File: libc.info, Node: I/O Primitives, Next: File Position Primitive, Prev: Opening and Closing Files, Up: Low-Level I/O Input and Output Primitives =========================== This section describes the functions for performing primitive input and output operations on file descriptors: `read', `write', and `lseek'. These functions are declared in the header file `unistd.h'. - Data Type: ssize_t This data type is used to represent the sizes of blocks that can be read or written in a single operation. It is similar to `size_t', but must be a signed type. - Fonction: ssize_t read (int FILEDES, void *BUFFER, size_t SIZE) The `read' function reads up to SIZE bytes from the file with descriptor FILEDES, storing the results in the BUFFER. (This is not necessarily a character string and there is no terminating null character added.) The return value is the number of bytes actually read. This might be less than SIZE; for example, if there aren't that many bytes left in the file or if there aren't that many bytes immediately available. The exact behavior depends on what kind of file it is. Note that reading less than SIZE bytes is not an error. A value of zero indicates end-of-file (except if the value of the SIZE argument is also zero). This is not considered an error. If you keep calling `read' while at end-of-file, it will keep returning zero and doing nothing else. If `read' returns at least one character, there is no way you can tell whether end-of-file was reached. But if you did reach the end, the next read will return zero. In case of an error, `read' returns `-1'. The following `errno' error conditions are defined for this function: `EAGAIN' Normally, when no input is immediately available, `read' waits for some input. But if the `O_NONBLOCK' flag is set for the file (*note File Status Flags::.), `read' returns immediately without reading any data, and reports this error. *Compatibility Note:* Most versions of BSD Unix use a different error code for this: `EWOULDBLOCK'. In the GNU library, `EWOULDBLOCK' is an alias for `EAGAIN', so it doesn't matter which name you use. On some systems, reading a large amount of data from a character special file can also fail with `EAGAIN' if the kernel cannot find enough physical memory to lock down the user's pages. This is limited to devices that transfer with direct memory access into the user's memory, which means it does not include terminals, since they always use separate buffers inside the kernel. This problem never happens in the GNU system. Any condition that could result in `EAGAIN' can instead result in a successful `read' which returns fewer bytes than requested. Calling `read' again immediately would result in `EAGAIN'. `EBADF' The FILEDES argument is not a valid file descriptor, or is not open for reading. `EINTR' `read' was interrupted by a signal while it was waiting for input. *Note Interrupted Primitives::. A signal will not necessary cause `read' to return `EINTR'; it may instead result in a successful `read' which returns fewer bytes than requested. `EIO' For many devices, and for disk files, this error code indicates a hardware error. `EIO' also occurs when a background process tries to read from the controlling terminal, and the normal action of stopping the process by sending it a `SIGTTIN' signal isn't working. This might happen if signal is being blocked or ignored, or because the process group is orphaned. *Note Job Control::, for more information about job control, and *Note Signal Handling::, for information about signals. The `read' function is the underlying primitive for all of the functions that read from streams, such as `fgetc'. - Fonction: ssize_t write (int FILEDES, const void *BUFFER, size_t SIZE) The `write' function writes up to SIZE bytes from BUFFER to the file with descriptor FILEDES. The data in BUFFER is not necessarily a character string and a null character is output like any other character. The return value is the number of bytes actually written. This may be SIZE, but can always be smaller. Your program should always call `write' in a loop, iterating until all the data is written. Once `write' returns, the data is enqueued to be written and can be read back right away, but it is not necessarily written out to permanent storage immediately. You can use `fsync' when you need to be sure your data has been permanently stored before continuing. (It is more efficient for the system to batch up consecutive writes and do them all at once when convenient. Normally they will always be written to disk within a minute or less.) You can use the `O_FSYNC' open mode to make `write' always store the data to disk before returning; *note Operating Modes::.. In the case of an error, `write' returns `-1'. The following `errno' error conditions are defined for this function: `EAGAIN' Normally, `write' blocks until the write operation is complete. But if the `O_NONBLOCK' flag is set for the file (*note Control Operations::.), it returns immediately without writing any data, and reports this error. An example of a situation that might cause the process to block on output is writing to a terminal device that supports flow control, where output has been suspended by receipt of a STOP character. *Compatibility Note:* Most versions of BSD Unix use a different error code for this: `EWOULDBLOCK'. In the GNU library, `EWOULDBLOCK' is an alias for `EAGAIN', so it doesn't matter which name you use. On some systems, writing a large amount of data from a character special file can also fail with `EAGAIN' if the kernel cannot find enough physical memory to lock down the user's pages. This is limited to devices that transfer with direct memory access into the user's memory, which means it does not include terminals, since they always use separate buffers inside the kernel. This problem does not arise in the GNU system. `EBADF' The FILEDES argument is not a valid file descriptor, or is not open for writing. `EFBIG' The size of the file would become larger than the implementation can support. `EINTR' The `write' operation was interrupted by a signal while it was blocked waiting for completion. A signal will not necessary cause `write' to return `EINTR'; it may instead result in a successful `write' which writes fewer bytes than requested. *Note Interrupted Primitives::. `EIO' For many devices, and for disk files, this error code indicates a hardware error. `ENOSPC' The device containing the file is full. `EPIPE' This error is returned when you try to write to a pipe or FIFO that isn't open for reading by any process. When this happens, a `SIGPIPE' signal is also sent to the process; see *Note Signal Handling::. Unless you have arranged to prevent `EINTR' failures, you should check `errno' after each failing call to `write', and if the error was `EINTR', you should simply repeat the call. *Note Interrupted Primitives::. The easy way to do this is with the macro `TEMP_FAILURE_RETRY', as follows: nbytes = TEMP_FAILURE_RETRY (write (desc, buffer, count)); The `write' function is the underlying primitive for all of the functions that write to streams, such as `fputc'.