Using FILE *f = stdin; - c

Question

I want(ed) to implement POSIX tail in C language. If the input is file (filename received from argv), then I used fseek to get at the end of file, so implementation was somehow easy.
But when the input is stdin, I can't use fseek. I somehow figured out that i can do:
FILE *f = stdin;
Then I can use stdin as file and fseek, everything works as intended (with some little work :D).
My question is only, is this okay ? I mean (for me) it's kinda unusual to use stdin like that. Wouldn't be there any 'security' or another errors with this ? I tested my tail a lot and it looks like it works good even with edge cases.

Answer 1

It is indeed a weird thing to do because it doesn't help. Whether fseek works or not isn't affected by the name of the variable used as an argument.
It can succeed if the handle is for a plain file.
It can't succeed if the handle isn't for a plain file.
This is true for fseek(stdin, ...). (Code below.)
$ ./fseek_stdin <file
fghij
$ cat file | ./fseek_stdin
fseek: Illegal seek
This is true for fseek(f, ...). (Code below.)
$ ./fseek_f <file
fghij
$ cat file | ./fseek_f
fseek: Illegal seek
But there is also no harm in assigning stdin to another variable. For example, you might do
FILE *f;
if (...) {
f = stdin;
} else {
f = fopen(...);
}
Or you might do
void some_func(FILE *f) {
...
}
some_func(stdin);
These are both perfectly legitimate assignments of stdin to another variable.
Here are the files used in the earlier tests:
file:
abcdefghij
fseek_stdin.c:
#include <stdio.h>
#include <stdlib.h>
int main(void) {
if (fseek(stdin, 5, SEEK_CUR) < 0) {
perror("fseek");
return EXIT_FAILURE;
}
char *line = NULL;
size_t n = 0;
if (getline(&line, &n, stdin) < 0) {
perror("getline");
free(line);
return EXIT_FAILURE;
}
printf("%s", line);
free(line);
return EXIT_SUCCESS;
}
fseek_f.c:
#include <stdio.h>
#include <stdlib.h>
int main(void) {
FILE *f = stdin;
if (fseek(f, 5, SEEK_CUR) < 0) {
perror("fseek");
return EXIT_FAILURE;
}
char *line = NULL;
size_t n = 0;
if (getline(&line, &n, f) < 0) {
perror("getline");
free(line);
return EXIT_FAILURE;
}
printf("%s", line);
free(line);
return EXIT_SUCCESS;
}
A diff of the two programs (slightly massaged for readability):
$ diff -y fseek_{stdin,f}.c
#include <stdio.h> #include <stdio.h>
#include <stdlib.h> #include <stdlib.h>
int main(void) { int main(void) {
> FILE *f = stdin;
>
if (fseek(stdin, 5, SEEK_CUR) < 0) { | if (fseek(f, 5, SEEK_CUR) < 0) {
perror("fseek"); perror("fseek");
return EXIT_FAILURE; return EXIT_FAILURE;
} }
char *line = NULL; char *line = NULL;
size_t n = 0; size_t n = 0;
if (getline(&line, &n, stdin) < 0) { | if (getline(&line, &n, f) < 0) {
perror("getline"); perror("getline");
free(line); free(line);
return EXIT_FAILURE; return EXIT_FAILURE;
} }
printf("%s", line); printf("%s", line);
free(line); free(line);
return EXIT_SUCCESS; return EXIT_SUCCESS;
} }