[PATCH] lib/autoconf/c.m4: fix NULL pointer dereference in _AC_LANG_IO_PROGRAM

[Nicolas Iooss]

The following program crashes (with a segmentation fault) when fopen
fails:

    #include <stdio.h>
    int main () {
        FILE *f = fopen ("conftest.out", "w");
        return ferror (f) || fclose (f) != 0;
    }

This is due to ferror dereferencing a NULL pointer (using GNU C Library
2.33).

* lib/autoconf/c.m4: return an error when f is NULL.
---
 lib/autoconf/c.m4 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index 9f881f0bc10c..0d9efa62d450 100644
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -114,7 +114,7 @@ $2
 m4_define([_AC_LANG_IO_PROGRAM(C)],
 [AC_LANG_PROGRAM([@%:@include <stdio.h>],
 [FILE *f = fopen ("conftest.out", "w");
- return ferror (f) || fclose (f) != 0;
+ return !f || ferror (f) || fclose (f) != 0;
 ])])
 
 
-- 
2.30.2


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