c-为什么它会返回我的分段错误(核心静音)

Question

放入两个字符串后，它返回mesegmention fault (core dumbed)

我正在用gcc运行代码。Valgrind写道没有错误。尝试编写使用Caesar方法解密文本的代码。首先，我输入要解码的文本，第二个文本显示我需要将第一个文本中的字母移动多远。也就是说，选择接收到的文本和第二输入的文本之间在字母上有更多对应关系的距离。我仔细检查了所有的东西，对我来说似乎真的没有错误，或者我只是不太擅长处理记忆。

以下是示例

输入

xUbbemehbT

XYlloworld

应返回

Helloworld

我真的不明白为什么它会返回给我，这是代码

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define INT_MSG_LEN 50;
enum {NO_ERROR = 0, 
ERROR_INPUT = 100, 
ERROR_LEN = 101};
static const char *error_texts[] = { "Error input!", 
"Error lenght"};
void shift(char *msgEnc, char *msg, char *msgRes, char *mainMsg, char *alphabet, int offset);
void report_error(int error);
void print_error(int error);
int get_sameletters(char *msg, char *msgRes, int offset);
int get_letter(char letter, char *alphabet);
int compare(char *msgEnc, char *msg, char *msgRes, char *alphabet, int offset);
char *read_Input_Msg(int *msglen);
int main(int argc, char *argv[])
{
int ret = NO_ERROR;
char *msgEnc, *msg, *msgRes, *mainMsg, alphabet[53] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
int msgEncLen, msgLen;
msgEnc = msg = msgRes = mainMsg = NULL;
msgEncLen = msgLen = 0;
msgEnc = read_Input_Msg(&msgEncLen);
if (msgEnc)
msg = read_Input_Msg(&msgLen);
if (msgEnc == NULL || msg == NULL)
ret = ERROR_INPUT;
else if (msgEncLen != msgLen)
ret = ERROR_LEN;

if (ret == NO_ERROR)
shift(msgEnc, msg, msgRes, mainMsg, alphabet, msgEncLen);
else
print_error(ret);
free(msgEnc);
free(msg);
free(msgRes);
free(mainMsg);
return ret;
}
void shift(char *msgEnc, char *msg, char *msgRes, char *mainMsg, char *alphabet, int offset)
{//function for decoding text by a defined offset
int dis;
dis = compare(msgEnc, msg, msgRes, alphabet, offset);
for (int i = 0; i<offset-1; ++i){
if ((msgEnc[i] >= 'a' && msgEnc[i] <= 'z') || (msgEnc[i] >= 'A' && msgEnc[i] <= 'Z')){
mainMsg[i] = msgEnc[i+dis];
break;
}
}
for(int i = 0; i<offset-1; ++i)
printf("%c", mainMsg[i]);
}
void report_error(int error)
{//prints error 
if (error >= ERROR_INPUT && error <= ERROR_LEN)
fprintf(stderr, "%sn", error_texts[error - ERROR_INPUT]);
}
void print_error(int error)
{//what error it is
switch (error){
case ERROR_INPUT:
report_error(ERROR_INPUT);
break;
case ERROR_LEN:
report_error(ERROR_LEN);
break;
}
}
int get_sameletters(char *msg, char *msgRes, int offset)
{//gets count of sameletters between two strings
int sameLetters = 0;
for (int i = 0; i<offset-1; ++i){
if (msg[i] == msgRes[i])
sameLetters++;
}
return sameLetters;
}
int get_letter(char letter, char *alphabet)
{   
int k;
for (int i=0; alphabet[i]; ++i){
if (letter == alphabet[i])
k = i;
}
return k;
}
int compare(char *msgEnc, char *msg, char *msgRes, char *alphabet, int offset)
{//calculate a distance between first input string and string what will get after decryption
int distance, max = 0;
for (int i = 0; alphabet[i]; ++i){
for (int j = 0; msgEnc[j]; ++j){
if ((msgEnc[i] >= 'a' && msgEnc[i] <= 'z') || (msgEnc[i] >= 'A' && msgEnc[i] <= 'Z'))
msgRes[j] = alphabet[(get_letter(msgEnc[j], alphabet) + i) % 52];
}
int sameLetters = get_sameletters(msg, msgRes, offset);
if (sameLetters > max){
max = sameLetters;
distance = i;
}
}
return distance;
}
char *read_Input_Msg(int *msglen)
{//input messages, at the same time counts the length of the entered string
int capacity = INT_MSG_LEN;
char *msg = malloc(capacity);
int c, len = 0;
while ((c = getchar()) != EOF && c != 'n'){
if (!((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'))) {
free(msg);
msg = NULL;
len = 0;
break;
}
if (len == capacity){
char *tmp = realloc(msg, capacity * 2);
if (tmp == NULL){
free(msg);
msg = NULL;
len = 0;
break;
}
capacity *= 2;
msg = tmp;
}
msg[len++] = c;
}
*msglen = len;
return msg;
}

Answer 1

您失败是因为您使用msgres==null 调用了shift

msgEnc = msg = msgRes = mainMsg = NULL; <<<<==========
msgEncLen = msgLen = 0;
msgEnc = read_Input_Msg(&msgEncLen);
if (msgEnc)
msg = read_Input_Msg(&msgLen);
if (msgEnc == NULL || msg == NULL)
ret = ERROR_INPUT;
else if (msgEncLen != msgLen)
ret = ERROR_LEN;
if (ret == NO_ERROR)
shift(msgEnc, msg, msgRes, mainMsg, alphabet, msgEncLen);  <<<<======

Answer 2

这是valgrind的抱怨，我知道这不是一个答案，但你的valgrind似乎有问题，也许这会帮助

==7662== Invalid write of size 1
==7662==    at 0x1096C9: compare (hello.c:110)
==7662==    by 0x1093CF: shift (hello.c:55)
==7662==    by 0x109338: main (hello.c:41)
==7662==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==7662==
==7662==
==7662== Process terminating with default action of signal 11 (SIGSEGV)
==7662==  Access not within mapped region at address 0x0
==7662==    at 0x1096C9: compare (hello.c:110)
==7662==    by 0x1093CF: shift (hello.c:55)
==7662==    by 0x109338: main (hello.c:41)

110这条线路是吗

msgRes[j] = alphabet[(get_letter(msgEnc[j], alphabet) + i) % 52];