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In this compliant solution, fseek() is called between the output and input, eliminating the the undefined behavior 156:

#include <stdio.h>
 
enum { BUFFERSIZE = 32 };
extern void initialize_data(char *data, size_t size);
 
void func(const char *file_name) {
  char data[BUFFERSIZE];
  char append_data[BUFFERSIZE];
  FILE *file;

  file = fopen(file_name, "a+");
  if (file == NULL) {
    /* Handle error */
  }

  initialize_data(append_data, BUFFERSIZE);
  if (fwrite(append_data, BUFFERSIZE, 1, file) != BUFFERSIZE) {
    /* Handle error */
  }

  if (fseek(file, 0L, SEEK_SET) != 0) {
    /* Handle error */
  }

  if (fread(data, BUFFERSIZE, 1, file) != 0) {
    /* Handle there not being data */
  }

  if (fclose(file) == EOF) {
    /* Handle error */
  }
}

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Alternately inputting and outputting from a stream without an intervening flush or positioning call is undefined behavior 156.

Automated Detection

Related Vulnerabilities

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

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CWE-664 = Union( FIO39-C, list) where list =

• Improper use of an object (besides alternating reading/writing a file stream without an intervening flush

This CWE is vague on what constitutes “improper control of a resource”. It could include any violation of an object’s method constraints (whether they are documented or not). Or it could be narrowly interpreted to mean object creation and object destruction (which are covered by other CWEs).

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