add optional argument to errno() for setting it, and update docs. fixes

#5514

base/error.jl

@@ -32,7 +32,8 @@ catch_backtrace() = ccall(:jl_get_backtrace, Array{Ptr{Void},1}, ())
 
 ## system error handling ##
 
-errno() = ccall(:jl_errno, Int32, ())
+errno() = ccall(:jl_errno, Cint, ())
+errno(e::Integer) = ccall(:jl_set_errno, Void, (Cint,), e)
 strerror(e::Integer) = bytestring(ccall(:strerror, Ptr{Uint8}, (Int32,), e))
 strerror() = strerror(errno())
 systemerror(p, b::Bool) = b ? throw(SystemError(string(p))) : nothing

doc/stdlib/base.rst

@@ -4844,6 +4844,18 @@ C Interface
 
    Close shared library referenced by handle.
 
+.. function:: find_library(names, locations)
+
+   Searches for the first library in ``names`` in the paths in the ``locations`` list, ``DL_LOAD_PATH``, or system
+   library paths (in that order) which can successfully be dlopen'd. On success, the return value will be one of
+   the names (potentially prefixed by one of the paths in locations). This string can be assigned to a ``global const``
+   and used as the library name in future ``ccall``'s. On failure, it returns the empty string.
+
+.. data:: DL_LOAD_PATH
+
+   When calling ``dlopen``, the paths in this list will be searched first, in order, before searching the
+   system locations for a valid library handle.
+
 .. function:: c_malloc(size::Integer)
 
    Call ``malloc`` from the C standard library.
@@ -4923,17 +4935,22 @@ C Interface
    Re-enable Ctrl-C handler during execution of a function. Temporarily
    reverses the effect of ``disable_sigint``.
 
-.. function:: find_library(names, locations)
+.. function:: errno([code])
 
-   Searches for the first library in ``names`` in the paths in the ``locations`` list, ``DL_LOAD_PATH``, or system
-   library paths (in that order) which can successfully be dlopen'd. On success, the return value will be one of
-   the names (potentially prefixed by one of the paths in locations). This string can be assigned to a ``global const``
-   and used as the library name in future ``ccall``'s. On failure, it returns the empty string.
+   Get the value of the C library's ``errno``. If an argument is specified, it is
+   used to set the value of ``errno``.
 
-.. data:: DL_LOAD_PATH
+   The value of ``errno`` is only valid immediately after a ``ccall`` to a C
+   library routine that sets it. Specifically, you cannot call ``errno`` at the next
+   prompt in a REPL, because lots of code is executed between prompts.
 
-   When calling ``dlopen``, the paths in this list will be searched first, in order, before searching the
-   system locations for a valid library handle.
+.. function:: systemerror(sysfunc, iftrue)
+
+   Raises a ``SystemError`` for ``errno`` with the descriptive string ``sysfunc`` if ``bool`` is true
+
+.. function:: strerror(n)
+
+   Convert a system call error code to a descriptive string
 
 .. data:: Cchar
 
@@ -5030,21 +5047,6 @@ Errors
    Get the backtrace of the current exception, for use within ``catch``
    blocks.
 
-.. function:: errno()
-
-   Get the value of the C library's ``errno``. As in C, ``errno()`` must be called
-   directly after a function that uses ``errno`` for error reporting. Specifically
-   you can not call ``errno`` on the next prompt in a REPL, because lots of code
-   is executed between REPL prompts.
-
-.. function:: systemerror(sysfunc, iftrue)
-
-   Raises a ``SystemError`` for ``errno`` with the descriptive string ``sysfunc`` if ``bool`` is true
-
-.. function:: strerror(n)
-
-   Convert a system call error code to a descriptive string
-
 .. function:: assert(cond, [text])
 
    Raise an error if ``cond`` is false. Also available as the macro ``@assert expr``.

src/julia.h

@@ -822,6 +822,7 @@ DLLEXPORT jl_value_t *jl_eqtable_get(jl_array_t *h, void *key, jl_value_t *deflt
 
 // system information
 DLLEXPORT int jl_errno(void);
+DLLEXPORT void jl_set_errno(int e);
 DLLEXPORT int32_t jl_stat(const char* path, char* statbuf);
 
 // environment entries

src/sys.c

@@ -297,6 +297,7 @@ void jl_free2(void *p, void *hint)
 // -- syscall utilities --
 
 int jl_errno(void) { return errno; }
+void jl_set_errno(int e) { errno = e; }
 
 // -- get the number of CPU cores --