Section Subclause 7.21.7.10 of the C Standard [ISO/IEC 9899:2011] defines ungetc() as follows:
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Likewise, for ungetwc(), C guarantees only one wide character of pushback (Section subclause 7.29.3.10). 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()).
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| Code Block | ||||
|---|---|---|---|---|
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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 */
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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 applies only if the input is seekable.
| Code Block | ||||
|---|---|---|---|---|
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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 */
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Remember to always call fgetpos() before fsetpos(). (See FIO44-C. Only use values for fsetpos() that are returned from fgetpos().)
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If used improperly, ungetc() and ungetwc() can cause data to be truncated or lost.
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
FIO13-C |
Medium |
Probable |
High | P4 | L3 |
Automated Detection
Tool | Version | Checker | Description | ||||||
|---|---|---|---|---|---|---|---|---|---|
| CodeSonar |
| (customization) | Users can implement a custom check that triggers a warning when ungetc() is called twice on the same stream without an intervening call to a read function or a file-positioning function. | ||||||
| Compass/ROSE |
Can detect simple violations of this recommendation. In particular, it warns when two calls to | ||||||||
| LDRA tool suite |
| 83 D |
Partially implemented | |||||||||
| PC-lint Plus |
| 2470 | Fully supported |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Bibliography
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