I have the following program that uses the ftd2xx library to write a byte to a USB device and then reads the response.
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdarg.h>
#include "../ftd2xx.h"
void fatal (const char *format,...) {
va_list argp;
fprintf(stderr, "FATAL: ");
va_start(argp, format);
vfprintf(stderr, format, argp);
va_end(argp);
fprintf(stderr, "\n");
exit(3);
}
int main(int argc, char* argv[]) {
int pid = fork();
if (pid > 0) {
FT_STATUS ftStatus;
printf("before FT_OpenEx\n");
FT_HANDLE ftHandle;
if ((ftStatus = FT_OpenEx("DA011SCV", FT_OPEN_BY_SERIAL_NUMBER, &ftHandle)) != FT_OK) fatal("FT_OpenEx failed");
printf("before FT_Write\n");
uint8_t buffer[1];
buffer[0] = 0x55;
DWORD bytesWritten;
if ((ftStatus = FT_Write(ftHandle, buffer, sizeof(buffer), &bytesWritten)) != FT_OK) fatal("FT_Write failed");
printf("before FT_Read\n");
DWORD bytesRead;
if ((ftStatus = FT_Read(ftHandle, buffer, 1, &bytesRead)) != FT_OK) fatal("FT_Read failed");
if (bytesRead > 0) {
printf("FT_Read data=0x%02X\n", buffer[0]);
} else {
printf("FT_Read no data\n");
}
printf("before FT_Close\n");
if ((ftStatus = FT_Close(ftHandle)) != FT_OK) fatal("FT_Close failed");
printf("press Enter to exit\n");
}
getchar();
exit(0);
}
The code shown below outputs this result:
//Output if (pid > 0)
before FT_OpenEx
before FT_Write
before FT_Read
FT_Read data=0x55
before FT_Close
press Enter to exit
However, if I change the condition of the first iffrom (pid > 0)to (pid == 0), i.e. if I do USB communication in a child process, the program freezes in a function FT_Read()and the output is:
//Output if (pid == 0)
before FT_OpenEx
before FT_Write
before FT_Read
Why is this happening?
Some information:
- The USB chip in the device is an FT240X with factory settings.
- The USB device acts like an echo: every byte it receives is immediately sent back.
- I checked with the protocol analyzer the correct values for the transmitted bytes.
- The version of the ftd2xx library is 1.1.12.