在文件中打印的額外字符

Question

我製作了一個程序，用vigenere密碼對消息進行加密和解密。

在對文本進行加密或解密時，會在其中打印一些額外的垃圾值。

它從一個名爲input.txt和輸出文件輸入到output.txt，你必須寫在input.txt文件的消息，並在運行，你必須給一個鍵（用字母數字字符的字）。

爲什麼會發生這種情況？

的代碼如下：

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



int Encrypt(char key[])   // CODE FOR ENCRYPTION 
{ 
    int sz = 0, i; 
    FILE *ifp, *ofp; 
    ifp = fopen("input.txt", "r"); 
    char *buffer; 
    char outputFilename[] = "output.txt"; 


    if (ifp == NULL) 
    { 
     fprintf(stderr, "Cant open input file\n"); 
     exit(1); 
    } 


    fseek(ifp, 0, SEEK_END); 
    sz = ftell(ifp); 

    // printf("%d",sz); 

    fseek(ifp, 0, SEEK_SET); 


    /* allocate memory for entire content */ 
    buffer = (char *)malloc(sizeof(char) * sz); 
    if (!buffer) 
     fclose(ifp), fputs("memory alloc fails", stderr), exit(1); 

    /* copy the file into the buffer */ 
    if (1 != fread(buffer, sz, 1, ifp)) 
     fclose(ifp), free(buffer), fputs("entire read fails", stderr), exit(1); 


    ofp = fopen(outputFilename, "w"); 

    if (ofp == NULL) 
    { 
     fprintf(stderr, "Can't open output file !\n"); 
    } 
    // fprintf(ofp,"%s",buffer); 

    int j = 0; 
    for (i = 0; i < strlen(buffer); i++) 
    { 
     if (j > strlen(key) - 1) 
      j = 0; 
     if (buffer[i] >= 65 && buffer[i] < 91) 
     { 

      int c = ((((buffer[i] - 65) + ((key[j] - 65) % 26))) % 26) + 65; 
      fprintf(ofp, "%c", c); 

     } 
     else if (buffer[i] >= 97 && buffer[i] < 123) 
     { 
      int c = ((((buffer[i] - 97) + ((key[j] - 65) % 26))) % 26) + 97; 

      fprintf(ofp, "%c", toupper(c)); 

     } 
     else 
     { 
      fprintf(ofp, "%c", buffer[i]); 
      continue; 
     } 
     j++; 
    } 
    printf("\n"); 

    fclose(ifp); 
    fclose(ofp); 

    return 0; 
} 

int Decrypt(char key[])   // CODE FOR DECRYPTION 
{ 

    int sz = 0, i; 
    FILE *ifp, *ofp; 
    ifp = fopen("output.txt", "r"); 
    char *buffer; 
    char outputFilename[] = "output2.txt"; 


    if (ifp == NULL) 
    { 
     fprintf(stderr, "Cant open input file\n"); 
     exit(1); 
    } 


    fseek(ifp, 0, SEEK_END); 
    sz = ftell(ifp); 

    // printf("%d",sz); 

    fseek(ifp, 0, SEEK_SET); 

    /* allocate memory for entire content */ 
    buffer = (char *)malloc(sizeof(char) * sz); 
    if (!buffer) 
     fclose(ifp), fputs("memory alloc fails", stderr), exit(1); 

    /* copy the file into the buffer */ 
    if (1 != fread(buffer, sz, 1, ifp)) 
     fclose(ifp), free(buffer), fputs("entire read fails", stderr), exit(1); 


    ofp = fopen(outputFilename, "w"); 

    if (ofp == NULL) 
    { 
     fprintf(stderr, "Can't open output file !\n"); 
    } 
    // fprintf(ofp,"%s",buffer); 

    int j = 0; 
    for (i = 0; i < strlen(buffer); i++) 
    { 
     if (j > strlen(key) - 1) 
      j = 0; 
     if (buffer[i] >= 65 && buffer[i] < 91) 
     { 
      if (buffer[i] > key[j]) 
      { 
       int c = 
        ((((buffer[i] - 65) - ((key[j] - 65) % 26))) % 26) + 65; 
       fprintf(ofp, "%c", tolower(c)); 
      } 
      else 
      { 
       int c = ((((buffer[i] - key[j]) + 26)) % 26) + 65; 
       fprintf(ofp, "%c", tolower(c)); 
      } 
     } 
     else if (buffer[i] >= 97 && buffer[i] < 123) 
     { 
      int c = ((((buffer[i] - 97) - ((key[j] - 65) % 26))) % 26) + 97; 

      fprintf(ofp, "%c", tolower(c)); 

     } 
     else 
     { 
      fprintf(ofp, "%c", buffer[i]); 
      continue; 
     } 
     j++; 
    } 
    printf("\n"); 

    fclose(ifp); 
    fclose(ofp); 

    return 0; 
} 

void main() 
{ 
    int ch; 
    char key[20]; 
    a:printf("0.Exit the Menu\n1.Encrypt\n2.Decrypt\n"); 
    printf("Enter your choice\n"); 
    scanf("%d", &ch); 


    switch (ch) 
    { 

    case 0: 
     printf("Goodbye\n"); 
     break; 

    case 1: 
     printf 
      ("-----------------------------Welcome to the encryption zone---------------------\n"); 
     printf("Enter the key to be used\n"); 
     scanf("%s", key); 

     Encrypt(key); 
     break; 

    case 2: 
     printf 
      ("-----------------------------Welcome to the decryption zone---------------------\n"); 
     printf("Enter the key to be used\n"); 
     scanf("%s", key); 

     Decrypt(key); 
     break; 

    default: 
     printf("Enter the correct choice\n"); 
     goto a; 
     break; 

    } 
} 

Answer 1

當你分配和複製在這裏

buffer = (char *)malloc(sizeof(char) * sz); 
if (!buffer) 
    fclose(ifp), fputs("memory alloc fails", stderr), exit(1); 

/* copy the file into the buffer */ 
if (1 != fread(buffer, sz, 1, ifp)) 
    fclose(ifp), free(buffer), fputs("entire read fails", stderr), exit(1); 

你分配的緩衝區sz字節，並複製sz字節，但是你沒有爲空終止符留下空間。因此，當您檢查strlen(buffer)並稍後進行加密時，您將進入您未分配的內存。

要解決這個問題，要麼必須爲'\0'分配一個額外的字節，要麼複製一個更少的字符並在最後加上'\0'。

Answer 2

錯誤的長度計算。

正如@丹尼斯·孟提到的，strlen（緩衝區）可能（* 1）會超過您分配的緩衝區的末端。您的malloc()沒問題。推薦的解決方案是不同的。而不是套結上一個NUL字符，2個for循環

for (i = 0; i < strlen(buffer); i++) 

改變

for (i = 0; i < sz; i++) 

這不僅會解決這個問題，它的運行速度更快，你將不再執行strlen(buffer)sz倍。

* 1如果您的文件包含NUL字符，它不會那麼遠。如果它確實超過了buffer的結尾，那麼UB就停止了。