grep that preserves the header line

hgrep.c

/*
 * This is free and unencumbered software released into the public domain.
 * 
 * Anyone is free to copy, modify, publish, use, compile, sell, or
 * distribute this software, either in source code form or as a compiled
 * binary, for any purpose, commercial or non-commercial, and by any
 * means.
 * 
 * In jurisdictions that recognize copyright laws, the author or authors
 * of this software dedicate any and all copyright interest in the
 * software to the public domain. We make this dedication for the benefit
 * of the public at large and to the detriment of our heirs and
 * successors. We intend this dedication to be an overt act of
 * relinquishment in perpetuity of all present and future rights to this
 * software under copyright law.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 * IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
 * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 * 
 * For more information, please refer to <http://unlicense.org/>
 */

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

#define MAX_DATA 4096

int main(int argc, char* argv[])
{
        // Unset FD_CLOEXEC
        fcntl(STDIN_FILENO, F_SETFD, fcntl(STDIN_FILENO, F_GETFD) & (~FD_CLOEXEC));
        fcntl(STDOUT_FILENO, F_SETFD, fcntl(STDOUT_FILENO, F_GETFD) & (~FD_CLOEXEC));
        fcntl(STDERR_FILENO, F_SETFD, fcntl(STDERR_FILENO, F_GETFD) & (~FD_CLOEXEC));

        // Disable buffering, print first line, enable line buffering
        setvbuf(stdin, NULL, _IONBF, 0);
        char buffer[MAX_DATA];
        if (fgets(buffer, MAX_DATA, stdin)) {
                fputs(buffer, stdout);
        }
        else {
                exit(1);
        }
        setlinebuf(stdin);

        // Flush stdout, exec grep
        fflush(stdout);
        return execvp("grep", argv);
}