了解 Valgrind 中的错误

Question

我对 C 还很陌生，所以我一直在通过研究这个程序来磨练我的技能，该程序通过搜索两个文本文件以查找匹配的哈希值来恢复密码。当我尝试编译这个程序时，出现以下错误。

Segmentation fault (core dumped)

因此，我尝试使用 valgrind 调试我的程序。由于我是 C 的新手，所以我不太了解程序中的错误。报错如下

错误 1：可能存在内存泄漏。

552 bytes in 1 blocks are still reachable in loss record 1 of 2
 at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-
 linux.so)
by 0x4EA7CDC: __fopen_internal (iofopen.c:69)
by 0x4008E7: matchfile (in /home/st2411/test)
by 0x400873: main (in /home/st2411/test)

错误2：大小1的读取无效。我觉得这个错误比较难理解，因为它没有指出错误发生在哪一行

Invalid read of size 1
at 0x4EE4070: __strstr_sse2_unaligned (strstr-sse2-unaligned.S:22)
by 0x4009DA: matchfile (in /home/st2411/test)
by 0x400873: main (in /home/st2411/test)
Address 0x0 is not stack'd, malloc'd or (recently) free'd

我的代码如下：

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#define MAXCHAR 1000
//Declaring Functions to match word in file
int matchfile(char *shadowfilename, char 
*hashtablefilename);//shadowfilename for shadow.txt hashtablefilename for hash table

void UsageInfo(char *shadowfile, char * hashtablefile );//Display usage info on arguments for program

void UsageInfo(char *shadowfile, char * hashtablefile) {
    printf("Usage: %s %s <shadowfile> <hashtable>\n", shadowfile, hashtablefile);
    
}

//main function.
int main(int argc, char *argv[]) {
    int result;
        int errcode;
//Display format for user to enter arguments and
//End program if user does not enter exactly 3 arguments
    if(argc < 3 || argc > 3) {
        UsageInfo(argv[1],argv[2]);
        exit(1);
    }

    
    system("cls");
//Pass command line arguments into searchstringinfile
    result = matchfile(argv[1], argv[2]);
    
//Display error message
    if(result == -1) {
        perror("Error");
        printf("Error number = %d\n", errcode);
        exit(1);
    }
    return(0);
}
//Declaring Functions to match word in file


int matchfile(char *shadowfilename, char *hashtablefilename){
    //Declare file containing user account and hashed password
    FILE *shadowfile;
    //Declare file containing list of words and corresponding hash values

    FILE *hashtable;
    //char variables to extract text from files

    char strshadow[MAXCHAR];

    char strhash[MAXCHAR];

//read from file containing user account and hashed password

shadowfile = fopen(shadowfilename, "r");

//error message if file does not exist

if (shadowfile == NULL){
    printf("Could not open file %s",shadowfilename);
    return 1;
}

//read from file containing list of words and corresponding hash values

hashtable = fopen(hashtablefilename, "r");

//error message if file does not exist

if (hashtable == NULL){
    printf("Could not open file %s",hashtablefilename);
    return 1;
}

const char ch = '$';
//char variables to extract hash values

//char *strshadowvalues for shadow file;

//char *strhashvalues for hash table file;
//Valgrind detected an error here
char *strshadowvalues;
char *strhashvalues;

    //variable to check line number for matched
int linenumber = 1;
    //Variable to count match results
int search_result = 0;
while (fgets(strshadow, MAXCHAR, shadowfile) != NULL && fgets(strhash, MAXCHAR, hashtable) != NULL){


    strshadowvalues = strchr(strshadow, ch);
    strhashvalues = strchr(strhash, ch);
            //Matching hashes line-by-line

    if((strstr(strshadowvalues,strhashvalues)) != NULL) {
        //Display lines in which matched hash is found
        
        printf("A match found on line: %d\n", linenumber);
        //Display matching hash in shadow file

                    printf("Shadow:\n%s\n", strshadow);
                    //Display matching hash in shadow file

        printf("Hash: \n%s\n", strhash);
        
        
        search_result++;
    }//Display message if no match

    if((strstr(strshadowvalues,strhashvalues)) == NULL|| strshadowvalues==NULL || strhashvalues==NULL) {
                printf("No password found ");
            }
    linenumber++;
}

//close file
fclose(shadowfile);
return 0;
}

如果有人能向我解释我哪里做错了并指导我如何解决这些问题，我将不胜感激。

Answer 1

想想这里发生了什么：

strshadowvalues = strchr(strshadow, ch);
strhashvalues = strchr(strhash, ch);
        //Matching hashes line-by-line

if((strstr(strshadowvalues,strhashvalues)) != NULL) {
    //Display lines in which matched hash is found

当在该行中找不到 $ 或当两个字符串之一（或两者，就此而言）为空时。答案是，strstr() 将 return 一个空指针。当您尝试使用 %s 格式字符串打印它时，您最终会陷入未定义行为和分段错误的境界。

我会留给您查找 Valgrind 提到的内存泄漏，但这就是它告诉您 Address 0x0 is not stack'd, malloc'd or (recently) free'd

的原因

了解 Valgrind 中的错误

Understanding errors in Valgrind

c

valgrind