GCC`scanf`分段故障

Question

I was playing around with C and the scanf function and came across this weird error that I can't seem to figure out. Given the following code:

#include <stdio.h>
#include <stdlib.h>

typedef struct {
  int a;
} sample;

void fn(sample *s) {
  char command;
  scanf("%[abc]", &command);
  printf("Read: %c\n", command);
  printf("In the sample function:, %i\n", s->a);
}

int main() {
  sample *s = malloc(sizeof(sample));
  s->a = 4;

  printf("Before sample function: %i\n", s->a);
  fn(s);
  printf("After sample function: %i\n", s->a);

  return 0;
}

It seems to seg fault. With the output:

$ ./sample
Before sample function: 4
a
Read: a
In the sample function:, 4
Segmentation fault (core dumped)

I used gdb and attached a watch to the struct, it seems that inside the scanf function, it seems to 'modify' the struct? Which is weird, because even after the scanf inside the sample function ' fn ', it is able to print out the struct fields fine. However, once returning from the fn and jumping back into main , it seg faults when it tries to print out the same information?

Interestingly, if you change the scanf to scanf("%c\\n", &command); (without the character set) it seems to work fine. For the record, the version of gcc I am using is 4.7.2, and I am compiling the code with: gcc -O0 -o sample sample.c .

My only thought is that perhaps character sets aren't supported by gcc? I'm not sure. Just wondering if anyone else could clear this up?

Answer 1

scanf("%[abc]", &command);

writes a string not a single character. The trailing null character of the string is being written in &command + 1 in your program.

You should pass to scanf something like:

command with command being:

char command[2];