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Section 2.9.1 of the System Interfaces volume of POSIX.1-2008 has a much longer list of functions that are not required to be thread-safe.

Noncompliant Code Example (POSIX)

Consider a multithreaded application that encounters an error while calling a system function. The strerror() function returns a human-readable error string given an error number. The C Standard, section 7.24.6.2, specifically states that strerror() is not required to avoid data races. Conventionally, it could rely on a static array that maps error numbers to error strings, and that array might be accessible and modifiable by other threads. (This code is specific to POSIX because fopen() is not guaranteed to set errno if an error occurs in C99 or C11.)

errno = 0;
FILE* fd = fopen( filename, "r");
if (fd == NULL) {
  char* errmsg = strerror(errno);
  printf("Could not open file because of %s\n", errmsg);
}

Note that this code first sets errno to 0 to comply with ERR30-C. Set errno to zero before calling a library function known to set errno, and check errno only after the function returns a value indicating failure.

Compliant Solution

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The compliant solution uses the POSIX strerror_r() function, which has the same functionality as strerror() but guarantees thread safety.

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Note that Linux provides two versions of strerror_r(), known as the XSI-compliant version and the GNU-specific version. This compliant solution assumes the XSI-compliant version. You can get the XSI-compliant version if you compile applications in the way POSIX requires (that is, by defining _POSIX_C_SOURCE or _XOPEN_SOURCE appropriately). Check your strerror_r() manual page to see which version(s) are available on your system.

Compliant Solution (strerror_s(), C11) 

This compliant solution uses the strerror_s() function from Annex K of the C Standard, which has the same functionality as strerror() but guarantees thread-safety. Furthermore, in C11, errno is a thread-local variable, so there is no race condition between when it is initialized and read by strerror_s().

errno = 0;
FILE* fd = fopen( filename, "r");
if (fd == NULL) {
  char errmsg[BUFSIZ];
  if (strerror_s(errno, errmsg, BUFSIZ) != 0) {
    /* handle error */
  }
  printf("Could not open file because of %s\n", errmsg);
}

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