问无法从管道读取，和另一个stdin问题。

EN

#define BUFSIZE 65536  // arbitrary choice, can be tuned for performance

ssize_t nread;
char buf[BUFSIZE]; // or char *buf = malloc(BUFSIZE);
while ((nread = read(filedes, buf, BUFSIZE)) > 0) {
    ssize_t written = 0;
    while (written < nread) {
        ssize_t ret = write(STDOUT_FILENO, buf + written, nread - written);
        if (ret <= 0)
            // handle error
        written += ret;
    }
}
if (nread < 0)
    // handle error

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <getopt.h>

#define ERR(_code, _fmt, ...) do {            \
        fprintf(stderr,"%s: " _fmt, progname, \
                ##__VA_ARGS__);               \
        if (_code) exit(_code);               \
    } while (0)

char *progname = "cat";

void process(FILE *f);

int main(int argc, char **argv)
{
    int opt;
    while ((opt = getopt(argc, argv, "u")) != EOF) {
        switch (opt) {
        case 'u': setbuf(stdout, NULL); break;
        }
    }

    /* for the case it has been renamed, calculate the basename
     * of argv[0] (progname is used in the macro ERR above) */
    progname = strrchr(argv[0], '/');
    progname = progname
             ? progname + 1
             : argv[0];

    /* shift options */
    argc -= optind;
    argv += optind;

    if (argc) {
        int i;
        for (i = 0; i < argc; i++) {
            FILE *f = fopen(argv[i], "r");
            if (!f) {
                ERR(EXIT_FAILURE,
                    "%s: %s (errno = %d)\n",
                    argv[i], strerror(errno), errno);
            }
            process(f);
            fclose(f);
        }
    } else {
        process(stdin);
    }
    exit(EXIT_SUCCESS);
}

/* you don't need to complicate here, fgetc and putchar use buffering as you stated in main
 * (no output buffering if you do the setbuf(NULL) and input buffering all the time).  The buffer
 * size is best to leave stdio to calculate it, as it queries the filesystem to get the best
 * input/output size and create buffers this size. and the processing is simple with a loop like
 * the one below. You'll get no appreciable difference between this and any other input/output.
 * you can believe me, I've tested it. */
void process(FILE *f)
{
    int c;
    while ((c = fgetc(f)) != EOF) {
        putchar(c);
    }
}