Thursday, December 3, 2015

Feeling More Stupid Than Usual

I need to write % into a file in C.  I tried putting %% in the format control string.  Did no\t work.  I tried %c with '%' in the data list.  Nope.  I tried fputs("%\n", fileHandle);  Nope.


Jay Kominek said...

Those all sound like they should do the job. Can't be sure what's going on without a complete chunk of code, but the first thing I wonder is if you're flushing/closing the file before checking to see if your single additional byte is there. (The stdio stuff, which you're using, does lots of buffering.)

hga said...

Memory + glibc man page on Ubuntu Trusty + Harbison and Steele all say %% is supposed to do the trick.

Rick C said...



void main()
FILE *fp = fopen("test.txt", "w");
fprintf(fp, "xx%%");

worked for me--test.txt was created with 'xx%' in it.

Clayton Cramer said...

Rick: I think you have it; need to fflush.

SJ said...

That is odd.

Inside printf/sprintf/fprintf, the "%%" in the format control string should be seen by the app as a control code for a "%" character in output.

I've never used fputs/puts, but they are supposed to allow single "%" characters in many situations.

Here's a thought: are you constructing the string in one function call, and then later running the result through another function that will also try to interpret the "%" character?

Something like this:
int pct_val = 1; // assumed for example
sprintf(array, "percent is %d%%", pct_val); // "percent is 1%" in array
fprintf(file, array); // "percent is 1%" passed to fprintf...and the "%" char disappears in output

Clayton Cramer said...

Actually the code was right. I had a makefile that rebuilt the binary, but didn't run the binary, so my output files weren't being recreated. Dumb.