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ComputingCStdio.h

strerror

System error messages

Interface

#include <stdio.h>
void perror (const char *string)
externint sys_nerr ()

#include <string.h>
char* strerror (int errnum)
int strerror_r (int errnum, char *strerrbuf, size_t buflen)

Description

The strerror, strerror_r and perror functions look up the error message string corresponding to an error number.

The strerror function accepts an error number argument errnum and returns a pointer to the corresponding message string.

The strerror_r function renders the same result into strerrbuf for a maximum of buflen characters and returns 0 upon success.

The perror function finds the error message corresponding to the current value of the global variable <span class="Va">errno</span> and writes it, followed by a newline, to the standard error file descriptor. If the argument \c string is non- <span class="Dv">NULL</span> and does not point to the null character, this string is prepended to the message string and separated from it by a colon and space; otherwise, only the error message string is printed.

If the error number is not recognized, these functions return an error message string containing "<span class="Dq"><span class="Li">Unknown error:</span></span>" followed by the error number in decimal. The strerror and strerror_r functions return <span class="Er">EINVAL</span> as a warning. Error numbers recognized by this implementation fall in the range 0 < errnum < sys_nerr.

If insufficient storage is provided in strerrbuf (as specified in buflen) to contain the error string, strerror_r returns <span class="Er">ERANGE</span> and strerrbuf will contain an error message that has been truncated and <span class="Dv">NUL</span> terminated to fit the length specified by buflen.

The message strings can be accessed directly using the external array <span class="Va">sys_errlist</span>. The external value <span class="Va">sys_nerr</span> contains a count of the messages in <span class="Va">sys_errlist</span>. The use of these variables is deprecated; strerror or strerror_r should be used instead.

Example 1

#include <stdio.h>
 
int main()
{
  char error_message[100] = {0};
  perror(error_message);
  return 0;
}

Output:
No error

History

The strerror and perror functions first appeared in 4.4BSD. The strerror_r function was implemented in FreeBSD 4.4 by <span class="An">Wes</span> <span class="An">Peters</span> <span class="An"><<span class="Aq">wes@FreeBSD.org</span>>.</span>

Bugs

For unknown error numbers, the strerror function will return its result in a static buffer which may be overwritten by subsequent calls.

The return type for strerror is missing a type-qualifier; it should actually be <span class="Vt">const</span> <span class="Vt">char</span> <span class="Vt">*</span>.

Programs that use the deprecated <span class="Va">sys_errlist</span> variable often fail to compile because they declare it inconsistently.

Last Modified: 18 Dec 11 @ 13:14     Page Rendered: 2022-03-14 15:45:08