通用
如果您正在使用Linux作为嵌入式操作系统编写打印机固件，则可能使用此驱动程序。这个驱动程序与在Linux主机系统上使用打印机没有任何关系。
您将需要一个USB设备控制器和Linux驱动程序，它接受一个使用Linux USB gadget API的gadget/“设备类”驱动程序。加载USB设备控制器驱动后，再加载printer gadget驱动。这将向USB设备端口连接的USB主机提供printer接口。
这个驱动程序是为在用户模式下运行的printer固件而设计的。用户模式printer固件将使用设备文件从内核模式printer gadget驱动程序读取和写入数据。当USB HOST发送获取打印机(printer)状态的设备请求时，打印机返回一个打印机状态字节。用户空间固件可以使用设备文件/dev/g_printer读取或写入这个状态字节。支持阻塞和非阻塞读写调用。
如何使用这个驱动程序
加载USB设备控制器驱动和打印机gadget驱动。以Netchip 2280 USB设备控制器驱动为例：
modprobe net2280
modprobe g_printer
可以在加载打印机gadget时使用如下命令行参数(例如:modprobe g_printer idVendor=0x0525 idProduct=0xa4a8)：
idVendor
这是设备描述符中使用的Vendor ID。默认是Netchip厂商id 0x0525。在发布产品之前，您必须更改为您自己的供应商id。如果您计划发布产品，但还没有供应商ID，请参阅www.usb.org了解如何获得供应商ID的详细信息。
idProduct
这是设备描述符中使用的Product ID。默认值是0xa4a8，您应该将其更改为一个不被任何其他USB产品使用的ID(如果您有任何USB产品)。从0x0001开始为产品编号是个好主意。
bcdDevice
这是您产品的版本号。把你的固件版本放在这里是个好主意。
iManufacturer
包含供应商名称的字符串
iProduct
包含产品名称的字符串。
iSerialNum
包含序列号的字符串。这应该为每个单位的产品而改变。
iPNPstring
用于此打印机的PNP ID字符串。您将希望在命令行或硬编码上设置用于打印机产品的PNP ID字符串。
qlen
每个端点要使用的8k缓冲区的数量。默认值是10，您应该针对您的产品进行优化。您可能还想为您的产品调整每个缓冲区的大小。
使用示例代码
此示例代码与标准输出对话，而不是与打印引擎对话。
编译下面的测试代码：
- 将其保存到一个名为prn_example.c的文件中
- 用下面的命令编译代码:
gcc prn_example.c -o prn_example
将打印机数据从主机读取到stdout:
# prn_example -read_data
将打印机数据从文件(data_file)写入主机:
# cat data_file | prn_example -write_data
要获取gadget驱动程序的当前打印机状态：
# prn_example -get_status Printer status is: Printer is NOT Selected Paper is Out Printer OK
将打印机设置为 Selected/On-line：
# prn_example -selected
将打印机设置为Not Selected/Off-line：
# prn_example -not_selected
将纸张状态设置为纸用完：
# prn_example -paper_out
将纸张状态设置为已载纸：
# prn_example -paper_loaded
将错误状态设置为打印机OK：
# prn_example -no_error
设置error状态为ERROR：
# prn_example -error
代码示例
#include <stdio.h> #include <stdlib.h> #include <fcntl.h> #include <linux/poll.h> #include <sys/ioctl.h> #include <linux/usb/g_printer.h> #define PRINTER_FILE "/dev/g_printer" #define BUF_SIZE 512 /* * ‘usage()‘ - Show program usage. */ static void usage(const char *option) /* I - Option string or NULL */ { if (option) { fprintf(stderr,"prn_example: Unknown option \"%s\"!\n", option); } fputs("\n", stderr); fputs("Usage: prn_example -[options]\n", stderr); fputs("Options:\n", stderr); fputs("\n", stderr); fputs("-get_status Get the current printer status.\n", stderr); fputs("-selected Set the selected status to selected.\n", stderr); fputs("-not_selected Set the selected status to NOT selected.\n", stderr); fputs("-error Set the error status to error.\n", stderr); fputs("-no_error Set the error status to NO error.\n", stderr); fputs("-paper_out Set the paper status to paper out.\n", stderr); fputs("-paper_loaded Set the paper status to paper loaded.\n", stderr); fputs("-read_data Read printer data from driver.\n", stderr); fputs("-write_data Write printer sata to driver.\n", stderr); fputs("-NB_read_data (Non-Blocking) Read printer data from driver.\n", stderr); fputs("\n\n", stderr); exit(1); } static int read_printer_data() { struct pollfd fd[1]; /* Open device file for printer gadget. */ fd[0].fd = open(PRINTER_FILE, O_RDWR); if (fd[0].fd < 0) { printf("Error %d opening %s\n", fd[0].fd, PRINTER_FILE); close(fd[0].fd); return(-1); } fd[0].events = POLLIN | POLLRDNORM; while (1) { static char buf[BUF_SIZE]; int bytes_read; int retval; /* Wait for up to 1 second for data. */ retval = poll(fd, 1, 1000); if (retval && (fd[0].revents & POLLRDNORM)) { /* Read data from printer gadget driver. */ bytes_read = read(fd[0].fd, buf, BUF_SIZE); if (bytes_read < 0) { printf("Error %d reading from %s\n", fd[0].fd, PRINTER_FILE); close(fd[0].fd); return(-1); } else if (bytes_read > 0) { /* Write data to standard OUTPUT (stdout). */ fwrite(buf, 1, bytes_read, stdout); fflush(stdout); } } } /* Close the device file. */ close(fd[0].fd); return 0; } static int write_printer_data() { struct pollfd fd[1]; /* Open device file for printer gadget. */ fd[0].fd = open (PRINTER_FILE, O_RDWR); if (fd[0].fd < 0) { printf("Error %d opening %s\n", fd[0].fd, PRINTER_FILE); close(fd[0].fd); return(-1); } fd[0].events = POLLOUT | POLLWRNORM; while (1) { int retval; static char buf[BUF_SIZE]; /* Read data from standard INPUT (stdin). */ int bytes_read = fread(buf, 1, BUF_SIZE, stdin); if (!bytes_read) { break; } while (bytes_read) { /* Wait for up to 1 second to sent data. */ retval = poll(fd, 1, 1000); /* Write data to printer gadget driver. */ if (retval && (fd[0].revents & POLLWRNORM)) { retval = write(fd[0].fd, buf, bytes_read); if (retval < 0) { printf("Error %d writing to %s\n", fd[0].fd, PRINTER_FILE); close(fd[0].fd); return(-1); } else { bytes_read -= retval; } } } } /* Wait until the data has been sent. */ fsync(fd[0].fd); /* Close the device file. */ close(fd[0].fd); return 0; } static int read_NB_printer_data() { int fd; static char buf[BUF_SIZE]; int bytes_read; /* Open device file for printer gadget. */ fd = open(PRINTER_FILE, O_RDWR|O_NONBLOCK); if (fd < 0) { printf("Error %d opening %s\n", fd, PRINTER_FILE); close(fd); return(-1); } while (1) { /* Read data from printer gadget driver. */ bytes_read = read(fd, buf, BUF_SIZE); if (bytes_read <= 0) { break; } /* Write data to standard OUTPUT (stdout). */ fwrite(buf, 1, bytes_read, stdout); fflush(stdout); } /* Close the device file. */ close(fd); return 0; } static int get_printer_status() { int retval; int fd; /* Open device file for printer gadget. */ fd = open(PRINTER_FILE, O_RDWR); if (fd < 0) { printf("Error %d opening %s\n", fd, PRINTER_FILE); close(fd); return(-1); } /* Make the IOCTL call. */ retval = ioctl(fd, GADGET_GET_PRINTER_STATUS); if (retval < 0) { fprintf(stderr, "ERROR: Failed to set printer status\n"); return(-1); } /* Close the device file. */ close(fd); return(retval); } static int set_printer_status(unsigned char buf, int clear_printer_status_bit) { int retval; int fd; retval = get_printer_status(); if (retval < 0) { fprintf(stderr, "ERROR: Failed to get printer status\n"); return(-1); } /* Open device file for printer gadget. */ fd = open(PRINTER_FILE, O_RDWR); if (fd < 0) { printf("Error %d opening %s\n", fd, PRINTER_FILE); close(fd); return(-1); } if (clear_printer_status_bit) { retval &= ~buf; } else { retval |= buf; } /* Make the IOCTL call. */ if (ioctl(fd, GADGET_SET_PRINTER_STATUS, (unsigned char)retval)) { fprintf(stderr, "ERROR: Failed to set printer status\n"); return(-1); } /* Close the device file. */ close(fd); return 0; } static int display_printer_status() { char printer_status; printer_status = get_printer_status(); if (printer_status < 0) { fprintf(stderr, "ERROR: Failed to get printer status\n"); return(-1); } printf("Printer status is:\n"); if (printer_status & PRINTER_SELECTED) { printf(" Printer is Selected\n"); } else { printf(" Printer is NOT Selected\n"); } if (printer_status & PRINTER_PAPER_EMPTY) { printf(" Paper is Out\n"); } else { printf(" Paper is Loaded\n"); } if (printer_status & PRINTER_NOT_ERROR) { printf(" Printer OK\n"); } else { printf(" Printer ERROR\n"); } return(0); } int main(int argc, char *argv[]) { int i; /* Looping var */ int retval = 0; /* No Args */ if (argc == 1) { usage(0); exit(0); } for (i = 1; i < argc && !retval; i ++) { if (argv[i][0] != ‘-‘) { continue; } if (!strcmp(argv[i], "-get_status")) { if (display_printer_status()) { retval = 1; } } else if (!strcmp(argv[i], "-paper_loaded")) { if (set_printer_status(PRINTER_PAPER_EMPTY, 1)) { retval = 1; } } else if (!strcmp(argv[i], "-paper_out")) { if (set_printer_status(PRINTER_PAPER_EMPTY, 0)) { retval = 1; } } else if (!strcmp(argv[i], "-selected")) { if (set_printer_status(PRINTER_SELECTED, 0)) { retval = 1; } } else if (!strcmp(argv[i], "-not_selected")) { if (set_printer_status(PRINTER_SELECTED, 1)) { retval = 1; } } else if (!strcmp(argv[i], "-error")) { if (set_printer_status(PRINTER_NOT_ERROR, 1)) { retval = 1; } } else if (!strcmp(argv[i], "-no_error")) { if (set_printer_status(PRINTER_NOT_ERROR, 0)) { retval = 1; } } else if (!strcmp(argv[i], "-read_data")) { if (read_printer_data()) { retval = 1; } } else if (!strcmp(argv[i], "-write_data")) { if (write_printer_data()) { retval = 1; } } else if (!strcmp(argv[i], "-NB_read_data")) { if (read_NB_printer_data()) { retval = 1; } } else { usage(argv[i]); retval = 1; } } exit(retval); }