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Section The C Standard, 7.1921.5.3 of C99 , paragraph 7 [ISO/IEC 9899:2011], places the following restrictions on update streams: [ISO/IEC 9899:1999]

When a file is opened with update mode both . . ., 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.

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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()

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• if the file is not at end-of-file
•  Outputting to a stream after receiving input from

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• that stream without a call to fseek(), fsetpos(), or rewind() if the file is not at end-of-file

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Consequently, a call to fseek(), fflush(), or fsetpos() is necessary between input and output to the same stream

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. See FIO07-CPP. Prefer fseek() to rewind() for more information on why fseek() is preferred over rewind().

Noncompliant Code Example

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

Code Block
bgColor #ffcccc lang cpp
#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]; char *file_name; FILE *file; /* Initialize file_name */ 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 */ } }

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

Compliant Solution

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

Code Block
bgColor #ccccff lang cpp
#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]; char *file_name; FILE *file; /* initialize file_name */ file = fopen(file_name, "a+"); if (file == NULL) { /* Handle error */ } /* Initialize 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 */ } }

Noncompliant Code Example (iofstream)

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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 */

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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.seekg(0, ios::beg);
  file >> data;
  // ...
} /* File gets closed here */

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

Automated Detection

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Related Vulnerabilities

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

Other Languages

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Related Guidelines

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Bibliography

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