Question

After opening a file in append update mode, is it necessary to execute a file positioning statement before each write to the file?

FILE *h;
int ch;
if ((h = fopen("data", "a+")) == NULL) exit(1);
if (fseek(h, 0 SEEK_SET)) exit(2);
ch = fgetc(h); /* read very first character */
if (ch == EOF) exit(3);

/* redundant? mandatory? */
fseek(h, 0, SEEK_END); /* call file positioning before output */

/* add 1st character to the end of file on a single line*/
fprintf(h, "%c\n", ch);

The C11 Standard says:

7.21.5.3/6 … all subsequent writes to the file to be forced to the then current end-of-file …

and

7.21.5.3/7 … input shall not be directly followed by output without an
intervening call to a file positioning function …

I take it the shall in 7.21.5.3/7 is stronger than the description in 7.21.5.3/6.

Answer 1

Probably not redundant in portable C. While the underlying file descriptor will always append (at least on Unix), the point of the fseek/fflush requirement is to get rid of the input buffer before writing to the output, so that the same buffer can be used for reading and writing. AFAIK you’re not even required to seek to end of file, you can seek anywhere, as long as you seek.

The second description is stronger than the first, but that is to be expected. The first only states that all writes go to EOF, i.e. that there’s no way to write anywhere else. The second establishes the rule that switching from reading to writing must be accompanied by a flush or seek, to ensure that read and write aspects of the buffer don’t get mixed up.