mkstemp() and fdopen() in Cygwin 1.7.28

Question

In the course of building some C++-based code under Cygwin (1.7.28-2, 64-bit) with GNU GCC 4.8.2, I ran into the following errors: ``` ... SortDetails.cpp: In function ‘FILE* create_tmpfile(const char*, char**)’: SortDetails.cpp:127:20: error: ‘mkstemp’ was not declared in this scope fd = mkstemp(tmpl); ^ SortDetails.cpp:133:24: error: ‘fdopen’ was not declared in this scope fp = fdopen(fd, "wb+"); ... ``` The specific chunk of code that fails to compile is: ``` FILE * create_tmpfile(char const* path, char** fileName) { FILE* fp; int fd; char* tmpl; if ( path == NULL ) { fileName = NULL; return tmpfile(); } tmpl = (char*)malloc(1 + strlen(path) + L_tmpnam); strcpy(tmpl, path); strcpy(tmpl+strlen(path), "/sb.XXXXXX"); fd = mkstemp(tmpl); /* <----- here... */ if(fd == -1) { fprintf(stderr, "unable to create temp file!
"); return NULL; } fp = fdopen(fd, "wb+"); /* <----- ...and here */ *fileName = (char*)malloc(strlen(tmpl) + 1); strcpy(*fileName, tmpl); free(tmpl); return fp; } ``` (Th…

Accepted Answer

When compiling with `g++` 4.8.2 on Cygwin, I logged the expansion of macros in three cases: ``` $ g++ -std=c++11 -E -Dd foo.cpp > foo.log.c++11 $ g++ -ansi -E -Dd foo.cpp > foo.log.ansi $ g++ -E -Dd foo.cpp > foo.log.noFlag ``` Comparing the logs was useful. There were "holes" in the `-std=c++11` and `-ansi` cases, while a block containing the `mkstemp()` declaration shows up in the "flagless" case. This let me zero in on the parts of the headers that were processed differently. In the file `/usr/include/stdlib.h`, declarations of `mkstemp()` and some other functions are rejected if `__STRICT_ANSI__` is defined — such as when we use the compile-time flags `-ansi` and `-std=c++11`. Likewise, in the file `/usr/include/stdio.h`, declarations of `fdopen()` will get skipped for the same reason. The C++ headers `` and `` both include the `stdlib.h` and `stdio.h` headers and leave declaration of those two functions (among others) up to those two headers. So if we use `-ansi` and/or `-std=c++11` then those two functions will not be declared and we get the compile errors. The solution that seems to work for the toy code samples is to undefine `__STRICT_ANSI__` before compiling: ``` $ g++ -std=c++11 -U__STRICT_ANSI__ foo.cpp ``` It's not clear what the side effects of this will be, but from googling, it seems like this is a common problem and a common fix applied by other developers who need to target Cygwin.

mkstemp() and fdopen() in Cygwin 1.7.28

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