__freadable() — Determine if a stream is open for reading

Standards

Standards / Extensions	C or C++	Dependencies
Language Environment®	both

Format

#include <stdio.h>
#include <stdio_ext.h>

int __freadable(FILE *stream);

General description

The __freadable() function determines if the specified stream has been opened for reading.

Returned value

The __freadable() function returns nonzero if the stream is opened for reading. Otherwise, the __freadable() function returns 0. If an error has occurred, __freadable() function returns 0 and sets errno to nonzero.

An application wishing to check for error situations should set errno to 0, then call __freadable(), and then check errno. If errno is nonzero, assume that an error has occurred.

Error Code: Description
EBADF: The stream specified by stream is not valid.

Example

CELEBF89

/* CELEBF89

   This example writes and reads data to a file while querying the
   stream for information about data in the I/O buffer.

*/

#include <stdio.h>
#include <stdio_ext.h>

void main() {
   FILE *f;
   char filename[FILENAME_MAX] = "myfile.dat";
   char data[128] = "There are 34 bytes in this buffer\n";
   int datalen = strlen(data);
   size_t n = 0;

   f = fopen(filename,"wb+");
   if (f == NULL) {
      perror("fopen() failed\n");
      return;
   }

   if (__fwritable(f)) printf("Writing is allowed on the open stream\n");
   if (__freadable(f)) printf("Reading is allowed on the open stream\n");

   n = fputs(data,f);
   if (n == EOF) {
      perror("fputs() failed\n");
      return;
   }

   n = __fpending(f);
   printf("There are %d bytes in the buffer pending to be written\n", n);

   if (__fwriting(f)) printf("The last operation on the stream was a write\n");

   rewind(f);

   n = fgetc(f);

   n = __freadahead(f);
   printf("There are %d bytes remaining to be read from the buffer\n", n);

   if (__freading(f)) printf("The last operation on the stream was a read\n");

   return;
}

Output

Writing is allowed on the open stream                  
Reading is allowed on the open stream                  
There are 34 bytes in the buffer pending to be written 
The last operation on the stream was a write           
There are 33 bytes remaining to be read from the buffer
The last operation on the stream was a read