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Calling 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 181.

Noncompliant Code Example

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

Non-Compliant Code Example

The following non-compliant code attempts to read two parameters from a file, then set the cursor position back to the beginning of the file and return to the caller.:

#include <stdio.h>
#include <string.h>
 
int
errno_t opener(FILE * file, int *width, int *height, int *data_offset) {
  int file_wrc;
  int file_h;
  int file_o;
  int rc;
  int offset = 0fpos_t offset;

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

  if (file == NULL) { return EINVAL; }
  if (fscanf(file, "%i %i %i", &file_w, &file_h, &file_o)  != 3) { return EIO; }
  if ((rc = fsetpos(file, &offset)) != 0 ) { return rc; return -1;
  }

  /*width = file_w;
  *height = file_h;
  *data_offset = file_o;

  return 0; 
}

int main(void) {
  int width;
  int height;
  int data_offset;
  FILE *file;
  ...Read in data from file */

  filerc = fopenfsetpos("myfile"file, "rb"&offset);
  if (opener(file, &width, &height, &data_offset) rc != 0 ) {
    return 0rc;
  }

  ...return 0;
}

However, since only Only the return value of a getposan fgetpos() call is a valid argument to be used with setposfsetpos(), ; passing a specified int in instead may not work. It is possible that the position will be set to an arbitrary location in the filevalue of type fpos_t that was created in any other way is 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 back to the beginning of the file in the later call to fsetpos().:

errno_t#include <stdio.h>
#include <string.h>
 
int opener(FILE * file, int *width, int *height, int *data_offset) {
  int file_w;
  int file_h;
  int file_o;
  int rc;
  fpos_t offset;

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

  if ((rc = fgetpos(file, &offset)) != 0 ) { return rc; }
  if (fscanf(file, "%i %i %i", &file_w, &file_h, &file_o)  != 3) { return EIO; }
  if ((rc = fsetpos(file, &offset)) != 0 ) { return rc; }

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

  return 0; 
 }

int main(void) {
  int width;
  int height;
  int data_offset;
  FILE *file;
  ...

  file = fopen("myfile", "rb" /* Read in data from file */

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

  return ...0;
}

Risk Assessment

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

References

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

Bibliography

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Image Added Image Added Image Added Wiki Markup\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Section 7.19.9.3, "The fsetpos function"