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#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_r(errno, errmsg, BUFFERSIZE) != 0) {
      /* Handle error */
    }
    printf("Could not get the file position because of: %s\n", errmsg);
  }
}

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, which is the default when an application is compiled as required by POSIX (that is, by defining _POSIX_C_SOURCE or _XOPEN_SOURCE appropriately). Thestrerror_r() manual page lists versions that are available on a particular system.

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