Page History

Wiki MarkupSection The C Standard, 7.1923.9.3 of C99 defines the following behavior for {{paragraph 2 [ISO/IEC 9899:2024], defines the following behavior for fsetpos()}}: \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]

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.

Consequently, using Invoking the fsetpos() function with any other values for pos will result in is undefined behavior and should be avoided181.

...

Noncompliant Code Example

The following non-compliant code This noncompliant code example attempts to read three values from a file and then set the cursor file position pointer back to the beginning of the file and return to the caller.:

#include <stdio.h>
enum { NO_FILE_POS_VALUES = 3 };
#include <string.h>
 
int opener(
 FILE FILE* file, 
  unsigned int *width, 
  unsigned int *height, 
  unsigned int *data_offset
) {
  unsigned int file_w;
  unsigned int file_h;
  unsigned int file_o;
  int rc;
  fpos_t offset;

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

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

  if (fscanf(file, "%u %u %u", &file_w, &file_h, &file_o)  
      != NO_FILE_POS_VALUES) { return EIO; }
/* Read in data from file */

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

  *width = file_w;
  *height = file_h;
  *data_offset = file_o;return rc;
  }

  return 0;
}

However, because only Only the return value of a an fgetpos() call is a valid argument to fsetpos(); passing an a value of type fpos_t value that was created in any other way may not work. It is possible that the position will be set to an arbitrary location in the fileis undefined behavior 181.

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>
enum { NO_FILE_POS_VALUES = 3 };
#include <string.h>
 
int opener(
 FILE FILE* file, 
  unsigned int *width, 
  unsigned int *height, 
  unsigned int *data_offset
) {
  unsigned int file_w;
  unsigned int file_h;
  unsigned int file_o;
  int rc;
  fpos_t offset;

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

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

  if (fscanf(file, "%u %u %u", &file_w, &file_h, &file_o)  
      != NO_FILE_POS_VALUES) { return EIO; }

/* Read in data from file */

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

  *width = return file_wrc;
  *height = file_h;
  *data_offset = file_o; }

  return 0;
}

Risk Assessment

The misuse Misuse of the fsetpos() function can move position a file stream read position indicator to a an unintended location in the file. If this location held input from the user, the user would then gain control of the variables being read from the file.

Automated Detection

...

...

...

Related Vulnerabilities

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

References

...

Related Guidelines

Key here (explains table format and definitions)

Bibliography

...

...

Image Added Image Added Image Added {{fsetpos}} function"FIO43-C. Do not create temporary files in shared directories      09. Input Output (FIO)       10. Environment (ENV)