fgets（）不保存c的第一行

Question

I am trying to remove a line from a file at load time.

Printing data in data_f works fine but when I use fseek to move pointer to the beginning of file and scan a line, it scans the last line and stores in s.

#include<stdio.h>
#include<stdlib.h>
int main(int argc, char **argv)
{
    FILE *fp1 = fopen("data_f", "w");
    FILE *fp = fopen(argv[1], "r");
    char s[100];
    int c=1, chk = atoi(argv[2]);

    while(fgets(s, 100, fp))  //counting number of lines
    {
        if(c != chk)
            fputs(s, fp1);
        c++;
    }
    fseek(fp1, 0, SEEK_SET);
    fgets(s, 100, fp1);
    printf("%s\n", s); 
}

I was expecting fgets to store first line of data_f but it stores last line, why?

Answer 1

You are trying to read from a file that was opened in write only mode.

FILE *fp1 = fopen("data_f", "w");
.
.  
.
fgets(s, 100, fp1);

Open the file in "w+" mode to allow both read&write.

You can read more on fopen(3) here: https://linux.die.net/man/3/fopen