c - 尝试从文件读取行返回不正确的行

Question

当从 C 语言的文件中读取一行时，尝试使用 printf 时会返回错误的行。我认为这是因为我返回 char* 而不是 char 但是返回 char 会导致崩溃。

char* getLineFromFile(const char name[], int lineNumber) 
{
    FILE *file = fopen(name, "r");
    int count = 0;
    if (file != NULL)
    {
        char line[256]; /* or other suitable maximum line size */
        while (fgets(line, sizeof line, file) != NULL) /* read a line */
        {
            if (count == lineNumber)
            {
                fclose(file);
                return line;
            }
            else
            {
                count++;
            }
        }
        fclose(file);
    }
    else
    {
        printf("Couldn't find file!");
    }
}

//Example call
printf(getLineFromFile("Include/index.db", 1));

应该获取文件的第二行，但返回格式错误的字符串。

Answer 1

问题是 getLineFromFile 中的 return line; - 您返回的是 local array当您尝试通过调用 printf 来访问它时，会调用未定义的行为。相反，将缓冲区传递给 getLineFromFile 并在找到行时将其复制到其中:

int getLineFromFile(const char name[], int lineNumber, char* out, size_t outsize)
{
    FILE* file = fopen(name, "r");
    int count = 0;
    if (file != NULL)
    {
        char line[256]; /* or other suitable maximum line size */
        while (fgets(line, sizeof line, file) != NULL) /* read a line */
        {
            if (count == lineNumber)
            {
                fclose(file);
                strncpy(out, line, outsize);
                out[outsize - 1] = '\0';
                return 1;
            }   
            else
            {
                count++;
            }
        }
        fclose(file);
        return 0;
    }
    else
    {
        printf("Couldn't find file!");
        return 0;
    }
}

//Example call
char lineBuffer[256];
if (getLineFromFile("Include/index.db", 1, lineBuffer, sizeof lineBuffer)) {
    printf("%s", lineBuffer);
}