为什么 Valgrind 抱怨 fgets？

Question

我有一个简单的 C 代码。

ma​​in.c

int main() {
    FILE *stream = fopen("../input.txt", "r");
    cube test_cube;
    cube_initialization(&test_cube, stream);

/* and some code with free memory which is not needed to understand the situation */

}

cube.c

/* just the function valgrind complains about */

void cube_initialization(cube *cube, FILE *input_file) {
    int side_length;
    char buffer[BUF_SIZE];
    char waste_buffer[BUF_SIZE];


    fgets(buffer, BUF_SIZE, input_file);
    fgets(waste_buffer, BUF_SIZE, input_file); /* empty line */
    side_length = (int) strtol(buffer, NULL, 10);
    cube->side_length = side_length;
    cube->cube_array = malloc(side_length * sizeof(int **));
    int z;
    for (z = 0; z < side_length; z++) {
        cube->cube_array[z] = malloc(side_length * sizeof(int *));
        int y;
        for (y = 0; y < side_length; y++) {
            cube->cube_array[z][y] = malloc(side_length * sizeof(int));
        }
    }
}

和valgrind输出

==8251== Invalid read of size 4
==8251==    at 0x48F3727: fgets (iofgets.c:47)
==8251==    by 0x1093C2: cube_initialization (cube.c:11)
==8251==    by 0x10928D: main (main.c:11)
==8251==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==8251== 
==8251== 
==8251== Process terminating with default action of signal 11 (SIGSEGV)
==8251==  Access not within mapped region at address 0x0
==8251==    at 0x48F3727: fgets (iofgets.c:47)
==8251==    by 0x1093C2: cube_initialization (cube.c:11)
==8251==    by 0x10928D: main (main.c:11)
==8251==  If you believe this happened as a result of a stack
==8251==  overflow in your program's main thread (unlikely but
==8251==  possible), you can try to increase the size of the
==8251==  main thread stack using the --main-stacksize= flag.
==8251==  The main thread stack size used in this run was 8388608.
==8251== 
==8251== HEAP SUMMARY:
==8251==     in use at exit: 0 bytes in 0 blocks
==8251==   total heap usage: 1 allocs, 1 frees, 488 bytes allocated
==8251== 
==8251== All heap blocks were freed -- no leaks are possible
==8251== 
==8251== For lists of detected and suppressed errors, rerun with: -s
==8251== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
Segmentation fault (core dumped)

我不明白 Valgrind 为什么抱怨 fgets()。我从文件中读取数据并使用带有指向大缓冲区（256）的指针的fgets，但它只需要我读取大约 1-6 个符号的短行（我需要 fgets，因为它在末尾找到 '\n' 时会停止线）。也许问题出在 fgets() 尝试读取 256 个符号的行并且它在 1-5 和 '\n' 之后停止？

Answer 1

Valgrind 投诉非法地址0x0 访问：

地址 0x0 没有被堆栈、malloc 或（最近）释放

在您的fgets() 电话中

fgets(buffer, BUF_SIZE, input_file);

buffer 参数很好，因为它是在堆栈中分配的。唯一负责的可能是input_file 参数，来自main()。该参数是从fopen() 调用获得的流。你在传递给cube_initialization()之前没有检查它！

检查一下，然后明白为什么它返回NULL（可能你试图打开一个不存在的路径）。

int main() {
    FILE *stream = fopen("../input.txt", "r");
    cube test_cube;
    if( stream != NULL )
    {
        cube_initialization(&test_cube, stream);

        /* Continue execution*/
    }
}

Answer 2

Valgrind 抱怨 fgets() 中的读取无效。问题不太可能是目标数组的大小，这可能会触发无效写入，但确实是 BUF_SIZE 字节的数组，您将其记录为 256 的合理值。问题更可能与流指针有关，正如 Valgrind 报告的那样，它可能为空：

Address 0x0 is not stack'd, malloc'd or (recently) free'd`

您应该修复代码中潜在的未定义行为：

• 验证fopen() 是否返回了有效的FILE*，以避免在您尝试从input_file 读取数据时出现未定义的行为。
• 验证fgets() 是否成功（不返回NULL），以避免从目标数组读取时出现未定义的行为。