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Some functions in the C standard library functions are not guaranteed to be reentrant with respect to threads. Some functions (such as strtok() and asctime()) return a pointer to the result stored in function-allocated memory on a per-process basis. Other functions (such as rand()) store state information in function-allocated memory on a per-process basis. Multiple threads invoking the same function can cause concurrency problems, which often result in abnormal behavior and can cause more serious vulnerabilities, such as abnormal termination, denial-of-service attack, and data integrity violations.

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#define __STDC_WANT_LIB_EXT1__ 1
#include <errno.h>
#include <stdio.h>
#include <string.h>
 
enum { BUFFERSIZE = 64 };
void f(FILE *fp) {
  fpos_t pos;
  errno = 0;

  if (0 != fgetpos(fp, &pos)) {
    char errmsg[BUFFERSIZE];
    if (strerror_s(errmsg, BUFFERSIZE, errno) != 0) {
      /* Handle error */
    }
    printf("Could not get the file position because of %s\n",
           errmsg);
  }
}

Note that because of the optional nature of Annex K is optional, strerror_s() may not be available in all implementations. 

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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 , which you will get if you compile applications in the way POSIX requires your application as required by POSIX (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.

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