fdopen() — Associates Stream With File Descriptor

Format

#include <stdio.h>
FILE *fdopen(int handle, char *type);

Language Level: XPG4

Threadsafe: Yes.

Integrated File System Interface: This function is not available when SYSIFCOPT(*NOIFSIO) is specified on the compilation command.

Description

The fdopen() function associates an input or output stream with the file that is identified by handle. The type variable is a character string specifying the type of access that is requested for the stream. The variable contains one positional parameter that is followed by optional keyword parameters.

The possible values for the positional parameters are:

Mode

Description

r

Create a stream to read a text file. The file pointer is set to the beginning of the file.

w

Create a stream to write to a text file. The file pointer is set to the beginning of the file.

a

Create a stream to write, in append mode, at the end of the text file. The file pointer is set to the end of the file.

r+

Create a stream for reading and writing a text file. The file pointer is set to the beginning of the file.

w+

Create a stream for reading and writing a text file. The file pointer is set to the beginning of the file.

a+

Create a stream for reading or writing, in append mode, at the end of the text file. The file pointer is set to the end of the file.

rb

Create a stream to read a binary file. The file pointer is set to the beginning of the file.

wb

Create a stream to write to a binary file. The file pointer is set to the beginning of the file.

ab

Create a stream to write to a binary file in append mode. The file pointer is set to the end of the file.

r+b or rb+

Create a stream for reading and writing a binary file. The file pointer is set to the beginning of the file.

w+b or wb+

Create a stream for reading and writing a binary file. The file pointer is set to the beginning of the file.

a+b or ab+

Create a stream for reading and writing to a binary file in append mode. The file pointer is set to the end of the file.

The specified type must be compatible with the access method you used to open the file. If the file was opened with the O_APPEND flag, the stream mode must be a, a+, ab, a+b, or ab+. To use the fdopen() function you need a file descriptor. To get a descriptor use the POSIX function open(). The O_APPEND flag is a mode for open(). Modes for open() are defined in QSYSINC/H/FCNTL. For further information see the APIs topic in the i5/OS Information Center.

The keyword parameters allowed for fdopen() are the same as those documented in fopen() — Open Files that are for the integrated file system.

If fdopen() returns NULL, use close() to close the file. If fdopen() is successful, you must use fclose() to close the stream and file.

Return Value

The fdopen() function returns a pointer to a file structure that can be used to access the open file. A NULL pointer return value indicates an error.

Example that uses fdopen()

This example opens the file sample.dat and associates a stream with the file using fdopen(). It then reads from the stream into the buffer.

/* compile with SYSIFCOPT(*IFSIO) */
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <string.h>
 
int main(void)
{
   long length;
   int fh;
   char buffer[20];
   FILE *fp;
 
   printf("\nCreating sample.dat.\n");
   if ((fp= fopen("/sample.dat", "w")) == NULL) {
       perror(" File was not created: ");
       exit(1);
   }
   fputs("Sample Program", fp);
   fclose(fp);
 
   memset(buffer, '\0', 20);                              /* Initialize buffer*/
 
   if (-1 == (fh = open("/sample.dat", O_RDWR|O_APPEND))) {
      perror("Unable to open sample.dat");
      exit(1);
   }
   if (NULL == (fp = fdopen(fh, "r"))) {
      perror("fdopen failed");
      close(fh);
      exit(1);
   }
   if (14 != fread(buffer, 1, 14, fp)) {
      perror("fread failed");
      fclose(fp);
      exit(1);
   }
   printf("Successfully read from the stream the following:\n%s.\n", buffer);
   fclose(fp);
   return 1;
 
   /****************************************************************
    * The output should be:
    *
    * Creating sample.dat.
    * Successfully read from the stream the following:
    * Sample Program.
    */
}

Related Information

• fclose() — Close Stream
• fopen() — Open Files
• fseek() — fseeko() — Reposition File Position
• fsetpos() — Set File Position
• rewind() — Adjust Current File Position
• <stdio.h>
• open API in the APIs topic in the i5/OS Information Center.
• close API in the APIs topic in the i5/OS Information Center.