Using fseek with stdout
Can fseek be used when the arg file pointer is stdout?
I tried this in a script where I was trying to overwrite the last "," in a comma separated list with ")"
eg.
fprintf(stdout, "1,2,3,4,");
fseek(stdout, -1, SEEK_CUR);
fprintf(stdout, ")");
for achievement:
1,2,3,4)
Unlike
1,2,3,4)
Unfortunately, I was outputting the last
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You can successfully use fseek
in stdout
if and only if the stdout
stream is referenced for searching.
If your program's standard output goes to a terminal or something similar (usually the default) or a pipe, it fseek
will fail. If it goes to a file because you ran a program with redirected output, then it fseek
may work the same as for any other file.
Each time you call fseek
, you must check its return value to see if it succeeds or fails.
It stdout
is probably bad practice to assume what can be found because it is a very bad guess.
Your goal is to print a sequence of elements with ", "
between elements, but not before the first or after the last. Neither fseek
printing nor printing '\b'
is a good approach to what you are trying to do. Instead of printing the final comma and then trying to erase it, you should only print the comma if you haven't just printed the last item in the list. Finding this out can be nontrivial.
But a simpler approach is to put a comma before each element, unless it's the first. You cannot always tell if you are printing the last value of the sequence. You can always tell if you are typing first.
Pseudo-code:
bool is_first_item = true
for each item
if is_first_item
is_first_item = false
else
print ", "
end if
print item
end for
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Using fseek
on is stdout
not possible. Instead, you can use the backspace character '\b'
to move the cursor back one space:
fprintf(stdout, "1,2,3,4,");
fprintf(stdout, "\b)");
Note: this will only work if it stdout
points to a terminal and does not redirect to a file or pipe. Also, if the cursor is at the beginning of a line, this is not guaranteed. In general, there are only a few cases where this will work.
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