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Subclause 7The C Standard, 7.21.5.3 of the C Standard , paragraph 7 [ISO/IEC 9899:2011], places the following restrictions on update streams:

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The following scenarios can result in undefined behavior (see undefined behavior 151):

•  Receiving input from a stream directly following an output to that stream without an intervening call to fflush(), fseek(), fsetpos(), or rewind() if the file is not at end-of-file
•  Outputting to a stream after receiving input from that stream without a call to fseek(), fsetpos(), or rewind() if the file is not at end-of-file

Consequently, a call to fseek(), fflush(), or fsetpos() is necessary between input and output to the same stream. See ERR07-C. Prefer functions that support error checking over equivalent functions that don't for more information on why fseek() is preferred over rewind().

Noncompliant Code Example

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#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, BUFFERSIZE1, 1BUFFERSIZE, file) != BUFFERSIZE) {
    /* Handle error */
  }
  if (fread(data, BUFFERSIZE1, 1BUFFERSIZE, file) !=< 0BUFFERSIZE) {
    /* Handle there not being data */
  }

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

However, because the stream is not flushed between the call Because there is no intervening flush or positioning call between the calls to fread() and fwrite(), the behavior is undefined.

Compliant Solution

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

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

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Related Guidelines

Bibliography

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