c - Strlen Valgrind 无效读取大小 1

Question

在这个程序中，我试图获取存储在文件中的信息，然后搜索它以找到存储的数字。我删除了遍历文件以查找存储的数字的部分，因为该部分工作正常。但是，在本节中，第 63 行(带有 strlen 的那个)有一个错误导致 valgrind 错误报告

==4149== Memcheck, a memory error detector
==4149== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==4149== Using Valgrind-3.12.0.SVN and LibVEX; rerun with -h for copyright info
==4149== Command: main.o
==4149== 
==4149== Invalid read of size 1
==4149==    at 0x4C2EDB4: strlen (vg_replace_strmem.c:454)
==4149==    by 0x108B9E: main (main.c:63)
==4149==  Address 0x54de5f7 is 0 bytes after a block of size 23 alloc'd
==4149==    at 0x4C2BBAF: malloc (vg_replace_malloc.c:299)
==4149==    by 0x108B62: main (main.c:56)
==4149== 

这是代码:

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

int findSize(char file_name[]) { 
    // opening the file in read mode 
    FILE *fp = fopen(file_name, "r"); 

    // checking if the file exist or not 
    if (fp == NULL) { 
        printf("File Not Found!\n"); 
        return -1; 
    } 

    fseek(fp, 0L, SEEK_END); 

    // calculating the size of the file 
    int res = ftell(fp); 

    // closing the file 
    fclose(fp); 

    return res; 
} 

int main() {
    char *buffer = 0;   //contents of the file
    long int is = 0; // thing that will be returned
    long int ie = 0;    // thing that will be returned
    int index1; //index of "s"
    int index2; //index of "e"
    char *a;    //phrase after s
    char *b;    //phrase after e
    int bufferlength;   //length of txt file
    int negativeCount1 = 0;
    int negativeCount2 = 0;
    long length;
    char file_name[] = { "filename" }; 
    FILE *f = fopen(file_name, "rb");

    int res = findSize(file_name); 
    if (res != -1) 
        printf("Size of the file is %d bytes \n", res); 
    if (res == 0) {
        printf("empty file detected");
        return 0;
    }

    if (f) {
        {
            fseek(f, 0, SEEK_END);
            length = ftell(f);
            fseek(f, 0, SEEK_SET);
            buffer = malloc(length);
            if (buffer) {
               fread(buffer, 1, length, f);
            }
            fclose(f);
        }
        bufferlength = strlen(buffer);
        printf("%d \n", bufferlength);
    }
}

Answer 1

在 C 字符串中是 零终止 - 字符串的最后一个元素必须为零。万一所有 length您从文件中读取的字符是非零的，strlen()搜索零字节时将读取缓冲区的末尾。

如果你有它，你可以使用 strnlen()限制缓冲区的最大长度 strlen()将尝试测量:

 bufferlength = strnlen(buffer, length);
 // you can also limit printf
 printf(".*s\n", (int)length, buffer); // pendantics would check if length <= INT_MAX

或者通常多分配一个字节并确保最后一个字节为零，以便 strlen()后面不读 buffer .

  buffer = malloc(length + 1); // allocate one more
  // your error handling of failed allocation is strange
  if (buffer != NULL) { 
      abort();
  }
  buffer[length] = '\0'; // null terminate buffer
  fread (buffer, 1, length, f);
  fclose (f);
  bufferlength = strlen(buffer); // will read at max `length + 1` characters, 
                                 // because `buffer[length] = '\0'`
  printf("%s\n", buffer); // buffer is zero terminated - valid