
{ 
    char change_case[1000]; 
    int loop = 0; 

    while(fgets(fdata,n,fp)!=NULL) 
    { 
     fgets(fdata,n,fp); 

     //printf("%s",fdata); 

     if ((*(fdata+loop) >= 65) && (*(fdata+loop) <= 90)) 
     { 
      printf("%c",*(fdata+loop)); 
      change_case[loop] = *(fdata+loop)+32; 
     } 
     else if ((*(fdata+loop) >= 97) && ((*fdata+loop) <= 122)) 
     { 
      printf("%c",*(fdata+loop)); 
      change_case[loop] = *(fdata+loop)-32; 
     } 
     else 
     { 
      change_case[loop] = *(fdata+loop); 
     } 
     loop++; 

     fpr = fopen("Text File 3.dat","w"); 

     if (fpr == NULL) 
     { 
      printf("Error in accessing/writing the target file"); 
     } 
     else 
     { 
      fputs(change_case,fpr); 
      printf("Line written successfully : %s\n", change_case); 
     } 
    } 
} 

int main() 
{ 
    FILE *sfp, *tfp; //declaring a file pointer for source file and target file 
    char *fdata; 
    int n = 1000; 
    sfp = fopen("Text File 1.txt","r"); 

    if (sfp == NULL) 
    { 
     printf("Error in reading the source file\n"); 
    } 

    else 
    { 
     merge(sfp,n,fdata,tfp); 
    } 

    fclose(sfp); 

return 0; 
} 

Answer 1

在功能void merge(FILE *fp,int n,char *fdata,FILE *fpr) -

fgets(fdata,n,fp);   // you tend to write at invalid location 

內存沒有分配給fdata。你需要爲它分配內存。

fdata=malloc(n);  // check its return and also remember to free it 

注 - 不要使用while!feof(fp)。你在哪裏分配到FDATA內存

while(fgets(fdata,n,fp)!=NULL) 
{ 
    // your code 
}