"segmemtaion fault" in C

Question

I'am beginner in C-programming and when trying to run this code - the result is "segmentation fault",  I suspect that GDB online compiler can not use "files" in C -  is that correct?

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#include <stdio.h>

int main () {
   FILE *fp;
   char str[80];
   
  //Open file
    if((fp = fopen("myfile.txt", "w")==NULL)){
        printf("Cannot open file\n");
        exit(1);
    }

  //writing to file
   fprintf("This is a system programming language.", fp);
   

   if((fp = fopen("myfile.txt", "r")==NULL)){
       printf("Cannot open file\n");
       exit(1);
   }

   fgets(str, 80, fp);
   printf("%s\n", str);

   fclose(fp);
   return(0);
}

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Answer 1

ok, got it, now it's right and its works!

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

int main () {
   FILE *fp;
   char str[80];
   
  //Open file
   fp = fopen("myfile.txt", "w");

  //writing to file
    fprintf(fp, "This is a system programming language.");
   

    fp = freopen("myfile.txt", "r", fp);

   fgets(str, 80, fp);
   
   printf("%s\n", str);

   fclose(fp);
   
   return(0);
}

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Answer 2

bye bro take care