CODEWITHSUNDEEP
cunistd.h
Poptart
Poptart

Reputation: 341

Unistd read() maximum size

In the following snippet, no matter how long of an input I put in (EDIT: I'm copy and pasting in a random string), say a string with 9998 characters, read() stops when i = 4095. It states it read in an EOF character, but my string does not have an EOF character (for example I tried a string of 9998 'a's). The return value also suggests there is no error from read(). Why does read() only read in only 4095 bytes?

#include <unistd.h>
#include <stdio.h>

int main() {
  char temp;
  char buf[10000];
  int i = 0;
  while(read(STDIN_FILENO, &temp, 1) > 0) {
    buf[i] = temp;
    i++;
  }
  printf("%d\n", i);
}

Edit: To clarify, read() doesn't literally state that it read in an EOF character, per https://linux.die.net/man/2/read read() returns 0 when it moves past the EOF.

Upvotes: 3

Views: 996

Answers (1)

Chris Dodd
Chris Dodd

Reputation: 126213

You're most likely seeing the terminal buffer limit -- terminals can only read a limited number of characters on a single line, and if you type in more than that (or simulate typing with a pseudo-terminal or cut-n-paste) without entering an NL, EOL, or EOL2 character, you'll get an error, which the terminal indicates with an EOF (read returning 0).

You can generally avoid this problem by putting the terminal into non-canonical mode (where it doesn't try to buffer lines to allow backspacing).

Upvotes: 2

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