如何正确释放内存？

Question

请帮忙，我不知道如何正确释放内存。下面有一段代码

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

#define INT_MSG_LEN 25;

enum {NO_ERROR = 0, 
      ERROR_INPUT = 100, 
      ERROR_LEN = 101};

static const char *error_texts[] = { "Error input!", 
                                     "Error lenghts!"};

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);
char rotate(char *original, int offset);

int main(int argc, char *argv[])
{
    int ret = NO_ERROR;
    char *msgEnc, *msg, alphabet[53] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    int msgEncLen, msgLen;

    msgEnc = msg = 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;
    
    char msgRes[msgEncLen], mainMsg[msgEncLen];
    
    if (ret == NO_ERROR)
        shift(msgEnc, msg, msgRes, mainMsg, alphabet, msgEncLen);
    else
        print_error(ret);

    free(msgEnc);
    free(msg);
    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; ++i)
            mainMsg[i] = msgEnc[i]+dis;

    rotate(mainMsg, offset);
    for(int i = 0; i<offset; ++i)
        printf("%c", mainMsg[i]);
    printf("\n");
}

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 = 0;
    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_letters = 0;
    for (int i = 0; alphabet[i]; ++i){
        for (int j = 0; msgEnc[j]; ++j){
                msgRes[j] = alphabet[(get_letter(msgEnc[j], alphabet) + i) % 52];
            }
            int sameLetters = get_sameletters(msg, msgRes, offset);
            if (sameLetters >= max_letters){
                max_letters = 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;
}

char rotate(char *original, int offset)
{
    for (int i = 0; i<offset; ++i){
        if (original[i] > 'Z' && original[i]<'a')
            original[i] += 6;
        else if (original[i] > 'z'){
            int k = (int)original[i];
            k -= 58;
            original[i] = (char)k;
        }
    }
    return *original;
}

当我通过 Valgrind 运行它时，它告诉我分配的内存有错误，说我没有释放它。在107.56和44行写到内存没有释放，更准确地说

==56665== Conditional jump or move depends on uninitialised value(s)
==56665==    at 0x4015BF: compare (main.c:107)
==56665==    by 0x401468: shift (main.c:56)
==56665==    by 0x4012C3: main (main.c:44)
==56665== 
�elloword
==56665== 
==56665== HEAP SUMMARY:
==56665==     in use at exit: 0 bytes in 0 blocks
==56665==   total heap usage: 4 allocs, 4 frees, 2,098 bytes allocated
==56665== 
==56665== All heap blocks were freed -- no leaks are possible
==56665== 
==56665== Use --track-origins=yes to see where uninitialised values come from
==56665== For lists of detected and suppressed errors, rerun with: -s
==56665== ERROR SUMMARY: 52 errors from 1 contexts (suppressed: 0 from 0)

看起来已经释放了内存，但这还不够，因为在一些函数调用之后还有未释放的内存。我想我只是不知道如何释放它。在每个请求额外内存的函数末尾，我尝试通过 free （函数调用的数组）释放它，但在这种情况下它显示分段错误。如果您至少让我知道一些信息，我将不胜感激。

这是输入的示例

xUbbemehbT
XYlloworld

Answer 1

valgrind 输出不会表明您存在内存泄漏或没有正确释放某些内容。

如果您查看这部分输出：

==56665== HEAP SUMMARY:
==56665==     in use at exit: 0 bytes in 0 blocks
==56665==   total heap usage: 4 allocs, 4 frees, 2,098 bytes allocated
==56665== 
==56665== All heap blocks were freed -- no leaks are possible

它指出所有已分配的内存块均已释放。所以你不存在分配问题。

重要的部分是这一部分：

==56665== Conditional jump or move depends on uninitialised value(s)
==56665==    at 0x4015BF: compare (main.c:107)
==56665==    by 0x401468: shift (main.c:56)
==56665==    by 0x4012C3: main (main.c:44)

这表示在第 107 行的 compare 函数（该函数是从第 56 行的 shift 调用的，而该函数又是从 main 在第 44 行），读取了一个从未初始化的值。这一行是：

for (int j = 0; msgEnc[j]; ++j){

所以这告诉我们您正在读取 msgEnc 数组中的一个从未写入过的字节。该数组是在 read_Input_Msg 函数中写入的，该函数一次将一个字符读入数组中。

由于这是数组应该是一个字符串，并且因为您一次读取一个字节，所以您需要手动添加一个您没有执行的终止空字节。因此，当最终调用 compare 时，上面的行将查找从未显式写入的空字节，因此您最终会读取未初始化的字节。

在 read_Input_Msg 中的主 while 循环之后，在字符串末尾添加空字节：

while ((c = getchar()) != EOF && c != '\n'){
    ...
}
msg[len]=0;