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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. C99, Section 7.22.6.2, 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.

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

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

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

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