C issue 0130: When is data read from a text stream guaranteed to match what was written to the stream?

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Sheng Yu, WG14
Date: 1993-12-03
Submitted against: C90
Status: Closed
Converted from: dr.htm, dr_130.html

Writing on a text stream might not cause characters to be overwritten exactly one for one, especially on fixed-length record based file systems. If the file is not truncated beyond the point where the data is written, there is no sure way to predict what will be read in after writing in the middle of a text stream because the data might just replace a character, a line, etc. Consider the following example:

#include <stdio.h>
 #include <string.h>
 int buf[99];
 unsigned int len;
 int main()
        {
        FILE *f = fopen("test data", "w");
        fwrite("abc\ndef\n", 8, 1, f);
        fseek(f, 0, SEEK_SET);
        fwrite("UWXYZ", 5, 1, f);
        fseek(f, 0, SEEK_SET);
        len = fread(buf, 1, 10, f);
        if (len == 8 && !memcmp(buf, "UWXYZef\n"))
                ;       /* Case 1: OK, acts like binary */
        else if (len == 5 && !memcmp(buf, "UWXYZ", 5))
                ;       /* Case 2: OK to truncate after write */
        else if (len > 5 && !memcmp(buf, "UWXYZ", 5))
                printf("len = %u, buf = %s\n", len, buf);
                        /* Case 3: Is this nonstandard? */
        else
                printf("This is obviously nonstandard.\n");
        }

Can a conforming implementation translate the above program and produce the following output (Case 3)?

len = 9, buf = UWXYZdef

Response

Yes, a conforming implementation may produce the “Case 3” output. However, there may be cases in some conforming implementations in addition to those shown in your example, so the printout “obviously nonstandard” may be inappropriate.

Issue 0130: When is data read from a text stream guaranteed to match what was written to the stream?

Response