为什么 Valgrind 抱怨 fgets？

Question

我有一个简单的 C 代码。

主文件

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 */

}

立方体.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)

我不明白为什么 Valgrind 抱怨fgets() 。 我从文件中读取数据并使用fgets和指向 big buffer(256) 的指针，但它只需要我读取大约 1-6 个符号的短行（我需要 fgets，因为它在行尾找到 '\\n' 时停止） . 也许问题在于 fgets() 尝试读取 256 个符号的行并在 1-5 和 '\\n' 之后停止？

Answer 1

Valgrind 抱怨非法地址0x0访问：

地址 0x0 未被堆栈、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。 唯一负责的可以是input_file参数，来自main() 。 该参数是从fopen()调用获得的流。 在传递给cube_initialization()之前你没有检查它！

检查它，然后理解为什么它返回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()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，但确实是一个BUF_SIZE字节的数组，您将其记录为合理的 256 值。问题更可能与流指针有关，它Valgrind 报告的可能为空：

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()返回了有效的FILE* ，以避免尝试从input_file读取时出现未定义的行为。
• 验证fgets()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。