使用printf然后在循环中使用scanf

Question

char *str;
while(1)
{
    printf("$$$$>");
    scanf("%s",str);
}

In this code I just want to print a command prompt. When user inputs something nothing happens and the command prompt is printed again. But the scanf() runs once and then $$$$> is printed in loop. The code runs when I tried to take a character array instead of str pointer. why?

Answer 1

char *str;

You used str without initializing it. Using an uninitialized pointer in another function like scanf is a recipe for program to crash.

You can fix the issue by malloc (dynamically allocating) for str pointer, but for such simple usage, you can use array instead.

char str[100] = "";
while(1)
{
    printf("$$$$>");
    // scanf("%s",str);  // not recommended
    fgets( str, sizeof( str ), stdin );  // fgets is better
}

Answer 2

如果您没有分配str可以指向的内存缓冲区，则在str declecle下添加以下行：

str=malloc(sizeof(char)*1000);

Answer 3

You must allocate the memory to the string after declaring the pointer. You can do it by using malloc or calloc functions.

char *str = (char*) malloc(sizeof(char) * 100);

or

char *str = (char*) calloc(100, sizeof(char));