Check if file exists, otherwise create a new one-CodePudding

I'm trying to create a function that checks if a file exists, if it does then it will open it up in order to update it, and if it doesn't exist it will create a new one.

void readFunc(void) {
  FILE *input;
  char filename[N];

  printf("Write textfile: ");
  scanf("%s", filename);

  if(input == NULL) {
    printf("\nFile doesn't exist. Creating a new one!\n\n");
    input = fopen(filename, "w ");

  } else {
    printf("\nFile exists!\n\n");
    input = fopen(filename, "r ");
  } 
}

This is what my code looks like now. I know it might be very wrong but I would like to know how to think in order for it to work.

I will insert pointers later in order for it to work well with the main function.

CodePudding user response：

Because of TOCTOU races, the best way to do this is to drop down a level and use open, which, unlike fopen, lets you specify "open this file for read and write, preserving existing contents, and create it if it doesn't already exist":

FILE *ensure_file(const char *fname) 
{
   int fd = open(fname, O_RDWR | O_CREAT, 0666);
   if (fd == -1) {
       perror(fname);
       return 0;
   }
   return fdopen(fd, "r ");
}

CodePudding user response：

The code is almost correct: you should for try with "r ", then "w if the file does not exist:

#include <errno.h>
#include <stdio.h>

void readFunc(void) {
    FILE *input;
    char filename[256];

    printf("Write textfile: ");
    if (scanf("%5s", filename) != 1)
        exit(1);

    input = fopen(filename, "r ");
    if (input != NULL) {
        printf("\nFile exists\n");
    } else {
        printf("\nFile doesn't exist. Creating a new one!\n\n");
        input = fopen(filename, "w ");
        if (input == NULL) {
            printf("Cannot create file: %s\n", strerror(errno));
            return;
        }
    }
    [...]
}