Read text file, save all digits into character string - c

Question

I am trying to read a text file containing the string "a3rm5t?7!z*&gzt9v" and put all the numeric characters into a character string to later convert into an integer.
I am currently trying to do this by using sscanf on the buffer after reading the file, and then using sprintf to save all characters found using %u in a character string called str.
However, the integer that is returning when I call printf on str is different each time I run the program. What am I doing right and what am I doing wrong?
This code works when the text file contains a string like "23dog" and returns 23 but not when the string is something like 23dog2.
EDIT: I now realize that i should be putting the numeric characters in a character ARRAY rather than just one string.
int main(int argc, const char **argv)
{
int in;
char buffer[128];
char *str;
FILE *input;
in = open(argv[1], O_RDONLY);
read(in, buffer, 128);
unsigned x;
sscanf(buffer, "%u", &x);
sprintf(str,"%u\n", x);
printf("%s\n",str);
close (in);
exit(0);
}

Answer 1

If you simply want to filter out any non-digits from your input, you need not use scanf, sprintf and the like. Simply loop over the buffer and copy the characters that are digits.
The following program only works for a single line of input read from standard input and only if it is less than 512 characters long but it should give you the correct idea.
#include <stdio.h>
#define BUFFER_SIZE 512
int
main()
{
char buffer[BUFFER_SIZE]; /* Here we read into. */
char digits[BUFFER_SIZE]; /* Here we insert the digits. */
char * pos;
size_t i = 0;
/* Read one line of input (max BUFFER_SIZE - 1 characters). */
if (!fgets(buffer, BUFFER_SIZE, stdin))
{
perror("fgets");
return 1;
}
/* Loop over the (NUL terminated) buffer. */
for (pos = buffer; *pos; ++pos)
{
if (*pos >= '0' && *pos <= '9')
{
/* It's a digit: copy it over. */
digits[i++] = *pos;
}
}
digits[i] = '\0'; /* NUL terminate the string. */
printf("%s\n", digits);
return 0;
}

Answer 2

A good approach to any problem like this is to read the entire line into a buffer and then assign a pointer to the buffer. You can then use the pointer to step through the buffer reading each character and acting on it appropriately. The following is one example of this approach. getline is used to read the line from the file (it has the advantage of allocating space for buffer and returning the number of characters read). You then allocate space for the character string based on the size of buffer as returned by getline. Remember, when done, you are responsible for freeing the memory allocated by getline.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main (int argc, const char **argv)
{
char *buffer = NULL; /* forces getline to allocate required space */
ssize_t read = 0; /* number of characters read by getline */
size_t n = 0; /* limit of characters to read, (0 no limit) */
char *str = NULL; /* string to hold digits read from file */
char *p = NULL; /* ptr to use with buffer (could use buffer) */
int idx = 0; /* index for adding digits to str */
int number = 0; /* int to hold number parsed from file */
FILE *input;
/* validate input */
if (argc < 2) { printf ("Error: insufficient input. Usage: %s filename\n", argv[0]); return 1; }
/* open and validate file */
input = fopen(argv[1], "r");
if (!input) { printf ("Error: failed to open file '%s\n", argv[1]); return 1; }
/* read line from file with getline */
if ((read = getline (&buffer, &n, input)) != -1)
{
str = malloc (sizeof (char) * read); /* allocate memory for str */
p = buffer; /* set pointer to buffer */
while (*p) /* read each char in buffer */
{
if (*p > 0x2f && *p < 0x3a) /* if char is digit 0-9 */
{
str[idx] = *p; /* copy to str at idx */
idx++; /* increment idx */
}
p++; /* increment pointer */
}
str[idx] = 0; /* null-terminate str */
number = atoi (str); /* convert str to int */
printf ("\n string : %s number : %d\n\n", buffer, number);
} else {
printf ("Error: nothing read from file '%s\n", argv[1]);
return 1;
}
if (input) fclose (input); /* close input file stream */
if (buffer) free (buffer); /* free memory allocated by getline */
if (str) free (str); /* free memory allocated to str */
return 0;
}
datafile:
$ cat dat/fwintstr.dat
a3rm5t?7!z*&gzt9v
output:
$ ./bin/prsint dat/fwintstr.dat
string : a3rm5t?7!z*&gzt9v
number : 3579