如何修复Windows下C文件复制程序的"copy: Bad file descriptor"错误？

Let's walk through the issues in your code that are causing that frustrating error, plus fix other bits to meet your original requirements:

1. Operator Precedence Bug (Main Cause of the Error) #

This is the root of your "Bad file descriptor" problem. Look at this line in your source file loop:

if (fds = _open(argv[i], _O_RDONLY) < 0)

The < operator has higher precedence than =, so this actually evaluates to:

fds = (_open(...) < 0)

That means fds gets assigned a boolean value (0 or 1) instead of the valid file descriptor from _open. When you try to read from this invalid descriptor, the error is thrown.

Fix: Wrap the assignment in parentheses to force the correct order:

if ((fds = _open(argv[i], _O_RDONLY)) < 0)

2. Reversed Parameters in copy_file Call #

Your copy_file function expects the destination file descriptor first, then the source:

int copy_file(int fdd, int fds) { // fdd = destination, fds = source
    while ((n = read(fds, buf, BUFSIZE)) > 0) {
        _write(fdd, buf, n);
    }
}

But in your loop, you're passing the source first and destination second:

if (copy_file(fds, fdd) < 0)

This swaps your read/write targets, which would cause errors even if the descriptor was valid.

Fix: Swap the parameters in the call:

if (copy_file(fdd, fds) < 0)

3. Loop Starts at the Wrong Index #

Your original loop starts at i=0, which points to argv[0]—the program's own executable (mycp.exe). You didn't intend to copy the executable itself, so this is a wasteful (and potentially confusing) mistake.

Fix: Start the loop at i=1, since source files begin at argv[1]:

for (i = 1; i < dest_idx; ++i) // dest_idx = argc - 1 (destination file index)

4. Incorrect Handling of Missing Source Files #

Your requirement says if a source file doesn't exist, the program should print a warning and continue—not exit. Right now, any failed _open triggers an immediate exit.

Fix: Check the errno value when _open fails. If it's ENOENT (file not found), print a warning and skip to the next file. For other errors, exit as intended:

if ((fds = _open(argv[i], _O_RDONLY)) < 0) {
    if (errno == ENOENT) {
        fprintf(stderr, "Warning: File %s does not exist, skipping.\n", argv[i]);
        continue;
    } else {
        remove_dest_file(fdd, argv, dest_idx);
        exit_sys("open source file");
    }
}

5. Minor Polish & Robustness #

• Check _write return value: Your copy_file function didn't verify if writes succeeded. Add a check to catch write failures:

  while ((n = read(fds, buf, BUFSIZE)) > 0) {
      if (_write(fdd, buf, n) != n) {
          return -1; // Indicate write failure
      }
  }
  

• Fix typo: printf("Succes"); → printf("Success!\n");
• Clarify usage message: Update the error message to reflect correct syntax more clearly.

Fixed Full Code #

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>

#define BUFSIZE 1024

void exit_sys(const char* msg) {
    perror(msg);
    exit(EXIT_FAILURE);
}

void exit_fail(const char* msg) {
    fprintf(stderr, "%s\n", msg);
    exit(EXIT_FAILURE);
}

void remove_dest_file(int fd, char** argv, int dest_idx) {
    _close(fd);
    unlink(argv[dest_idx]);
}

int copy_file(int fdd, int fds) {
    char buf[BUFSIZE];
    int n;
    while ((n = read(fds, buf, BUFSIZE)) > 0) {
        if (_write(fdd, buf, n) != n) {
            return -1;
        }
    }
    return n < 0 ? -1 : 0;
}

int main(int argc, char** argv) {
    int fds, fdd;
    int ch;
    int flags;
    int dest_idx;
    int i;

    flags = _O_WRONLY | _O_CREAT;
    if (argc < 3) {
        exit_fail("usage: mycp.exe source1.txt [source2.txt ...] dest.txt");
    }

    dest_idx = argc - 1;
    if (!access(argv[dest_idx], F_OK)) {
        printf("The file %s exists. Do you want to overwrite? [y/N]\n", argv[dest_idx]);
        ch = getchar();
        if (ch == 'y' || ch == 'Y') {
            flags |= _O_TRUNC;
        } else {
            exit(EXIT_SUCCESS);
        }
    }

    if ((fdd = _open(argv[dest_idx], flags, _S_IREAD | _S_IWRITE)) < 0) {
        exit_sys("open destination file");
    }

    for (i = 1; i < dest_idx; ++i) {
        if ((fds = _open(argv[i], _O_RDONLY)) < 0) {
            if (errno == ENOENT) {
                fprintf(stderr, "Warning: File %s not found, skipping.\n", argv[i]);
                continue;
            } else {
                remove_dest_file(fdd, argv, dest_idx);
                exit_sys("open source file");
            }
        }

        if (copy_file(fdd, fds) < 0) {
            remove_dest_file(fdd, argv, dest_idx);
            exit_sys("copy file");
        }

        _close(fds);
    }

    printf("Success!\n");
    _close(fdd);
    return 0;
}

内容的提问来源于stack exchange，提问作者user13575644