Why is fread() not working, but fwrite() works fine?

Question

I am working with an NXP microcontroller, which has stdio functionality when connected to the debugger. I've had no issues using fwrite() to write data to a binary file, so that I can monitor and plot values with an external program. I'm now trying to go the other way, but I can't get fread() to work. I've reduced the code to the simplest example I can think of, and it still doesn't read the file:

#include <stdio.h>
int main (void)
{
 FILE *fp;
 int16_t writeData[5] = {1, 2, 3, 4, 5};
 int16_t readData[5] = {0, 0, 0, 0, 0};
 int16_t nData;
 // write to file
 fp = fopen("testFile.dat", "wb");
 nData = fwrite(&writeData, sizeof(uint16_t), 5, fp);
 fclose(fp);
 // read from file
 fp = fopen("testFile.dat", "rb");
 nData = fread(&readData, sizeof(uint16_t), 5, fp);
 fclose(fp);
}

I'm stepping through the code line-by-line. after writing the file, nData = 5 and the file contents are as expected "01 00 02 00 03 00 04 00 05 00" (observed using Windows PowerShell command format-hex "testFile.dat"). After reading the file, nData = 0 and readData is not changed.

I've checked if (fp = NULL) after calling fopen(), that's not the issue. I've tried calling ferror(fp) after each line in the read section, it always returns 0. I've also had no luck with the agruments readData, &readData[0], and readData[0] (doesn't make sense/seem necessary respectively, but I've seen these variations in some examples). The "required supporting OS subroutines" are the same for both fread() and fwrite(), so that's not the issue either.

Any suggestions?

• Mark Ransom: Why the mismatch between the type used in the array and the sizeof? Wouldn't expect it to make a difference, but could lead to UB...
• Barmar: You don't need & before readData or writeData. Arrays automatically decay to pointers when used as function arguments.
• Eugene Sh.: It sounds like you are using semihosting functionality. It is possible that only the output part is implemented by the library.
• Shane Allen: @EugeneSh. You are correct, I am using semihosting. This was my fear as well, perhaps it's not supported. As far as I can tell fread is implemented, it's at least declared in stdio.h and has doxygen documentation when I mouse over it in my IDE (MCUXpresso). I'll take a look for any programs that use it in the NXP-provided example code for my development board to be sure.
• Eugene Sh.: You could try reading from stdin for starters

Answer 1

• To be sure the correct byte size is used:

try:

(sizeof(int16_t)

instead of

sizeof(uint16_t))

• return values should be checked.
• Error checking is also a good practice.

• Shane Allen: Good eye, I didn't notice this. Unfortunately changing the type in sizeof (or changing the types of the arrays for that matter) doesn't fix the issue.
• Alexandre Vinçon: Well, since int16_t and uint16_t are both 16-bits integer types, it won't make any difference on the outcome.