FILE *fdopen(int fildes, const char *mode);
The fdopen() function shall associate a stream with a file descriptor.
The mode argument is a character string having one of the following values:
The meaning of these flags is exactly as specified in fopen(), except that modes beginning with w shall not cause truncation of the file.
Additional values for the mode argument may be supported by an implementation.
The application shall ensure that the mode of the stream as expressed by the mode argument is allowed by the file access mode of the open file description to which fildes refers. The file position indicator associated with the new stream is set to the position indicated by the file offset associated with the file descriptor.
The error and end-of-file indicators for the stream shall be cleared. The fdopen() function may cause the st_atime field of the underlying file to be marked for update.
If fildes refers to a shared memory object, the result of the fdopen() function is unspecified.
If fildes refers to a typed memory object, the result of the fdopen() function is unspecified.
The fdopen() function shall preserve the offset maximum previously set for the open file description corresponding to fildes.
Upon successful completion, fdopen() shall return a pointer to a stream; otherwise, a null pointer shall be returned and errno set to indicate the error.
The fdopen() function may fail if:
The following sections are informative.
File descriptors are obtained from calls like open(), dup(), creat(), or pipe(), which open files but do not return streams.
The file descriptor may have been obtained from open(), creat(), pipe(), dup(), or fcntl(); inherited through fork() or exec; or perhaps obtained by implementation-defined means, such as the 4.3 BSD socket() call.
The meanings of the mode arguments of fdopen() and fopen() differ. With fdopen(), open for write (w or w+) does not truncate, and append (a or a+) cannot create for writing. The mode argument formats that include a b are allowed for consistency with the ISO C standard function fopen(). The b has no effect on the resulting stream. Although not explicitly required by this volume of IEEE Std 1003.1-2001, a good implementation of append (a) mode would cause the O_APPEND flag to be set.
Interaction of File Descriptors and Standard I/O Streams , fclose() , fopen() , open() , the Base Definitions volume of IEEE Std 1003.1-2001, <stdio.h>