Thread: Why Isent My Code Executing?

I have some code that writes to a file, if the file already exists it needs to update bytes 5-8. This is not happening tho, so I added some printfs to try and figure out whats going wrong:

Code:

int PolygonSave(Polygon p, const char* filename,  int append)
{
	FILE *file = fopen(filename, (append)? "a+b": "w+b");
	
	int num_poly = 1;;	

	if(ftell(file) < 9) //Write header info	 
	{
		fwrite("POLY", 1, 4, file);
		fwrite(&num_poly, sizeof(int), 1, file);
	}
	else // Read the number of polygons, increment & write back
	{
		fseek(file, 4, SEEK_SET);
		fread(&num_poly, sizeof(int), 1, file);
		num_poly++;
		fseek(file, 4, SEEK_SET);
		fwrite(&num_poly, sizeof(int), 1, file); // This should overwrite - hopefully
		fseek(file, 0, SEEK_END);
		printf("ISDOINGSOMETHING\n");
	}
	printf("FS: %u\n", ftell(file));	
	printf("NP: %u\n", num_poly);

Heres what I dont get: If the file exists then ftell gives the correct size, which will be >8. This means that the code in the else block should be executed; however, it never prints ISDOINGSOMETHING. I'm getting no errors or warnings with gcc -Wall -std=c99

Add some printf() statements in the if part and make sure it's not executing that part. BTW, also add a check to see if the file has been opened or not.

Ok I changed the code a little. Now it seems the problem was that ftell was returning 0 before appending to the file, but it returns the correct size after. I Still dont know how to fix it tho o_0 now I get all 0's.

Even if you seek to the start, you still end up writing to the end of the file.

That sounds like the answer, but then how can I open a file to read then append to? I cant seem to find a suitable option in the fopen manpage :/

Edit: Ignore the above - its "r+"

Its basically just a truth check. It would be the equivelant to writing:

Code:

if(append != 0)
    file = fopen(filename, "a+b");
else
    file = fopen(filename, "w+b");

For more info: http://en.wikipedia.org/wiki/%3F: