Input and output are mapped into logical data streams, whose properties are more uniform than their various inputs and outputs. Two forms of mapping are supported, for text streams and for binary streams [C99]. They differ in the actual representation of data as well as in the functionality of some C99 functions.
Text streams
Representation
Characters may have to be altered to conform to differing conventions for representing text in the host environment. As a consequence, data read/written to or from a text stream will not necessarily compare equal to the stream's byte content.
The following code opens the file myfile as a text stream:
FILE *file = fopen("myfile", "w");
/* Check for errors */
fputs("\n", file);
Some architectures might model line breaks differently. For example, on Windows, the above code will write two bytes (a carriage return and then a newline) to the file, whereas on *nix systems, it will only write one byte (a newline).
ungetc()
The ungetc() function causes the file position indicator to be "unspecified" until all pushed back characters are read therefore, care must be taken that file position related functions are not used while this is true.
Binary streams
Representation
A binary stream is an ordered sequence of characters that can transparently record internal data. As a consequence, data read/written to or from a binary stream will necessarily compare equal to the stream's byte content.
The following code opens the file myfile as a binary stream:
FILE *file = fopen("myfile", "wb");
/* Check for errors */
fputs("\n", file);
Regardless of architecture, this code will write exactly one byte (a newline).
ungetc()
The ungetc() function causes the file position indicator to be decremented by one for each successful call, with the value being indeterminate if it is zero before any call. Therefore, it must never be called on a binary stream where the file position indicator is zero.
Risk Assessment
Failure to understand file stream mappings can result in unexpectedly formatted files.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
FIO07-A |
1 (low) |
2 (probable) |
2 (medium) |
P4 |
L3 |
References
[[ISO/IEC 9899-1999:TC2]] Section 7.19.2, "Streams"