MSC23-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

API	Description	Version
Variable-length arrays	Absent	2012
`static` and type qualifiers in parameter array declarators	Absent	2012
`_Static_assert`	Absent; can use `static_assert`, as in C++11	2012
`_Noreturn`	Absent; can use `__declspec(noreturn)`	2012
`inline`	Absent; can use `__inline`	2012
Hexadecimal floating-point constants	Absent	2012
Compound literals	Absent	2012
Designated initializers	Absent	2012
Implicit function declarations	Support was removed in C11, but it is still supported in MSVC (with diagnostic)	2012
Mixed declarations and code	Absent	2012
_Pragma	Absent; can use `__pragma`	2012
`_Bool`	Absent	2012
`_Complex`	Absent	2012
`__func__`	Absent; can use `__FUNCTION__`	2012
idempotent type qualifiers	Diagnosed, but functional	2012
`_Thread_local`	Absent; can use `__declspec(thread)`
<complex.h>	Absent; `__STDC_NO_COMPLEX__` is not defined	2012
`isblank()`	Absent	2012
<fenv.h>	Absent	2012
<inttypes.h>	Absent	2012
`float_t`, `double_t`, `HUGE_VALF, HUGE_VALL, INFINITY, NAN`	Absent	2012
#pragma `FP_CONTRACT`	Spelled `fp_contract` instead of `FP_CONTRACT`	2012
`fpclassify()`	Absent, use `_fpclass()` instead	2012
`isfinite()`, `isinf()`	Absent, use `_finite()` or `_finitef()`instead	2012
`isnan()`	Absent, use `_isnan()` or `_isnanf()` instead	2012
`isnormal()`	Absent, use `_fpclass()` instead	2012
`signbit()`	Absent	2012
`acosh()`, `asinh()`, `atanh()`	Absent	2012
`exp2()`, `expm1()`, `ilogb()`, `log1p()`, `log2()`	Absent	2012
`scalbn()`	Absent; use `_scalb()` (or `_scalbf()` on x64 targets only) instead	2012
`cbrt()`	Absent	2012
`erf()`, `erfc()`, `lgamma()`, `tgamma()`	Absent	2012
`nearbyint()`, `rint()`, `lrint()`, `round()`, `lround()`, `trunk()`	Absent	2012
`remainder()`, `remquo()`	Absent	2012
`copysign()`	Absent; use `_copysign()` instead	2012
`nan()`, `nexttoward()`	Absent	2012
`nextafter()`	Absent; use `_nextafter()` or `_nextafterf()` instead	2012
`fdim()`, `fmax(),` `fmin()`	Absent	2012
`fma()`	Absent	2012
`isgreater()`, `isgreaterequal()`, `isless()`, `islessequal()`, `isunordered()`	Absent	2012
`<stdalign.h>`	Absent
`va_copy()`	Absent	2012
`<stdatomic.h>`	Absent; `__STDC_NO_ATOMICS__` is not defined	2012
`<stdbool.h>`	Absent; `_Bool` is not supported in `/TC` mode, but `bool` is supported in `/TP` mode	2012
`max_align_t`	Absent	2012
Printing format specifier type field	`S` is MSVC-specific; `c`, `s`, and `z` are not conforming [ISO/IEC 9899-2011]; `F` is unsupported	2012
Printing format specifier size field	`I`, `I32` and `I64` are MSVC-specific; `h`, `w` and `l` are not conforming [ISO/IEC 9899-2011]; `hh`, `j`, `z`and `t` unsupported	2012
Scanning format specifier type field	`S` is MSVC-specific; `c` and `s` are not conforming [ISO/IEC 9899-2011]; `p` is unsupported	2012
Scanning format specifier size field	`I64` is MSVC-specific; `h`, `l`, and `L` prefixes are not conforming [ISO/IEC 9899-2011]; `hh`, `j`, `z`, and `t` are unsupported	2012
`snprintf()`	Absent; beware that `_snprintf()`'s semantics are sufficiently different that it is not advisable as a replacement	2012
`vfscanf()`, `vscanf()`, `vsscanf()`	Absent	2012
`fopen()`, `freopen()`	Mode parameter not conforming [ISO/IEC 9899-2011]; `t`, `c`, `n`, `N`, `S`, `R`, `T`, `D` and `css` are MSVC extensions; `x` is unsupported; see remarks	2012
`atoll()`	Absent; use `_atoi64()` instead	2012
`strtof()`, `strtold()`	Absent	2012
`strtoll()`	Absent; use `_strtoi64()` instead	2012
`strtoull()`	Absent; use `_strtoui64()` instead	2012
`aligned_alloc()`	Absent; use `_aligned_malloc()` instead (beware, the parameter order is reversed)	2012
`at_quick_exit()`, `quick_exit()`	Absent	2012
`_Exit()`	Absent; use `_exit()` instead	2012
<stdnoreturn.h>	Absent; use `__declspec(noreturn)` instead	2012
`<tgmath.h>`	Absent	2012
`<threads.h>`	Absent; `__STDC_NO_THREADS__` is not defined	2012
`TIME_UTC,` `struct timespec,` `timespec_get()`	Absent	2012
`strftime(), wcsftime()`	`z` is not conforming [ISO/IEC 9899-2011]; `C`, `D`, `e`, `F`, `g`, `G`, `h`, `n`, `r`, `R`, `t`, `T`, `u`, `V` unsupported	2012
`<uchar.h>`	Absent	2012
`vfwscanf()`, `vswscanf()`, `vwscanf()`	Absent	2012
`fwide()`	Unsupported	2012
`wcstof()`, `wcstold()`	Absent	2012
`wcstoll()`	Absent; use `_wcstoi64()` instead	2012
`wcstoull()`	Absent; use `_wcstoui64()` instead	2012
`iswblank()`	Absent	2012
`FLT_EVAL_METHOD`, `_HAS_SUBNORM`, `_DECIMAL_DIG`, `*_TRUE_MIN`	Absent	2012
__STDC_LIB_EXT1__, __STDC_WANT_LIB_EXT1__	Not defined; instead, MSVC uses `__STDC_SECURE_LIB__` and `__STDC_WANT_SECURE_LIB__`	2012
`fopen_s(), freopen_s()`	Uses the same mode strings as `fopen()`; `u` prefix is unsupported; see remarks	2012
`vfscanf_s()`, `vscanf_s()`, `vsscanf_s()`	Absent	2012
`constraint_handler_t`, `set_constraint_handler_s(), abort_handler_s(), ignore_handler_s()`	Absent; use `_invalid_parameter_handler` and `_set_invalid_parameter_handler()`instead. Beware that the `_invalid_parameter_handler` signature is considerably different than that of `constraint_handler_t`. No replacement for `abort_handler_s()` or `ignore_handler_s()`	2012
`bsearch_s()`	Not conforming [ISO/IEC 9899-2011]; beware that the comparison function pointer's signature is different from the standard; namely that the context parameter comes first in MSVC but last in ISO C	2012
`qsort_s()`	Not conforming [ISO/IEC 9899-2011]; beware that the comparison function's pointer signature is different from the standard; namely that the context parameter comes first in MSVC but last in ISO C	2012
`strtok_s()`	Not conforming [ISO/IEC 9899-2011]; the function signature is missing the `rsize_t *` parameter	2012
`memset_s(), strerrorlen_s()`	Absent	2012
`gmtime_s(), localtime_s()`	Not conforming [ISO/IEC 9899-2011]; the function signature has the parameters reversed and returns `errno_t` instead of `struct tm *`	2012
`snwprintf_s()`	Absent; use `_snwprintf_s()` instead. Note that the parameters differ from the ISO C signature	2012
`vfwscanf_s()`, `vswscanf_s()`, `vwscanf_s()`	Absent	2012
`vsnwprintf_s()`	Absent, use `_vsnwprintf_s()` instead. Note that the parameters differ from the ISO C signature	2012
`wcstok_s()`	Not conforming [ISO/IEC 9899-2011]; the function signature is missing the `rsize_t *` parameter	2012

`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.

Risk Assessment

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
MSC23-C	High	Probable	Medium	P12	L1

Automated Detection

Tool	Version	Checker	Description
Astrée	24.04		Supported: Astrée reports non-standard language elements.
CodeSonar	8.1p0	BADFUNC.ABORT_HANDLER_S BADFUNC.ATOLL BADFUNC.BSEARCH_S BADFUNC.CONSTRAINT_HANDLER_T BADFUNC.FENV_H BADFUNC.FOPEN_S BADFUNC.FREOPEN_S BADFUNC.GMTIME_S BADFUNC.IGNORE_HANDLER_S BADFUNC.LOCALTIME_S BADFUNC.MEMSET_S BADFUNC.SET_CONSTRAINT_HANDLER_S BADFUNC.SNWPRINTF_S BADFUNC.STRERRORLEN_S BADFUNC.STRTOK_S BADFUNC.VFSCANF_S BADFUNC.VFWSCANF_S BADFUNC.VSCANF_S BADFUNC.VSNWPRINTF_S BADFUNC.VSSCANF_S BADFUNC.VSWSCANF_S BADFUNC.VWSCANF_S BADFUNC.WCSTOK_S CONCURRENCY.C_ATOMIC CONCURRENCY.THREADLOCAL LANG.FUNCS.NORETURN LANG.PREPROC.INCL.TGMATH_H LANG.STRUCT.ALIGNAS LANG.STRUCT.ALIGNAS.EZA LANG.STRUCT.ALIGNAS.IAS LANG.STRUCT.ALIGNAS.TMAS LANG.STRUCT.ALIGNOF LANG.STRUCT.DECL.IMPFN LANG.STRUCT.DECL.VLA LANG.STRUCT.INIT.UADI LANG.TYPE.VMAT MATH.RANGE.GAMMA	Use of abort_handler_s Use of atoll Use of bsearch_s Use of constraint_handler_t Use of <fenv.h> Exception Handling Function Use of fopen_s Use of freopen_s Use of gmtime_s Use of ignore_handler_s Use of localtime_s Use of memset_s Use of set_constraint_handler_s Use of snwprintf_s Use of strerrorlen_s Use of strtok_s Use of vfscanf_s Use of vfwscanf_s Use of vscanf_s Use of vsnwprintf_s Use of vsscanf_s Use of vswscanf_s Use of vwscanf_s Use of wcstok_s Use of C Atomic Use of Thread Local Use of Noreturn Use of <tgmath.h> Use of Alignas Explicit Zero Alignment Inconsistent Alignment Specifications Too Many Alignment Specifiers Use of Alignof Implicit Function Declaration Declaration of Variable Length Array Unspecified Array Size with Designator Initialization Pointer to Variably-modified Array Type Gamma on Zero

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Implementation-Specific Deviations

Microsoft Visual Studio

`fopen()` & `freopen()`

Risk Assessment

Automated Detection

2 Comments

Robert Seacord (Manager)

Aaron Ballman

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MSC23-C. Beware of vendor-specific library and language differences

Implementation-Specific Deviations

Microsoft Visual Studio

fopen() & freopen()

Risk Assessment

Automated Detection

2 Comments

Robert Seacord (Manager)

Aaron Ballman

`fopen()` & `freopen()`