Function read [src]

Returns the number of bytes that were read, which can be less than buf.len. If 0 bytes were read, that means EOF. If fd is opened in non blocking mode, the function will return error.WouldBlock when EAGAIN is received. Linux has a limit on how many bytes may be transferred in one read call, which is 0x7ffff000 on both 64-bit and 32-bit systems. This is due to using a signed C int as the return value, as well as stuffing the errno codes into the last 4096 values. This is noted on the read man page. The limit on Darwin is 0x7fffffff, trying to read more than that returns EINVAL. The corresponding POSIX limit is maxInt(isize).

Prototype

pub fn read(fd: fd_t, buf: []u8) ReadError!usize

Parameters

fd: fd_tbuf: []u8

Possible Errors

AccessDenied

In WASI, this error occurs when the file descriptor does not hold the required rights to read from it.

BrokenPipe

Canceled

reading a timerfd with CANCEL_ON_SET will lead to this error when the clock goes through a discontinuous change

ConnectionResetByPeer

ConnectionTimedOut

InputOutput

IsDir

LockViolation

Unable to read file due to lock.

NotOpenForReading

OperationAborted

ProcessNotFound

This error occurs in Linux if the process to be read from no longer exists.

SocketNotConnected

SystemResources

Unexpected UnexpectedError

The Operating System returned an undocumented error code.

This error is in theory not possible, but it would be better to handle this error than to invoke undefined behavior.

When this error code is observed, it usually means the Zig Standard Library needs a small patch to add the error code to the error set for the respective function.

WouldBlock

This error occurs when no global event loop is configured, and reading from the file descriptor would block.

Source

  pub fn read(fd: fd_t, buf: []u8) ReadError!usize {
    if (buf.len == 0) return 0;
    if (native_os == .windows) {
        return windows.ReadFile(fd, buf, null);
    }
    if (native_os == .wasi and !builtin.link_libc) {
        const iovs = [1]iovec{iovec{
            .base = buf.ptr,
            .len = buf.len,
        }};

        var nread: usize = undefined;
        switch (wasi.fd_read(fd, &iovs, iovs.len, &nread)) {
            .SUCCESS => return nread,
            .INTR => unreachable,
            .INVAL => unreachable,
            .FAULT => unreachable,
            .AGAIN => unreachable,
            .BADF => return error.NotOpenForReading, // Can be a race condition.
            .IO => return error.InputOutput,
            .ISDIR => return error.IsDir,
            .NOBUFS => return error.SystemResources,
            .NOMEM => return error.SystemResources,
            .NOTCONN => return error.SocketNotConnected,
            .CONNRESET => return error.ConnectionResetByPeer,
            .TIMEDOUT => return error.ConnectionTimedOut,
            .NOTCAPABLE => return error.AccessDenied,
            else => |err| return unexpectedErrno(err),
        }
    }

    // Prevents EINVAL.
    const max_count = switch (native_os) {
        .linux => 0x7ffff000,
        .macos, .ios, .watchos, .tvos, .visionos => maxInt(i32),
        else => maxInt(isize),
    };
    while (true) {
        const rc = system.read(fd, buf.ptr, @min(buf.len, max_count));
        switch (errno(rc)) {
            .SUCCESS => return @intCast(rc),
            .INTR => continue,
            .INVAL => unreachable,
            .FAULT => unreachable,
            .NOENT => return error.ProcessNotFound,
            .AGAIN => return error.WouldBlock,
            .CANCELED => return error.Canceled,
            .BADF => return error.NotOpenForReading, // Can be a race condition.
            .IO => return error.InputOutput,
            .ISDIR => return error.IsDir,
            .NOBUFS => return error.SystemResources,
            .NOMEM => return error.SystemResources,
            .NOTCONN => return error.SocketNotConnected,
            .CONNRESET => return error.ConnectionResetByPeer,
            .TIMEDOUT => return error.ConnectionTimedOut,
            else => |err| return unexpectedErrno(err),
        }
    }
}  