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Section 7.19.5.3 of C99 places the following restrictions on update streams: \[[ISO/IEC 9899:1999|AA. C++ References#ISO/IEC 9899-1999]\]
<blockquote><p>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 <code>fflush</code> function or to a file positioning function (<code>fseek</code>, <code>fsetpos</code>, or <code>rewind</code>), 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.</p></blockquote>Receiving input from a stream directly following an output to that stream without an intervening call to {{fflush()}}, {{fseek()}}, {{fsetpos()}}, or {{rewind()}}, or outputting to a stream after receiving input from it without a call to {{fseek()}}, {{fsetpos()}}, {{rewind()}} if the file is not at end-of-file results in [undefined behavior|BB. Definitions#undefined behavior]. Consequently, a call to {{fseek()}}, {{fflush()}} or {{fsetpos()}} is necessary between input and output to the same stream (see [FIO07-CPP. Prefer fseek() to rewind()|FIO07-CPP. Prefer fseek() to rewind()]). |
Noncompliant Code Example (FILE*)
This noncompliant code example appends data to a file and then reads from the same file.
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However, because the stream is not flushed in between the call to fread() and fwrite(), the behavior is undefined.
Compliant Solution (FILE*)
In this compliant solution, fseek() is called in between the output and input, eliminating the undefined behavior.
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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);
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Noncompliant Code Example (iofstream)
This noncompliant code example uses C++ iostreams, but makes the same mistake as the previous noncompliant code example.
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{ char data[BUFFERSIZE]; char append_data[BUFFERSIZE]; char *file_name; * Initialize file_name and append_data */ fstream file( file_name, fstream::in | fstream::out | fstream::app); file << append_data << ends; file >> data; // ... } /* File gets closed here */ |
Compliant Solution (seekg)
In this compliant solution, the seekg() member function is called in between the output and input, eliminating the undefined behavior.
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char data[BUFFERSIZE];
{
char data[BUFFERSIZE];
char append_data[BUFFERSIZE];
char *file_name;
* Initialize file_name and append_data */
fstream file( file_name, fstream::in | fstream::out | fstream::app);
file << append_data << ends;
file.seekg(0, ios::beg);
file >> data;
// ...
} /* File gets closed here */
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Risk Assessment
Alternately inputting and outputting from a stream without an intervening flush or positioning call results in undefined behavior.
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