malloc 看起来在我的程序中不起作用

Question

我有一个打印文件全部内容的函数，该函数似乎工作正常，但 valgring 抱怨 条件跳转或移动取决于未初始化的值 未初始化的值是由堆分配创建的:

==7876== Memcheck, a memory error detector
==7876== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==7876== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==7876== Command: ./program
==7876== 
==7876== Conditional jump or move depends on uninitialised value(s)
==7876==    at 0x4E864B2: vfprintf (vfprintf.c:1642)
==7876==    by 0x4E8CC38: printf (printf.c:33)
==7876==    by 0x40074C: main (program.c:45)
==7876==  Uninitialised value was created by a heap allocation
==7876==    at 0x4C2BBA0: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7876==    by 0x4008A7: printFile (program.c:23)
==7876==    by 0x40073A: main (program.c:43)
==7876== 
The world is not enought and michi is the only one who's not agree.
==7876== 
==7876== HEAP SUMMARY:
==7876==     in use at exit: 0 bytes in 0 blocks
==7876==   total heap usage: 2 allocs, 2 frees, 621 bytes allocated
==7876== 
==7876== All heap blocks were freed -- no leaks are possible
==7876== 
==7876== For counts of detected and suppressed errors, rerun with: -v
==7876== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

程序如下：

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

char *printFile(char *fileName){
    size_t length=0,size=0;
    char *buffer;
    FILE *file;

    file = fopen (fileName , "r" );

    if (file==NULL){
        printf("\n");
        printf("\tThe file %s does not Exists\n", fileName);
        exit(1);
    }

    fseek (file , 0 , SEEK_END);
    length = (size_t)ftell (file);
    fseek (file , 0 , SEEK_SET);


    buffer = malloc(length+1);

    if (!buffer){
        fputs ("Memory error",stderr);
        exit (2);
    }

    size = fread (buffer,1,length+1,file);

    if (size != length){
        fputs ("Reading error",stderr);
        exit(3);
    }

    fclose (file);
    return buffer;
}

int main (void) {
    char *fileName = "test.txt";
    char *fileContent = printFile(fileName);

    printf("%s",fileContent);
    free(fileContent);
    return 0;
}

快速修复是使用 calloc 而不是 malloc，因为它将返回的字节归零所以我更换了：

buffer = malloc(length+1);

与：

buffer = calloc(length,sizeof(char*));

而且 valgrind 没有抱怨：

==7897== Memcheck, a memory error detector
==7897== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==7897== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==7897== Command: ./program
==7897== 
The world is not enought and michi is the only one who's not agree.
==7897== 
==7897== HEAP SUMMARY:
==7897==     in use at exit: 0 bytes in 0 blocks
==7897==   total heap usage: 2 allocs, 2 frees, 1,096 bytes allocated
==7897== 
==7897== All heap blocks were freed -- no leaks are possible
==7897== 
==7897== For counts of detected and suppressed errors, rerun with: -v
==7897== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

我的问题是，为什么 malloc 会产生该错误以及如何避免 calloc。我这里有编码问题还是只是 malloc？ . . 编辑：如果我改变：

size = fread (buffer,1,length+1,file);

与：

size = fread (buffer,1,length,file);

我得到：

==7985== Memcheck, a memory error detector
==7985== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==7985== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==7985== Command: ./program
==7985== 
==7985== Invalid read of size 1
==7985==    at 0x4E864B2: vfprintf (vfprintf.c:1642)
==7985==    by 0x4E8CC38: printf (printf.c:33)
==7985==    by 0x40074C: main (program.c:44)
==7985==  Address 0x52022f4 is 0 bytes after a block of size 68 alloc'd
==7985==    at 0x4C2BBA0: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7985==    by 0x4008A6: printFile (program.c:22)
==7985==    by 0x40073A: main (program.c:42)
==7985== 
The world is not enought and michi is the only one who's not agree.
==7985== 
==7985== HEAP SUMMARY:
==7985==     in use at exit: 0 bytes in 0 blocks
==7985==   total heap usage: 2 allocs, 2 frees, 620 bytes allocated
==7985== 
==7985== All heap blocks were freed -- no leaks are possible
==7985== 
==7985== For counts of detected and suppressed errors, rerun with: -v
==7985== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

Answer 1

您将读取到缓冲区的文件内容打印出来，但无法确保缓冲区包含 NUL 字符，因此 valgrind 会抱怨，因为 printf 会解析您的数据直到 NUL (jump or move depends on uninitialised value(s))。使用 calloc 告诉 valgrind 你更谨慎了...

Answer 2

通过malloc()获得的内存未初始化。那是故意的。解决方法是改用 calloc() 或在读取之前初始化通过 malloc() 获得的内存。在某些情况下，您只能初始化其中的一部分，而且通常您可以使用比全零更好的初始值。

但是，您的特定错误仅与此相关。您 do 通过 fread() 初始化大部分缓冲区，但是您分配的字节比文件长多了一个字节，并且 fread() 不会在最后一个字节中存储任何内容字节。看起来您可能打算添加一个 '[=15=]' 终止符，但忘记了。这是 valgrind 抱怨的最后一个字节。

在这种情况下，calloc() 执行的内存清除大部分没有用，因为您将覆盖缓冲区中的所有字节，只有一个字节除外。但它确实也初始化了最后一个字节，当您无法以任何其他方式初始化它时，它会为您省去多种麻烦。

Answer 3

您的字符串必须以 NUL 结尾。没有它，程序有未定义的行为，valgrind 正确地报告了这一点。

以 NUL 终止字符串的最简单方法是：

size = fread (buffer,1,length,file); /* no need to specify useless extra char */
                                     /* it will never be read */
...                                  /* check for errors here */
buffer[length] = '[=10=]';               /* <--- null termination */

calloc用NUL字符填充整个buffer，但是很浪费循环。你只需要一个。

malloc 看起来在我的程序中不起作用

malloc doesn't look to work in my program

c

malloc

calloc