帮我写一段c语言在Linux下基于libssh的sftp登录和下载文件代码

好的，以下是基于libssh的sftp登录和下载文件的C语言代码，适用于Linux系统：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <libssh/libssh.h>
#include <libssh/sftp.h>

int sftp_download(ssh_session session, const char *remote_path, const char *local_path) {
    sftp_session sftp = sftp_new(session);
    if (sftp == NULL) {
        fprintf(stderr, "Error: Failed to create SFTP session.\n");
        return SSH_ERROR;
    }

    if (sftp_init(sftp) != SSH_OK) {
        fprintf(stderr, "Error: Failed to initialize SFTP session: %s.\n", ssh_get_error(session));
        sftp_free(sftp);
        return SSH_ERROR;
    }

    sftp_file remote_file = sftp_open(sftp, remote_path, O_RDONLY, 0);
    if (remote_file == NULL) {
        fprintf(stderr, "Error: Failed to open remote file: %s.\n", ssh_get_error(session));
        sftp_free(sftp);
        return SSH_ERROR;
    }

    FILE *local_file = fopen(local_path, "w");
    if (local_file == NULL) {
        fprintf(stderr, "Error: Failed to open local file: %s.\n", strerror(errno));
        sftp_close(remote_file);
        sftp_free(sftp);
        return SSH_ERROR;
    }

    char buffer[1024];
    ssize_t bytes_read;
    while ((bytes_read = sftp_read(remote_file, buffer, sizeof(buffer))) > 0) {
        fwrite(buffer, 1, bytes_read, local_file);
    }

    if (bytes_read < 0) {
        fprintf(stderr, "Error: Failed to read remote file: %s.\n", ssh_get_error(session));
        fclose(local_file);
        sftp_close(remote_file);
        sftp_free(sftp);
        return SSH_ERROR;
    }

    fclose(local_file);
    sftp_close(remote_file);
    sftp_free(sftp);

    printf("Downloaded file from %s to %s.\n", remote_path, local_path);

    return SSH_OK;
}

int main(int argc, char **argv) {
    if (argc != 5) {
        fprintf(stderr, "Usage: %s <host> <port> <user> <password>\n", argv[0]);
        return EXIT_FAILURE;
    }

    const char *host = argv[1];
    const int port = atoi(argv[2]);
    const char *user = argv[3];
    const char *password = argv[4];

    ssh_session session = ssh_new();
    if (session == NULL) {
        fprintf(stderr, "Error: Failed to create SSH session.\n");
        return EXIT_FAILURE;
    }

    ssh_options_set(session, SSH_OPTIONS_HOST, host);
    ssh_options_set(session, SSH_OPTIONS_PORT, &port);
    ssh_options_set(session, SSH_OPTIONS_USER, user);

    if (ssh_connect(session) != SSH_OK) {
        fprintf(stderr, "Error: Failed to connect: %s.\n", ssh_get_error(session));
        ssh_free(session);
        return EXIT_FAILURE;
    }

    if (ssh_userauth_password(session, NULL, password) != SSH_AUTH_SUCCESS) {
        fprintf(stderr, "Error: Failed to authenticate: %s.\n", ssh_get_error(session));
        ssh_disconnect(session);
        ssh_free(session);
        return EXIT_FAILURE;
    }

    const char *remote_path = "/path/to/remote/file";
    const char *local_path = "/path/to/local/file";

    if (sftp_download(session, remote_path, local_path) != SSH_OK) {
        ssh_disconnect(session);
        ssh_free(session);
        return EXIT_FAILURE;
    }

    ssh_disconnect(session);
    ssh_free(session);

    return EXIT_SUCCESS;
}

请注意将 `<host>`, `<port>`, `<user>` 和 `<password>` 替换为实际的值，以及将 `"/path/to/remote/file"` 和 `"/path/to/local/file"` 替换为实际的远程和本地文件路径。