# perror

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           perror — write error messages to standard error
    


## SYNOPSIS



           #include <stdio.h>
    
           void perror(const char *_s_);
    


## DESCRIPTION



           The functionality described on this reference page is aligned with
           the ISO C standard. Any conflict between the requirements
           described here and the ISO C standard is unintentional. This
           volume of POSIX.1‐2017 defers to the ISO C standard.
    
           The _perror_() function shall map the error number accessed through
           the symbol _errno_ to a language-dependent error message, which
           shall be written to the standard error stream as follows:
    
            *  First (if _s_ is not a null pointer and the character pointed to
               by _s_ is not the null byte), the string pointed to by _s_
               followed by a <colon> and a <space>.
    
            *  Then an error message string followed by a <newline>.
    
           The contents of the error message strings shall be the same as
           those returned by _strerror_() with argument _errno_.
    
           The _perror_() function shall mark for update the last data
           modification and last file status change timestamps of the file
           associated with the standard error stream at some time between its
           successful completion and _exit_(), _abort_(), or the completion of
           _fflush_() or _fclose_() on _stderr_.
    
           The _perror_() function shall not change the orientation of the
           standard error stream.
    
           On error, _perror_() shall set the error indicator for the stream to
           which _stderr_ points, and shall set _errno_ to indicate the error.
    
           Since no value is returned, an application wishing to check for
           error situations should call _clearerr_(_stderr_) before calling
           _perror_(), then if _ferror_(_stderr_) returns non-zero, the value of
           _errno_ indicates which error occurred.
    


## RETURN VALUE



           The _perror_() function shall not return a value.
    


## ERRORS



           Refer to fputc(3p).
    
           _The following sections are informative._
    


## EXAMPLES



       **Printing an Error Message for a Function**
           The following example replaces _bufptr_ with a buffer that is the
           necessary size. If an error occurs, the _perror_() function prints a
           message and the program exits.
    
               #include <stdio.h>
               #include <stdlib.h>
               ...
               char *bufptr;
               size_t szbuf;
               ...
               if ((bufptr = malloc(szbuf)) == NULL) {
                   perror("malloc"); exit(2);
               }
               ...
    


## APPLICATION USAGE



           Application writers may prefer to use alternative interfaces
           instead of _perror_(), such as _strerror_r_() in combination with
           _fprintf_().
    


## RATIONALE



           None.
    


## FUTURE DIRECTIONS



           None.
    


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