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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

Undefined behavior ;

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successful call to the fgetpos function on a stream associated with the same file.

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Invoking the fsetpos() function with any other values for pos is undefined behavior 181.

Noncompliant

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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 add3opener(int x, int y,FILE *file) {
  int z)
{
&nbsp;&nbsp; return x+y+z;
}

void add2to3()
{
int (*function_pointer) (int, int) ;
function_pointer = &add3;
&nbsp;&nbsp;&nbsp; i = function_pointer(2,3);
&nbsp;&nbsp;&nbsp; printf(" ans = %d \n",i);
}

Compliant Solution

int main(int argc, char \*argv\[\])
{
  FILE \*info = fopen( "widget.txt", "r");
  fpos_t  pos;
  int a,b,c;
  if(info == NULL)
  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 ) {
      perror("Could not open file")return rc;
  }

     return 0;
  }
  if(fgetpos(info, &pos)\!=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 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>
#include <string.h>
 
int opener(FILE *file) {
  int  perror("fgetpos failed");rc;
  fpos_t offset;

  if  fclose(info);(file == NULL) {
    return return 0-1;
   }
 if(fscanf(info,"%i %i %i", &a, &b, &c)\!=3)
 {
   fprintf(stderr, "Expected but did not find three integers \n");
   fclose(info);
   return 0;
 }
 else
 {
   printf("abc is %i%i%i\n",a,b,c);
 }

if(fsetpos(info, &pos)\!=0)
{
  perror("fsetpos failed");
  fclose(info);
  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;
}

if(fscanf(info,"%i %i %i", &a, &b, &c)\!=3)
{
  fprintf(stderr, "Expected but did not find %i %i %i \n");
}
else
{
  printf("abc is %i%i%i \n",a,b,c);
}

fclose(info);
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

Key here (explains table format and definitions)

Bibliography

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