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Proper understanding of Understanding the difference between text mode and binary mode is important when using functions that operate on file streams. (See FIO14-C. Understand the difference between text mode and binary mode with file streams for more information.)

Section Subclause 7.21.9.2 of the C standard Standard [ISO/IEC 9899:2011] states specifies the following specific behavior for fseek() when opening a binary file in binary mode:

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In addition, footnote 268 of Section subclause 7.21.3 has this to saysays:

Setting the file position indicator to end-of-file, as with fseek(file, 0, SEEK_END), has undefined behavior for a binary stream (because of possible trailing null characters) or for any stream with state-dependent encoding that does not assuredly end in the initial shift state.

Seeking to the end of a binary stream in binary mode with fseek() is not meaningfully supported and , as a result, is not a recommended method for computing the size of a file.

Section Subclause 7.21.9.4 of the C standard Standard [ISO/IEC 9899:2011] states specifies the following specific behavior for ftell() when opening a text file in text mode:

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Consequently, the return value of ftell() for streams opened in text mode should never be used for offset calculations other than in calls to fseek().

POSIX [Open Group 2008IEEE Std 1003.1:2013] provides several guarantees that the problems described in the C standard Standard do not occur on POSIX systems.

First, the fopen() page page says:

The character 'b' shall have no effect, but is allowed for ISO C standard conformance.

This guarantees that binary files are treated the same as text files in POSIX.

Second, the fwrite() page page says:

For each object, size calls shall be made to the fputc() function, taking the values (in order) from an array of unsigned char exactly overlaying the object. The file-position indicator for the stream (if defined) shall be advanced by the number of bytes successfully written.

This indicates means that the file position indicator, and hence consequently the file size, is directly based on the number of bytes actually written to a file.

Noncompliant Code Example (

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Binary File)

This noncompliant code example attempts to open a binary file in binary mode and use fseek() and ftell() to obtain the file size. This code is noncompliant on systems that do not provide the same guarantees as POSIX. On these systems, setting the file position indicator to the end of the file using fseek() is not guaranteed to work for a binary stream, and consequently, the amount of memory allocated may be incorrect, leading to a potential vulnerability.

FILE *fp;
long file_size;
char *buffer;

fp = fopen("foo.bin", "rb");
if (fp == NULL) {
  /* Handle Errorerror */
}

if (fseek(fp, 0 , SEEK_END) != 0) {
  /* Handle Errorerror */
}

file_size = ftell(fp);
if (file_size == -1) {
  /* Handle Errorerror */
}

buffer = (char*)malloc(file_size);
if (buffer == NULL) {
  /* Handle Errorerror */
}

/* ... */ 

However, setting the file position indicator to the end of the file using fseek() has undefined behavior for a binary stream, and consequently, the amount of memory allocated may be incorrect, leading to a potential vulnerability.

Compliant Solution (POSIX)

Compliant Solution (POSIX ftello())

If the code needs to handle large files, it is preferable to use fseeko() and ftello() because, for some implementations, they can handle larger file offsets than fseek() and ftell() can handle. If they are used, the file_size variable should have type off_t to avoid the possibility of overflow when assigning the return value of ftello() to it. This solution works only with regular filesThe above code is, however, perfectly compliant when run on a regular file in POSIX.

FILE* *fp;
int fd;
longoff_t file_size;
char *buffer;
struct stat st;
  
fpfd = fopenopen("foo.bin", O_RDONLY);
if (fd == -1) {
  /* Handle error */
}

fp = fdopen(fd, "rbr");
if (fp == NULL) {
  /* Handle Errorerror */
}
 
/* Ensure that the file is a regular file */
if ((fstat(fpfd, &st) != 0) || (!S_ISREG(st.st_mode))) {
  /* Handle Errorerror */
}
 
if (fseekfseeko(fp, 0 , SEEK_END) != 0) {
  /* Handle Errorerror */
}
  
file_size = ftellftello(fp);
if (file_size == -1) {
  /* Handle Errorerror */
}
 
buffer = (char*)malloc(file_size);
if (buffer == NULL) {
  /* Handle Errorerror */
}

/* ... */ 

Compliant Solution

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(POSIX fstat())

This compliant solution uses the size provided by the POSIX fstat() function, rather than by fseek() and ftell(), to obtain the size of the binary file. This solution works only with regular files.

FILE *fp;
long off_t file_size;
char *buffer;
struct stat stbuf;
int fd;
 
fd = open("foo.bin", O_RDONLY);
if (fd == -1) {
  /* Handle Errorerror */
}
 
fp = fdopenif ((fstat(fd, "rb");
if (fp == NULL) {
  /* Handle Error */
}

if (fstat(fd, &stbuf) == -1&stbuf) != 0) || (!S_ISREG(stbuf.st_mode))) {
  /* Handle Errorerror */
}
 
file_size = stbuf.st_size;
 
buffer = (char*)malloc(file_size);
if (buffer == NULL) {
  /* Handle Errorerror */
}

/* ... */ 

Noncompliant Code Example (text file)

Compliant Solution (Windows)

This compliant solution uses the Windows _filelength() function to determine the size of the file on a 32-bit operating system. For a 64-bit operating system, consider using _filelengthi64 insteadThis noncompliant code example attempts to open a text file in text mode and use fseek() and ftell() to obtain the file size.

FILEint *fpfd;
long file_size;
char *buffer;

fp = fopen(_sopen_s(&fd, "foo.txt", "r"bin", _O_RDONLY, _SH_DENYRW, _S_IREAD);
if (fpfd == NULL-1) {
  /* Handle Errorerror */
}

if (fseek(fp, 0 , SEEK_END) != 0) {
  /* Handle Error */
}

file_size = ftell_filelength(fpfd);
if (file_size == -1) {
  /* Handle Errorerror */
}

buffer = (char*)malloc(file_size);
if (buffer == NULL) {
  /* Handle Errorerror */
}

/* ... */ 

However, the file position indicator returned by ftell() with a file opened in text mode is only useful in calls to fseek(). As such, the value of file_size may not necessarily be a meaningful measure of the number of characters in the file, and consequently, the amount of memory allocated may be incorrect, leading to a potential vulnerability.

Implementation Details

[MSDN] has the following to say about ftell():

The value returned by ftell may not reflect the physical byte offset for streams opened in text mode, because text mode causes carriage return-linefeed translation. Use ftell with fseek to return to file locations correctly.

Again, this indicates that the return value of ftell() for streams opened in text mode is useful only in calls to fseek() and should not be used for any other purpose.

The following code example attempts to read in 10 bytes from a text file opened in text mode. It obtains the current value of the file-position indicator by using ftell().

The code was compiled with Microsoft Visual Studio 2008 SP1 on Windows XP Professional SP3:

#include <stdio.h>
#include <stdlib.h>

int main() {

  FILE *fp;
  char a[11];
  long offset;

  /* The contents of foo.txt are: 0123456789 repeated 10 times separated by line feeds (ie \n) */
  fp = fopen( "foo.txt", "r" );
  if (fp == NULL) {
    perror("fopen() error");
    exit(EXIT_FAILURE);
  }

  /* Read 10 (n-1) bytes */
  if (fgets(a, 11, fp) == NULL) {
    perror("fgets() error");
    fclose(fp);
    exit(EXIT_FAILURE);
  }

  offset = ftell(fp);
  if (offset == -1) {
    perror("ftell() error");
    fclose(fp);
    exit(EXIT_FAILURE);
  }

  printf("offset = %ld\n", offset);   /* Prints out 0.*/ 

  return 0;
}

The following is printed when this example is run with the preceding setup:

offset = 0

However, 0 is incorrect; the correct value should be 10, as evident when the file is opened in binary mode rather than text mode.

Compliant Solution (Windows, POSIX)

This compliant solution uses fstat() instead to the size of the text file.

Compliant Solution (Windows)

This compliant solution uses the Windows GetFileSizeEx() function to determine the size of the file on a 32- or 64-bit operating system:

HANDLE file;
LARGE_INTEGER file_size;
char *buffer;

 
file = CreateFile(TEXT("foo.bin"), GENERIC_READ, 0, NULL, 
                   OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
if (INVALID_FILE_HANDLE == file) {
  /* Handle error */
}
 
if (!GetFileSizeEx(file, &file_size)) {
  /* Handle error */
}
 
/* 
 * Note: 32-bit portability issue with LARGE_INTEGER
 * truncating to a size_t.
 */
buffer = (char*)malloc(file_size);
if (buffer == NULL) {
  /* Handle error */
}

/* ... */ 

Noncompliant Code Example (Text File)

This noncompliant code example attempts to open a text file in text mode and use fseek() and ftell() to obtain the file size:

FILE *fp;
long file_size;
char *buffer;
struct stat stbuf;
int fd;

fd
fp = openfopen("foo.txt", O_RDONLY"r");
if (fdfp == -1NULL) {
  /* Handle Errorerror */
}

if (fseek(fp, =0 fdopen(fd, "r");
if (fp == NULLSEEK_END) != 0) {
  /* Handle Errorerror */
}

file_size = ftell(fp);
if (fstat(fd, &stbuf)file_size == -1) {
  /* Handle Errorerror */
}
file_size = stbuf.st_size;

buffer = (char*)malloc(file_size);
if (buffer == NULL) {
  /* Handle Errorerror */
}

/* ... */ 

However, the file position indicator returned by ftell() with a file opened in text mode is useful only in calls to fseek(). As such, the value of file_size may not necessarily be a meaningful measure of the number of characters in the file, and consequently, the amount of memory allocated may be incorrect, leading to a potential vulnerability.

The Visual Studio documentation for ftell() [MSDN] states:

The value returned by ftell may not reflect the physical byte offset for streams opened in text mode, because text mode causes carriage return-linefeed translation. Use ftell with fseek to return to file locations correctly.

Again, this indicates that the return value of ftell() for streams opened in text mode is useful only in calls to fseek() and should not be used for any other purpose.

Compliant Solution (Windows)

The compliant solution used for binary files on Windows can also be used for text files.

Compliant Solution (POSIX)

Because binary files are treated the same as text files in POSIX, either compliant solution can be used for determining the size of a binary file under POSIX to determine the size of a text file as well.

Risk Assessment

Understanding the difference between text mode and binary mode with file streams is critical when working with functions that operate on them. Setting the file position indicator to end-of-file with fseek() has undefined behavior for a binary stream. In addition, the return value of ftell() for streams opened in text mode is useful only in calls to fseek(), not for determining file sizes or for any other use. As such, fstat(), or other platform-equivalent functions , should be used to determine the size of a file.

Related Guidelines

Automated Detection

Bibliography

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