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"segmentation fault: 11" with sprintf in C

Hi,

I have problems with the strings size.
My program is the following:

Code:

#include <stdio.h>

#include <stdlib.h>

#include <string.h>



int main() {



  FILE * fp;

  char input[20];

  char *str;



  printf("Enter the file name: ");

  scanf("%s", input);

  printf("The file name is %s\n", input);

  printf("Sizeof %s is %lu\n", input, sizeof(input));



  fp = fopen("file.txt", "w");

  fprintf(fp, "%s", input);

  fclose(fp);



  sprintf(str, "%s", input);

  puts(str);



  return 0;



}

Why fprintf works well whereas the sprintf causes the segmentation fault?
I noticed that changing the size of the input array from 20 to (at least) 41 solves the problem and sprintf works as well. Why?

Hi.

Quote:

Why fprintf works well whereas the sprintf causes the segmentation fault?

The problem is the str pointer. You did not initialized it to point to a valid memory area, hence segfault when sprintf() tries to write to this (invalid) memory area.

You should allocate a big enough buffer before trying to write to it:

Code:

char *str = malloc(sizeof(char)*20);

Code:

char str[20]; // static allocation

Quote:

I noticed that changing the size of the input array from 20 to (at least) 41 solves the problem and sprintf works as well. Why?

Probably a compiler/operating system quirk. On my machine increasing length of input does not prevents segfault (fortunately).

Few links: C pointers and arrays, C memory management.

It is true.
Thanks a lot!