如何使用 scanf() 获取整个句子作为输入

Question

I want to get a character and a string and a sentence then write each one of them in a new line. I don't know what is wrong with my code.

#include <stdio.h>
#include <string.h>

int main(){

    /* Enter your code here. Read input from STDIN. Print output to STDOUT */
    const int MAX_LEN=100;
    char ch;
    char s[MAX_LEN];
    char sentence[MAX_LEN];
    scanf ("%c",&ch);
    scanf("%s",&s);
    scanf("%[^\n]%*c", &sentence);

    printf("%c\n",ch);
    printf("%s\n", s);
    printf("%s\n",sentence);


    return 0;
}

Answer 1

Regarding

char s[MAX_LEN];
...
scanf("%s",&s);

Use of the & in this case is wrong. s is created here as an array type, so its symbol by itself already points to the location of the first element, (its address) and has type char * , which agrees with the "%s" format specifier used in the scanf statement. Prepending s with the & changes its type to char(*)[100] , which is no longer compatible with "%s" . Using mismatched format specifiers with scanf() invokes undefined behavior , which is far worse than your program simply being mal-formed, it is unpredictable.

Note that with warnings turned on (eg, in GCC by compiling with -Wall ) it should result in a message describing all of this. On my system (not GCC) the following message is issued:

Likewise, when in debug mode with appropriate settings during runtime I see the following assert:

"Parameter type mismatch, expecting pointer to char but found pointer to aggregate or user defined."

By the way, when I allowed execution to continue after this warning message the code appeared to work normally, ie it output the content entered into stdin . But that is the nature of undefined behavior, anything can happen. .

The fix here is to simply replace this:

scanf("%s",&s);

With either this:

scanf("%s",s);  //okay

or this;

fgets(s, sizeof(s), stdin);//better
s[strcspn(s, "\n")] = 0; // optional, if need to eliminate the newline

either approach works as a replacement for those places in your code where you use an & on an array.

Aside: It is also a good practice to use variable names that self describe themselves. Consider using something more descriptive than s , eg: char string[MAX_LEN]; or char string_array[MAX_LEN]; .