為什么它回傳我分段錯誤（核心愚蠢）？

放兩個字串后，它回傳我 segmention fault (core dumbed)

我正在用 gcc 運行代碼。Valgrind 寫道，沒有錯誤。嘗試撰寫使用 Caesar 方法解密文本的代碼。首先我輸入要解碼的文本，第二個文本顯示我需要將第一個文本中的字母移動多遠。即，選擇接收文本與第二輸入文本之間的字母對應更多的距離。我仔細檢查了所有內容，對我來說似乎真的沒有錯誤，或者我只是不精通記憶體。

這是示例

輸入

xUbbemehbT

XYlloworld

應該回傳

Helloworld

我真的不明白為什么它會回傳給我，這是代碼

#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, "%s\n", 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;
}

uj5u.com熱心網友回復：

你失敗了，因為你用 msgres == null 呼叫 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);  <<<<======

uj5u.com熱心網友回復：

這是 valgrind 抱怨，我知道這不是答案，但您的 valgrind 似乎有問題，也許這會有所幫助

==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是這條線

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

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