用逗号读取输入（使用 sscanf）

Question

Hi I'm trying to read the input of a user in the following form:

param1,param2,param3

that I get with fgets . The problem is that I can't seem to understand why it doesn't, after the first comma it shows me a weird behaviour. Ex:

char input[60];
char a1, a2, a3, a4;


printf("Enter info");
fgets(input, 60, stdin);
sscanf(input,"%[^,\n] %[^,\n] %[^,\n] %[^,\n]", &a1, &a2, &a3, &a4);
printf("%c %c %c %c", a1, a2,a3, a4);

If I type for example a,b,c,d I get: a � What am I missing here?

Answer 1

Adding to the answer. Your format string is missing the actual , commas, so 1,2,3,4 cannot match it.

You can do something like this

#include <stdio.h>

int main()
{
  char input[60];
  char a1[2], a2[2], a3[2], a4[2];


  printf("Enter info");
  fgets(input, 60, stdin);
  if (4 != sscanf(input,"%1[^,\n], %1[^,\n], %1[^,\n], %1[^,\n]", a1, a2, a3, a4))
    return printf("Error"), 1;

  printf("%c %c %c %c", a1[0], a2[0],a3[0], a4[0]);
  return 0;
}

Note the error checking with sscanf if it was successful it should return the number of variables which it filled with data. It must be 4 if it worked.

Change the array size of your parameter and the number after the % in the sscanf format string to match your needs.

Edit: To make it throw an error if the input has too much params you can do this

#include <stdio.h>

int main()
{
  char input[60];
  char a1[2], a2[2], a3[2], a4[2];
  char temp[2];

  printf("Enter info");
  fgets(input, 60, stdin);
  if (4 != sscanf(input,"%1[^,\n], %1[^,\n], %1[^,\n], %1[^,\n] %1[^\n]", a1, a2, a3, a4, temp))
    return printf("Error"), 1;

  printf("%c %c %c %c", a1[0], a2[0],a3[0], a4[0]);
  return 0;
}

By adding %1[^\n] to the end of the format sscanf will try to read in another non whitespace character after the four parameter. If it succeed that mean there is too much parameter in the input for ex: 1,2,3,4,5 . It will return 5 which is the number of data it was able to fill. But this last format specifier should not match anything if the input is valid, so sscanf should return exactly 4 in case of a valid input.

Answer 2

The problem is that the %[ format specifier appends a null character as it consumes a sequence of characters.

It needs to have an appropriate argument of type char[] capable of holding at least the character plus the null character, but the relative arguments of each %[ is only of type char at your example.

This is causing undefined behavior, as the appending \0 is written beyond the bounds of the array.

If you want to read and print comma separated word you can use:

#include <stdio.h>

int main()
{
  char input[60];
  char a1[15], a2[15], a3[15], a4[15];

  for(int i; i < 10; i++)
  {
      a1[i] = a2[i] = a3[i] = a4[i] = 0;  // Initialize each character with `\0`.
  }

  printf("Enter info\n");
  fgets(input, 60, stdin);
  sscanf(input,"%[^,\n],%[^,\n],%[^,\n],%[^\n]", a1, a2, a3, a4);


  printf("%s %s %s %s", a1, a2, a3, a4);
  return 0;
}

Execution:

/a.out
hello,mister,tamborine,man
hello mister tamborine man