[英]Why Valgrind complains about fgets?
我有一個簡單的 C 代碼。
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 */
}
立方體.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 輸出
==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)
我不明白為什么 Valgrind 抱怨fgets()
。 我從文件中讀取數據並使用fgets
和指向 big buffer(256) 的指針,但它只需要我讀取大約 1-6 個符號的短行(我需要 fgets,因為它在行尾找到 '\\n' 時停止) . 也許問題在於 fgets() 嘗試讀取 256 個符號的行並在 1-5 和 '\\n' 之后停止?
Valgrind 抱怨非法地址0x0
訪問:
地址 0x0 未被堆棧、malloc 或(最近)釋放
在您的fgets()
電話中
fgets(buffer, BUF_SIZE, input_file);
buffer
參數很好,因為它是在堆棧中分配的。 唯一負責的可以是input_file
參數,來自main()
。 該參數是從fopen()
調用獲得的流。 在傳遞給cube_initialization()
之前你沒有檢查它!
檢查它,然后理解為什么它返回NULL(可能你試圖打開一個不存在的路徑)。
int main() {
FILE *stream = fopen("../input.txt", "r");
cube test_cube;
if( stream != NULL )
{
cube_initialization(&test_cube, stream);
/* Continue execution*/
}
}
Valgrind 抱怨fgets()
讀取無效。 問題不太可能是目標數組的大小,這可能會觸發無效寫入,但確實是一個BUF_SIZE
字節的數組,您將其記錄為合理的 256 值。問題更可能與流指針有關,它Valgrind 報告的可能為空:
Address 0x0 is not stack'd, malloc'd or (recently) free'd`
您應該修復代碼中潛在的未定義行為:
fopen()
返回了有效的FILE*
,以避免嘗試從input_file
讀取時出現未定義的行為。fgets()
成功(不返回NULL
),以避免從目標數組讀取時出現未定義的行為。
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