我编写了一个使用vigenere密码加密和解密消息的程序。加密部分工作正常,问题是解密部分:当我想使用free()取消分配char *时,我的程序崩溃了。当我不解除它时,该程序工作正常,但不会崩溃。 这是程序代码:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>
int main(void) {
int choice, check, error;
char ch;
printf("What do you want to do?\n"
"1...Decode a message that is saved in a ciphertext.txt file.\n"
"2...Encode a message and save it in a ciphertext.txt file.\n");
do {
error = 0;
check = scanf("%d%c", &choice, &ch);
if (check != 2 || ch != '\n' || choice < 1 || choice > 2) {
printf("Error: Invalid input!\n");
error = 1;
}
fflush(stdin);
} while (error);
if (choice == 2) {
FILE *fp = fopen("ciphertext.txt", "w+");
char buffer[1000];
char *p, *q, *r;
char *encryption_text;
char *encrypted;
char *decrypted;
printf("What are bad characters?\n");
printf("-Everything except A-Z and a-z.\n");
printf("Plaintext: ");
gets(buffer);
decrypted = (char *)malloc(strlen(buffer));
strcpy(decrypted, buffer);
printf("Key: ");
gets(buffer);
encryption_text = (char *)malloc(strlen(buffer));
strcpy(encryption_text, buffer);
encrypted = (char *)malloc(strlen(decrypted));
strcpy(encrypted, decrypted);
putchar('\n');
p = decrypted;
q = encryption_text;
r = encrypted;
if (strlen(q) > strlen(p)) {
printf("The key has to be shorter or equal length as the plaintext.");
return 0;
}
while (*p != '\0') {
if (!(*p >= 'A' && *p <= 'Z') && !(*p >= 'a' && *p <= 'z')) {
printf("Bad characters.");
return 0;
}
if (*q == '\0') {
q = encryption_text;
} else if (!(*q >= 'A' && *q <= 'Z') && !(*q >= 'a' && *q <= 'z')) {
printf("Bad characters.");
return 0;
}
if (*p >= 'a' && *p <= 'z') *p -= ' ';
if (*q >= 'a' && *q <= 'z') *q -= ' ';
*r = (*p - 'A' + *q - 'A') % 26 + 'A';
p++;
q++;
r++;
}
printf("Ciphertext: ");
puts(encrypted);
fputs(encryption_text, fp);
fprintf(fp, ";");
fputs(encrypted, fp);
free(decrypted);
free(encrypted);
free(encryption_text);
fclose(fp);
} else {
FILE *fp = fopen("ciphertext.txt", "r+");
char buffer[1000];
char *encryption_text;
char *encrypted;
char *decrypted;
char delimeter[2] = ";";
char *token;
char *p, *q, *r;
fgets(buffer, 1000, fp);
encrypted = (char *)calloc(strlen(buffer) * sizeof(char), sizeof(char));
encryption_text = (char *)calloc(strlen(buffer) * sizeof(char), sizeof(char));
token = strtok(buffer, delimeter);
strcpy(encryption_text, token);
encrypted = strrchr(buffer, '\0');
if (encrypted != NULL) {
strcpy(encrypted, encrypted + 1);
}
decrypted = (char *)calloc(strlen(buffer) * sizeof(char), sizeof(char));
for (p = decrypted, q = encryption_text, r = encrypted;
*r != '\0'; p++, r++, q++) {
if (*q == '\0') {
q = encryption_text;
}
*p = (*r - 'A' - (*q - 'A') + 26) % 26 + 'A';
}
printf("Plaintext: ");
puts(decrypted);
printf("Key: ");
puts(encryption_text);
printf("Ciphertext: ");
puts(encrypted);
if (decrypted) free(decrypted);
if (encrypted) free(encrypted);
if (encryption_text) free(encryption_text);
fclose(fp);
}
return 0;
}
我希望你能告诉我它崩溃的原因。谢谢!
答案 0 :(得分:0)
崩溃的主要原因可能是以下一行,
encrypted = strrchr(buffer, '\0');
,其中encrypted
指向由局部变量buffer
表示的内存中的某个位置。这很可能是堆栈上的内存,但至少这个内存尚未通过malloc/calloc
分配。
因此,稍后释放encrypted
很可能会在free
- 操作时崩溃。
顺便说一句:正如评论中所述,在使用malloc/calloc
时重新考虑您的strcpy
语句,以便为终止'\0'
留出空间。
此外,请注意strrchr(buffer, '\0')
返回指向buffer
中包含的字符串末尾的指针;在使用'\0'
终止strcpy
后立即从某个位置复制内存可能很关键,因为不能保证会有第二个字符串终止字符。
encrypted = strrchr(buffer, '\0');
if (encrypted != NULL) {
strcpy(encrypted, encrypted + 1);
}