MSC41-C. Beware of vendor-specific library and language differences

When compiling with a specific vendor's implementation of the C language, and related libraries, be aware that, unfortunately, standards conformance can differ from vendor to vendor.  Be certain to read your vendor's documentation to reduce the likelihood of accidentally relying on implementation-specific behavior or deviations.

Implementation-Specific Deviations

Implementation-specific deviations are listed below.  The Version column lists the latest version of the compiler or library that exhibits the behavior.

Microsoft Visual Studio

fopen() & freopen()

The C standard does not specify what happens with Windows newline characters (CRLF), and so care should be taken when working with text files.  For instance:

#include <stdio.h>
 
void func( void ) {
  FILE *fp = fopen("text_file.txt", "r");
  if (fp) {
    int counter = 0;
    while (!feof(fp) && !ferror(fp)) {
      ++counter;
      (void)fgetc(fp);
    }
    fclose(fp);
    printf("Number of characters read: %d\n", counter);
  }
}

 
// Contents of text_file.txt
This has
CRLF newlines
in it.

If you save the contents of text_file.txt with Windows line endings (CRLF) and run the program on Windows, it will print 30.  However, if you compile the application on a platform which does not use CRLF as its line endings, it will print 32.  This is because MSVC's text translation mode will translate the CRLF characters into a single LF character on input, and translate a single LF character to CRLF on output.  To ensure consistent behavior between platforms, consider opening the file in binary translation mode explicitly.

#include <stdio.h>
 
void func( void ) {
  FILE *fp = fopen("text_file.txt", "rb");
  if (fp) {
    int counter = 0;
    while (!feof(fp) && !ferror(fp)) {
      ++counter;
      (void)fgetc(fp);
    }
    fclose(fp);
    printf("Number of characters read: %d\n", counter);
  }
}

 This program will print 32 with the given text, regardless of platform.