The feof trap in C

feof reports that a read already failed, so testing it first reads one line too many.

#include <stdio.h>

/* Wrong: feof is only true after a read has failed. */
int overcounts(FILE *f) {
    char line[128];
    int n = 0;
    while (!feof(f)) {
        fgets(line, (int) sizeof line, f);
        n++;
    }
    return n;
}

/* Right: the read result is the loop condition. */
int counts(FILE *f) {
    char line[128];
    int n = 0;
    while (fgets(line, (int) sizeof line, f) != NULL) {
        n++;
    }
    return ferror(f) ? -1 : n;
}

How it works

1. while (!feof(f)) processes the last line twice.
2. The fix is to loop on the read call's own result.
3. ferror afterwards distinguishes a real error from the end.

Keywords and builtins used here

• FILE
• NULL
• char
• counts
• int
• overcounts
• return
• sizeof
• while

The run, in numbers

Lines

22

Characters to type

391

Tokens

109

Three-star pace

95 tpm

At the three-star pace of 95 tokens a minute, this run takes about 69 seconds.

Type this snippet

Step 3 of 4 in Opening & reading, step 3 of 12 in Files & I/O.

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