passing string to scanf - c

Question

I am trying to make a program that generates a string given user input, and then passes that string to a function that will change stdin to a dummy file, write the string to the file, use scanf on said file, then delete the file, but I'm having trouble redirecting stdin to the dummy file, any help on the best action that will only extend into the scope of the function?
int scan(const char* __restrict__ _format, ...){
FILE* original = stdin, *mod = calloc(1, sizeof(FILE));
mod = freopen("testFile.txt", "w+", stdin);
fputs(_format, stdin);
int a, b;
scanf("%d %d", &a, &b);
printf("%d, %d", a, b);
// freopen(orig)
return 1;
}
void swap(char* a, char* b) {
if (*a != ' ' && *b != ' ') {
char temp = *a;
*a = *b;
*b = temp;
}
}
void permiate(char* str, int start, int end){
int i;
if(start == end){
printf("%s\n", str);
}else{
for(i = start; i<=end; i++){
swap(str+start, str + i);
permiate(str, start + 1, end);
swap(str + start, str + i);
}
}
}
int main(){
int a, b;
char str[] = "1 3";
//function to put string to stdio
scan(str);
scanf("%d %d", &a, &b);
printf("%d, %d", a, b);
return 0;
}
after someone pointed out fscanf, a function i was never aware of becasue my teacher never covered it, i have found a working solution to the scan function:
int scan(const char* __restrict__ _format, ...){
int *a = malloc(sizeof(int)), i = 0;
FILE *fp1 = fopen("testfile.txt", "w");
fputs(_format, fp1);
freopen("testFile.txt", "r", fp1);
while(fscanf(fp1, "%d", &a[i]) != EOF){
i++;
a = realloc(a, sizeof(int)*i);
}
for(int j = 0; j < i; j++){
printf("%d, ", a[j]);
}
fclose(fp1);
return 1;
}
but whenever i give str a value like "1 2 3 4 5 6 ..." or anything that has more than 5 numbers, the 5th number is always 0 if i leave realloc in, if i comment that line out, then it is fine. any idea on what that is about? ps my labs at uni only got to basic uses of arrays, no dynamic memory or anything, so if im using anything wrong it would b greatly appriciated

Answer 1

Alright, I think I understand what you are attempting to do, and it is doable, but I certainly would not recommend it. Whenever you freopen() one of the standard streams, there is no portable way to restore the original standard stream. You can remove original from your function as it will not work. Also do not allocate for mod.
With that said, you can write your scan function as:
int scan(const char* __restrict__ format)
{
int a, b;
FILE *mod = freopen ("dat/freopenab.txt", "w", stdin);
if (!mod) {
perror ("freopen-mod-write");
return 0;
}
fputs (format, stdin); /* write to file as stdin */
stdin = freopen (NULL, "r", mod); /* reopen file "r" as stdin */
if (!stdin) {
perror ("freopen-stdin-read");
return 0;
}
if (scanf("%d %d", &a, &b) != 2) { /* read values from file */
fputs ("error: read of int values failed.\n", stderr);
return 0;
}
printf("%d, %d\n", a, b); /* output resutls */
return 1;
}
stdin is reopened as file "dat/freopenab.txt" above. The format string is written to the file via the reopened stdin. Then freopen() is called again with NULL as the pathname which allows you to change the mode of the existing stream mod to read and assign the return to stdin (making stdin read from the file)
(note: variable names with leading underscores are best avoided)
A full example would be:
#include <stdio.h>
int scan(const char* __restrict__ format)
{
int a, b;
FILE *mod = freopen ("dat/freopenab.txt", "w", stdin);
if (!mod) {
perror ("freopen-mod-write");
return 0;
}
fputs (format, stdin); /* write to file as stdin */
stdin = freopen (NULL, "r", mod); /* reopen file "r" as stdin */
if (!stdin) {
perror ("freopen-stdin-read");
return 0;
}
if (scanf("%d %d", &a, &b) != 2) { /* read values from file */
fputs ("error: read of int values failed.\n", stderr);
return 0;
}
printf("%d, %d\n", a, b); /* output resutls */
return 1;
}
int main (void) {
scan ("12 34\n");
}
Example Use/Output
$ ./bin/freopenab
12, 34
Resulting Output/Input File
$ cat dat/freopenab.txt
12 34
Again, there is no portable way to restore stdin to it original state, though on Linux you can look at using "/dev/tty" or "/dev/stdin". Look things over, and give it a try. Then avoid freopen() unless there is no other way to do what you need to do in favor of just using fopen, fclose, and fopen again.