Reading data from files in C

Question

So I have a big text file with data in it and I would like to rearrange it. The data has a combination of integers and floats per each line but I'm only interested in grabbing the first integer, which is either a 1 or 0, and putting it at the end of the line.

For example, in my data file, I have the following line
1 0.41 1 44
and I would like to be
0.41 1 44 1

This is what I have so far and can't get it to work right. Thanks.

void main() {
FILE *fp;
FILE *out;

char str[15];
char temp;

fp = fopen("dust.txt", "r+");
out = fopen("dust.dat", "w");

while(fgets(str, sizeof(str), fp) != NULL) {
    temp = str[0];
    str[strlen(str)] = ' ';
    str[strlen(str)+1] = temp;
    str[strlen(str)+2] = '\r';
    str[strlen(str)+3] = '\n';

fwrite(str, 1, strlen(str), out);
}   

fclose(fp);
    fclose(out);
}

Answer 1

This treats the output as a text file (same as input), not a binary. I've put code comments where appropriate. Your most serious error was in calling strlen after overwriting the string terminator. There is only need to call it once anyway.

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

int main(void)  {                           // main must be type int
    FILE *fp;
    FILE *out;
    char str[100];                          // be generous
    size_t len;

    fp = fopen("dust.txt", "r");
    out = fopen("dust2.txt", "w");          // text file
    if(fp == NULL || out == NULL)
        return 1;

    while(fgets(str, sizeof(str)-3, fp) != NULL) {
        str [ strcspn(str, "\r\n") ] = 0;   // remove trailing newline etc
        len = strlen(str);
        str[len] = ' ';                     // overwrites terminator
        str[len+1] = str[0];                // move digit from front
        str[len+2] = 0;                     // terminate string
        fprintf(out, "%s\n", str + 2);      // write as text
    }   

    fclose(fp);
    fclose(out);
    return 0;
}

Input file:

1 0.41 1 44
0 1.23 2 555

Output file:

0.41 1 44 1
1.23 2 555 0