C 程序吃掉了我所有的 RAM+SWAP 资源

Question

我做了一个简单的程序来找出用户输入的md5校验和(输入[1;6]) 问题是这些 for 循环似乎耗尽了我所有的 RAM + 交换资源，导调用脑挂起。我做错了什么？管理系统内存的更好方法是什么？

/* 

C Example w/o mpi 

mpicc md5.c -o md5 -lcrypto -lssl
./md5

Single process on c2d laptop
String finding matchin md5 of string "hello" tooks  

*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <unistd.h>
#include <time.h>
#if defined(__APPLE__)
#  define COMMON_DIGEST_FOR_OPENSSL
#  include <CommonCrypto/CommonDigest.h>
#  define SHA1 CC_SHA1
#else
#  include <openssl/md5.h>
#endif


char *str2md5(const char *str, int length) {
    int n;
    MD5_CTX c;
    unsigned char digest[16];
    char *out = (char*)malloc(33);

    MD5_Init(&c);

    while (length > 0) {
        if (length > 512) {
            MD5_Update(&c, str, 512);
        } else {
            MD5_Update(&c, str, length);
        }
        length -= 512;
        str += 512;
    }

    MD5_Final(digest, &c);

    for (n = 0; n < 16; ++n) {
        snprintf(&(out[n*2]), 16*2, "%02x", (unsigned int)digest[n]);
    }

    return out;
}

int main (int argc, char* argv[])
{

    #ifdef COUNT // Very bad name, not long enough, too general, etc..
    static int const count = COUNT;
    #else
    static int const count = 6; // default value
    #endif
    clock_t begin, end;
    double time_spent;
    begin = clock();

    int bflag = 0;
    int sflag = 0;
    int index;
    int c;
    char input[count];
    char action[2]; // char + \n
    char *inputResult = (char*)malloc(33);
    char *tmpResult = (char*)malloc(33);
    char inputGuess[6];
    int i,j,k,l,m,n;
    char letters[] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

    printf("Please enter string to guess (max 6 char - latin letters and numbers): ");
    scanf("%s", input);
    inputResult = str2md5(input, strlen(input));
    printf("Md5 to find = %s \n", inputResult);
    printf("Continue? (Y/n) ");
    scanf("%s", action);
    if(action == "n\n"){
        return 0;
    }


    /* for 1 char input */
    for(i=0; i<sizeof(letters); i++){ //letters + numbers
        char guess1[] = {letters[i], '\0'};
/*      printf("%s\n", guess1);*/
        if(strcmp(str2md5(guess1, strlen(guess1)),inputResult) == 0){       
            printf("We guessed it! Your input was - %s \n", guess1);
            end = clock();
            time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
            printf("Time spent: %f seconds\n", time_spent);
            free(inputResult);
            return 0;
        }
    }

    /* for 2 char input */
    for(i=0; i<sizeof(letters); i++){ //letters + numbers
        for(j=0; j<sizeof(letters); j++){
            char guess2[] = {letters[i], letters[j], '\0'};
            if(strcmp(str2md5(guess2, strlen(guess2)),inputResult) == 0){       
                printf("We guessed it! Your input was - %s \n", guess2);
                end = clock();
                time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
                printf("Time spent: %f seconds\n", time_spent);
                free(inputResult);
                return 0;
            }
        }
    }

    /* for 3 char input */
    for(i=0; i<sizeof(letters); i++){ //letters + numbers
        for(j=0; j<sizeof(letters); j++){
            for(k=0; k<sizeof(letters); k++){
                char guess3[] = {letters[i], letters[j], letters[k], '\0'};
/*              printf("%s\n", guess3);*/
                if(strcmp(str2md5(guess3, strlen(guess3)),inputResult) == 0){       
                    printf("We guessed it! Your input was - %s \n", guess3);
                    end = clock();
                    time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
                    printf("Time spent: %f seconds\n", time_spent);
                    free(inputResult);
                    return 0;
                }
            }
        }
    }

    /* for 4 char input */
    for(i=0; i<sizeof(letters); i++){ //letters + numbers
        for(j=0; j<sizeof(letters); j++){
            for(k=0; k<sizeof(letters); k++){
                for(m=0; m<sizeof(letters); m++){
                    char guess4[] = {letters[i], letters[j], letters[k], letters[m], '\0'};
/*                  printf("%s\n", guess4);*/
                    if(strcmp(str2md5(guess4, strlen(guess4)),inputResult) == 0){       
                        printf("We guessed it! Your input was - %s \n", guess4);
                        end = clock();
                        time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
                        printf("Time spent: %f seconds\n", time_spent);
                        free(inputResult);
                        return 0;
                    }
                }
            }
        }
    }

    /* for 5 char input */
    for(i=0; i<sizeof(letters); i++){ //letters + numbers
        for(j=0; j<sizeof(letters); j++){
            for(k=0; k<sizeof(letters); k++){
                for(m=0; m<sizeof(letters); m++){
                    for(n=0; n<sizeof(letters); n++){
                        char guess5[] = {letters[i], letters[j], letters[k], letters[m], letters[n], '\0'};
/*                      printf("%s\n", guess5);*/
                        if(strcmp(str2md5(guess5, strlen(guess5)),inputResult) == 0){       
                            printf("We guessed it! Your input was - %s \n", guess5);
                            end = clock();
                            time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
                            printf("Time spent: %f seconds\n", time_spent);
                            free(inputResult);
                            return 0;
                        }
                    }
                }
            }
        }
    }

    /* for 6 char input */
    for(i=0; i<sizeof(letters); i++){ //letters + numbers
        for(j=0; j<sizeof(letters); j++){
            for(k=0; k<sizeof(letters); k++){
                for(m=0; m<sizeof(letters); m++){
                    for(n=0; n<sizeof(letters); n++){
                        for(l=0; l<sizeof(letters); l++){
                            char guess6[] = {letters[i], letters[j], letters[k], letters[m], letters[n], letters[l], '\0'};
/*                          printf("%s\n", guess6);*/
                            if(strcmp(str2md5(guess6, strlen(guess6)),inputResult) == 0){       
                                printf("We guessed it! Your input was - %s \n", guess6);
                                end = clock();
                                time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
                                printf("Time spent: %f seconds\n", time_spent);
                                free(inputResult);
                                return 0;
                            }
                        }
                    }
                }
            }
        }
    }

    return 0;
}

Answer 1

在不检查整个代码的情况下:您至少有一个 malloc没有相应的free .

str2md5为 out 分配 33 个字节返回的。有几行 strcmp(str2md5(guess3, strlen(guess3)),inputResult) 形式其中 malloc 产生的指针对于 out是暂时的、丢失的，因此永远无法再次释放。

考虑到您在多个嵌套循环中调用这些行，这可能会导致您的问题。

通过分配str2md5的结果可以避免这个问题。到本地指针并在执行 strcmp 后释放它。这是前面提到的行的示例:

char* str2md5_ret = str2md5(guess3, strlen(guess3));
int cmp = strcmp(str2md5_ret, inputResult);
free(str2md5_ret);
if(cmp == 0)
  //...

再次查看代码，我发现了其他几个问题:

您分配给inputResult就在声明处，但随后您从 str2md5 分配一个新数据 block 到它。所以又有 33 个字节被泄露。然后你分配 tmpResult它没有被释放，实际上甚至从未被再次使用过。这又是 33 字节的泄露。 inputResult如果没有任何循环中断，则不会被释放。

正如其他人指出的那样，深度嵌套循环是不好的风格。实际上，您可以使用单个循环将它们全部编写在单个递归函数中。这可能更具可读性、可调试性、可维护性和灵 active 。

您还可以删除几乎所有 malloc是通过保留一个 char*所有 str2md5 的输出长度为 33 个字节调用并将其作为参数传递而不是返回它。