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Section 7.19.5.3 of C99 places the following restrictions on update streams [ISO/IEC 9899:1999]:

When a file is opened with update mode both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations.

The following scenarios will be diagnosed because either can result in undefined behavior:
â€¢ 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
â€¢ 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

(See also undefined behavior 143 in Annex J of C99.) Consequently, a call to fseek(), fflush() or fsetpos() is necessary between input and output to the same stream. (See recommendation FIO07-C. Prefer fseek() to rewind().)

Noncompliant Code Example

This noncompliant code example appends data to a file and then reads from the same file.

char data[BUFFERSIZE];
char append_data[BUFFERSIZE];
char *file_name;
FILE *file;

/* Initialize file_name */

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

/* initialize append_data */

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

fclose(file);

However, because the stream is not flushed in between the call to fread() and fwrite(), the behavior is undefined.

Compliant Solution

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

char data[BUFFERSIZE];
char append_data[BUFFERSIZE];
char *file_name;
FILE *file;

/* initialize file_name */

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

/* Initialize append_data */

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 */
}

fclose(file);