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.
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+");
}
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("%255s", 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;
}
}
[...]
}