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pngfix: ensure fclose happens last on file close

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This reverts the code to a variant of my old style of doing an 'fclose'
correctly and adds comments explaining why it is so difficult.  Thanks
to @ihsinme for pointing out the error on github.com

Signed-off-by: John Bowler <jbowler@acm.org>
This commit is contained in:
John Bowler 2023-12-30 11:06:34 -08:00
parent 9f9f8d8e63
commit 71b9b5e16e
1 changed files with 12 additions and 3 deletions
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15
contrib/tools/pngfix.c
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@ -1005,10 +1005,18 @@ file_end(struct file *file)
if (file->out != NULL) if (file->out != NULL)
{ {
/* NOTE: this is bitwise |, all the following functions must execute and /* On some systems 'fclose' deletes the FILE struct (making it
* must succeed. * inaccessbile). There is no guarantee that fclose returns an error
* code from fflush or, indeed, from the FILE error indicator. There is
* also no explicit (or clear) guarantee in the standard that anything
* other than a read or write operation sets the error indicator; fflush
* is not a read or write operation, so both conditions must be checked
* to ensure the close succeeded and in ANSI-C conformant code they must
* be checked before the fclose call.
*/ */
if (ferror(file->out) | fflush(file->out) | fclose(file->out)) const int err = fflush(file->out) || ferror(file->out);
if (fclose(file->out) || err)
{ {
perror(file->out_name); perror(file->out_name);
emit_error(file, READ_ERROR_CODE, "output write error"); emit_error(file, READ_ERROR_CODE, "output write error");
@ -4054,3 +4062,4 @@ main(void)
return 77; return 77;
} }
#endif /* PNG_SETJMP_SUPPORTED */ #endif /* PNG_SETJMP_SUPPORTED */
/* vi: set textwidth=80 shiftwidth=3 softtabstop=-1 expandtab: */