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RoyalUp
Asked: 2020-05-17 08:16:18 +0800 CST 2020-05-17 08:16:18 +0800 CST

Doubt about stderr and fprintf

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Hi, I have a question and I don't know how stderr from the stdio.h library and fprintf() from string.h are used . For example I have this:

fprintf(stderr, "Error en planta de Centro: %d\n", numeroPlantas);

Where numberPlants is an int . I would like you to clarify when they are usually used and their meaning if possible.

c

2 Answers

  1. Best Answer

    fprint I don't think you'll have a problem knowing that, it allows the output of printf to be written to any file.

    fprint This is often used by programmers to print errors, but it can work with any file opened with the fopen function.

    stderr is a way to print the errors, stderr contains the possible errors that the user wants to report .

    stderr is, as its name suggests, the standard error output. this is useful when, for example, you redirect the output of your program to a file. If in that case an error occurs in the execution of the program and you have set its output to stderr, the error will appear in the terminal instead of in the file.

    Here is a documentation in Spanish

    as an example:

    #include <stdio.h>
#include <errno.h>
#include <string.h>

extern int errno ;

int main () {

   FILE * pf;
   int errnum;
   pf = fopen ("archivo_no_existente.txt", "rb");

   if (pf == NULL) { //Archivo no existe!
      errnum = errno;
      //Imprime salida formateada del error.
      fprintf(stderr, "Error al tratar de abrir el archivo es : %s\n", strerror( errnum ));
   }
   else {
      fclose (pf);
   }

   return 0;
}

    It has as output:

    enter image description here

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  2. When you run a program, the output you see on the console is usually a combination of stdout and stderr. But it is possible to redirect both outputs to different files.

    So if you run "./myprogram 1>log 2>errorLog" you will get everything you print to stderr in errorlog and not in log. This is assuming that the Operating System associates 0 with stdin, 1 with stdout and 2 with stderr, which is normal.

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