FIO13-C. Never push back anything other than one read character

Section 7.19.7.11 of C99 defines ungetc() as follows: [ISO/IEC 9899:1999]

The ungetc function pushes the character specified by c (converted to an unsigned char) back onto the input stream pointed to by stream. Pushed-back characters will be returned by subsequent reads on that stream in the reverse order of their pushing. A successful intervening call (with the stream pointed to by stream) to a file positioning function (fseek, fsetpos, or rewind) discards any pushed-back characters for the stream. The external storage corresponding to the stream is unchanged.

One character of pushback is guaranteed.

Consequently, multiple calls to ungetc() on the same stream must be separated by a call to a read function or a file-positioning function (which will discard any data pushed by ungetc()).

Likewise, for ungetwc() C99 only guarantees one wide character of pushback (section 7.24.3.10). Consequently, multiple calls to ungetwc() on the same stream must be separated by a call to a read function or a file-positioning function (which will discard any data pushed by ungetwc()).

Noncompliant Code Example

In this noncompliant code example, more than one character is pushed back on the stream referenced by fp.

FILE *fp;
char *file_name;

/* initialize file_name */

fp = fopen(file_name, "rb");
if (fp == NULL) {
  /* Handle error */
}

/* read data */

if (ungetc('\n', fp) == EOF) {
  /* Handle error */
}
if (ungetc('\r', fp) == EOF) {
  /* Handle error */
}

/* continue on */

Compliant Solution

If more than one character needs to be pushed by ungetc(), then fgetpos() and fsetpos() should be used before and after reading the data instead of pushing it back with ungetc(). Note that this solution only applies if the input is seekable.

FILE *fp;
fpos_t pos;
char *file_name;

/* initialize file_name */

fp = fopen(file_name, "rb");
if (fp == NULL) {
  /* Handle error */
}

/* read data */

if (fgetpos(fp, &pos)) {
  /* Handle error */
}

/* read the data that will be "pushed back" */

if (fsetpos(fp, &pos)) {
  /* Handle error */
}

/* Continue on */

Remember to always call fgetpos() before fsetpos() (see FIO44-C. Only use values for fsetpos() that are returned from fgetpos()).

Risk Assessment

If used improperly, ungetc() and ungetwc() can cause data to be truncated or lost.

Automated Detection

Compass/ROSE can detect simple violations of this recommendation. In particular, it warns when two calls to ungetc() on the same stream are not interspersed with a file positioning or file read function. It is unable to handle cases where ungetc() is called from inside a loop.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Bibliography

[ISO/IEC 9899:1999] Section 7.19.7.11, "The ungetc function"

FIO12-C. Prefer setvbuf() to setbuf()      09. Input Output (FIO)