Check whether stdin/std::cin support seeking

EDIT: I had a stupid bug in the code and fixed this, so things are explicable now. I updated my post accordingly.

The following code shows that seeking within the line buffer works.

First, the result when piping something into the program.

$ echo 'asdf' | ./seektest
stdin does not support fseek()
c == s
std::cin does not support seekg()
Second read to std::cin failed!
c == ?
Second read to std::cin failed!
c == ?

Second, I typed "a[enter]s[enter]d[enter]f[enter]".

$ ./seektest 
a
stdin supports fseek().
s
c == s
std::cin supports seekg().
d
c == d
c == 

Third, I typed "asdf" for each getc/get() call.

$ ./seektest 
asdf
stdin supports fseek().
asdf
c == a
std::cin supports seekg().
asdf
c == a
c == s

Seeking appears to work within the line buffer.

Here is the code.

#include <iostream>
#include <cstdio>

int main(int argc, char ** argv)
{
    // Try cstdio.
    int x = fgetc(stdin);
    if (x < 0) {
        fprintf(stderr, "First read to stdin failed!.");
    }
    int res = fseek(stdin, -1, SEEK_CUR);
    if (!res) {
        fprintf(stdout, "stdin supports fseek().\n");
    } else {
        fprintf(stdout, "stdin does not support fseek()\n");
    }
    x = fgetc(stdin);
    if (x < 0) {
        fprintf(stderr, "Second read to stdin failed!\n");
    }
    char c = x;
    fprintf(stdout, "c == %c\n", c);

    // Try iostream.
    x = std::cin.get();
    if (std::cin.fail()) {
        fprintf(stderr, "First read to std::cin failed!\n");
    }
  开发者_C百科  std::cin.seekg(-1, std::ios::cur);
    if (std::cin.fail()) {
        fprintf(stdout, "std::cin does not support seekg()\n");
    } else {
        fprintf(stdout, "std::cin supports seekg().\n");
    }
    c = std::cin.get();
    if (std::cin.fail()) {
        fprintf(stderr, "Second read to std::cin failed!\n");
    }
    fprintf(stdout, "c == %c\n", c);
    c = std::cin.get();
    if (std::cin.fail()) {
        fprintf(stderr, "Second read to std::cin failed!\n");
    }
    fprintf(stdout, "c == %c\n", c);

    return 0;
}

---

Looks to me like you have your condition reversed here:

if (std::cin.fail()) {
    fprintf(stdout, "std::cin supports seekg().\n");
} else {
    fprintf(stdout, "std::cin does not support seekg().\n");
}

std::cin "supports seekg()" if it failed?