FIO44-C. Only use values for fsetpos() that are returned from fgetpos()

The C Standard, 7.23.9.3 paragraph 2 [ISO/IEC 9899:2024], defines the following behavior for fsetpos():

The fsetpos function sets the mbstate_t object (if any) and file position indicator for the stream pointed to by stream according to the value of the object pointed to by pos, which shall be a value obtained from an earlier successful call to the fgetpos function on a stream associated with the same file.

Invoking the fsetpos() function with any other values for pos is undefined behavior.

Noncompliant Code Example

This noncompliant code example attempts to read three values from a file and then set the file position pointer back to the beginning of the file:

#include <stdio.h>
#include <string.h>
 
int opener(FILE *file) {
  int rc;
  fpos_t offset;

  memset(&offset, 0, sizeof(offset));

  if (file == NULL) { 
    return -1;
  }

  /* Read in data from file */

  rc = fsetpos(file, &offset);
  if (rc != 0 ) {
    return rc;
  }

  return 0;
}

Only the return value of an fgetpos() call is a valid argument to fsetpos(); passing a value of type fpos_t that was created in any other way is undefined behavior.

Compliant Solution

In this compliant solution, the initial file position indicator is stored by first calling fgetpos(), which is used to restore the state to the beginning of the file in the later call to fsetpos():

#include <stdio.h>
#include <string.h>
 
int opener(FILE *file) {
  int rc;
  fpos_t offset;

  if (file == NULL) {
    return -1;
  }

  rc = fgetpos(file, &offset);
  if (rc != 0 ) {
    return rc;
  }

  /* Read in data from file */

  rc = fsetpos(file, &offset);
  if (rc != 0 ) {
    return rc;
  }

  return 0;
}

Risk Assessment

Misuse of the fsetpos() function can position a file position indicator to an unintended location in the file.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

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Bibliography