SEI CERT C Coding Standard

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The C Standard identifies specific strings to use for the mode on calls to fopen() and fopen_s(). C11 provides a new mode flag, x, that provides the mechanism needed to determine if the file that is to be opened exists. To be strictly conforming and portable, one of the strings from the following table (adapted from the C Standard, subclause 7.21.5.2 [ISO/IEC 9899:2011]) must be used:

Strings to Use for the Mode on Calls to fopen() and fopen_s()

mode String

Result

r

Open text file for reading

w

Truncate to zero length or create text file for writing

wx

Create text file for writing

a

Append; open or create text file for writing at end-of-file

rb

Open binary file for reading

wb

Truncate to zero length or create binary file for writing

wbx

Create binary file for writing

ab

Append; open or create binary file for writing at end-of-file

r+

Open text file for update (reading and writing)

w+

Truncate to zero length or create text file for update

w+x

Create text file for update

a+

Append; open or create text file for update, writing at end-of-file

r+b or rb+

Open binary file for update (reading and writing)

w+b or wb+

Truncate to zero length or create binary file for update

w+bx or wb+x

Create binary file for update

a+b or ab+

Append; open or create binary file for update, writing at end-of-file

If the mode string begins with one of these sequences, the implementation might choose to ignore the remaining characters, or it might use them to select different kinds of files.

When calling fopen_s(), any of the mode strings used for writing (w or a) may be prefixed with the u character to give the file system default access permissions.

An implementation may define additional mode strings, but only the modes shown in the table are fully portable and C compliant. Beware that Microsoft Visual Studio 2012 and earlier do not support the x or u mode characters [MSDN].

Risk Assessment

Using a mode string that is not recognized by an implementation may cause the call to fopen() to fail.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

FIO11-C

Medium

Probable

Medium

P8

L2

Automated Detection

Tool

Version

Checker

Description

Compass/ROSE




LDRA tool suite
9.7.1

590 S

Partially implemented

PC-lint Plus

1.4

2472, 2473

Fully supported

Polyspace Bug Finder

R2023b

CERT C: Rec. FIO11-C

Checks for bad file access mode or status (rec. fully covered)

Related Vulnerabilities

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

Related Guidelines

SEI CERT C++ Coding StandardVOID FIO11-CPP. Take care when specifying the mode parameter of fopen()

Bibliography

[ISO/IEC 9899:2011]Subclause 7.21.5.3, "The fopen Function"


7 Comments

  1. Jonathan Leffler

    Someone mentioned a GNU C Library extension with the letter 'x' for the O_EXCL flag in a comment in a previous item.

    Another previous item mentioned the letter 'u' as modifying the permissions on files - possibly in conjunction with fopen_s().

    These seem somewhat inconsistent with the advice given here. 

    1. Douglas A. Gwyn

      The point is that using anything beyond this list is nonportable, which might be okay but the programmer needs to consider whether it fits the project goals, and should warn loudly about it in his source code.

  2. Alex Volkovitsky

    Section 7.19.3 of C99 says

    A stream is associated with an external file (which may be a physical device) by opening a file, which may involve creating a new file. Creating an existing file causes its former contents to be discarded, if necessary. If a file can support positioning requests (such as a disk file, as opposed to a terminal), then a file position indicator associated with the stream is positioned at the start (character number zero) of the file, unless the file is opened with append mode in which case it is implementation-defined whether the file position indicator is initially positioned at the beginning or the end of the file. The file position indicator is maintained by subsequent reads, writes, and positioning requests, to facilitate an orderly progression through the file.

    How does this reconcile with the definition of the flags given above? Is this a problem with the standard?

    1. Shaun Hedrick

      There is no "problem" in the standard. The idea is that by opening something in append mode, you will only be appending to it. Therefore, the point of having a file position indicator (knowing where you are in a file and being able to move around within a file) is unnecessary and, in fact, possibly dangerous (what validity is there in seeking around on something that is by definition append-only?).

      If you carefully read the definitions of all of the output functions, you can notice that the C standard weaves around these problems by making the file position indicator basically useless on an append stream, e.g.:

      7.19.7.3
      The fputc function writes the character specified by c (converted to an unsigned char) to the output stream pointed to by stream, at the position indicated by the associated file position indicator for the stream (if defined), and advances the indicator appropriately. If the file cannot support positioning requests, or if the stream was opened with append mode, the character is appended to the output stream.

    2. Alex Volkovitsky

      ok... shaun and I talked about this... looks the file position indicator isn't defined to do anything useful and has no meaning whatsoever in append mode since you can't seek

    3. Robert Seacord

      I recently had this same question.

      > 1. append mode
      >
      > section 7.19.3 Files paragraph 1 states:
      >
      > If a file can support positioning requests (such as a
      > disk file, as opposed to a terminal), then a file position indicator associated with the
      > stream is positioned at the start (character number zero) of the file, unless the file is
      > opened with append mode in which case it is implementation-defined whether the file
      > position indicator is initially positioned at the beginning or the end of the file.
      >
      > while section 7.19.5.3 The fopen function, paragraph 3 seemingly contradicts this:
      >
      > a append; open or create text file for writing at end-of-file
      >
      > i'm guessing section section 7.19.3 is correct and that section 7.19.5.3 is simply a misleading simplification, yes?

      I don't see any obvious contradiction.

      The first part describes the initial location of the
      "file position indicator" when a file is opened with
      append, the second says that writes always occur at
      the end of the file.

      They are separate things. The first, in effect, says
      that what you get returned from ftell() just after
      opening with append is implementation defined. The
      second says how opening with append differs from a
      regular opened-for-writing file.

  3. David Svoboda

    This recommendation could use a noncompliant code example and compliant solution.