Fprintf不会将数组写入文件

Question

I have a code which should read an array, write it to a binary and to a text file, then print the files. However, the fprintf function returns an error and i have no idea why. This is my code :

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

int main()
{ 
    FILE *f,*b;

    if (f=fopen("fis.txt","w+")==NULL) { 
        printf ("Error\n");
        exit(1);
    }
    if(b=fopen("binar.txt","w+b")==NULL) { 
        printf ("Error\n");
        exit(1);
    }

    float *v; int n;
    char s[1000];
    scanf("%d",&n);
    v=malloc(n*sizeof(float)); 

    int x,y;
    for (int i=0;i<=n;i++) { 
        scanf("%f",&v[i]);
        printf("%f",v[i]);

        x=fprintf(f,"%f",v[i]); 
        if (x<0) printf("err\n");

        y=fprintf(b,"%f",v[i]); 
        if (y<0) printf ("err2\n");
    }

    fgets(s,sizeof(s),f); 
    puts(s); 
    printf("\n");

    fgets(s,sizeof(s),b);
    puts(s);
    printf("\n");

    free(v);
    fclose(f);
    fclose(b);
}

Answer 1

The main issue is how you're opening the files:

if (f=fopen("fis.txt","w+")==NULL) { 

The equality operator == has higher precedence than the assignment operator = . So first the result of fopen is compared to NULL, then the result of that comparison, ie either 0 or 1, is assigned to f . So f doesn't point to a valid location, and that is why your fprintf calls fail. If you have warnings turned up on your compiler, it should have warned about assigning an integer to a pointer.

Add parenthesis to get the proper ordering:

if ((f=fopen("fis.txt","w+"))==NULL) {

And:

if ((b=fopen("binar.txt","w+b"))==NULL) { 

Also, your loop condition is incorrect:

for (int i=0;i<=n;i++) { 

The array v has n elements, meaning its indexes go from 0 to n-1 , but you loop from 0 to n . Change the loop condition to account for this:

for (int i=0;i<n;i++) { 

You also need to call rewind on each file descriptor before reading back from them so that you can read what you just wrote:

rewind(f);
fgets(s,sizeof(s),f); 
puts(s); 
printf("\n");

rewind(b);
fgets(s,sizeof(s),b);
puts(s);
printf("\n");