diff options
authorRich Felker <>2013-06-22 17:11:17 -0400
committerRich Felker <>2013-06-22 17:11:17 -0400
commitc20804500deebaabc56f383d48dd1ac77dce8349 (patch)
parenta494171a5a2778fc7b4d24d673d950f3e9864063 (diff)
fix major scanf breakage with unbuffered streams, fmemopen, etc.
the shgetc api, used internally in scanf and int/float scanning code to handle field width limiting and pushback, was designed assuming that pushback could be achieved via a simple decrement on the file buffer pointer. this only worked by chance for regular FILE streams, due to the linux readv bug workaround in __stdio_read which moves the last requested byte through the buffer rather than directly back to the caller. for unbuffered streams and streams not using __stdio_read but some other underlying read function, the first character read could be completely lost, and replaced by whatever junk happened to be in the unget buffer. to fix this, simply have shgetc, when it performs an underlying read operation on the stream, store the character read at the -1 offset from the read buffer pointer. this is valid even for unbuffered streams, as they have an unget buffer located just below the start of the zero-length buffer. the check to avoid storing the character when it is already there is to handle the possibility of read-only buffers. no application-exposed FILE types are allowed to use read-only buffers, but sscanf and strto* may use them internally when calling functions which use the shgetc api.
1 files changed, 1 insertions, 0 deletions
diff --git a/src/internal/shgetc.c b/src/internal/shgetc.c
index 96f72b6a..e878b00a 100644
--- a/src/internal/shgetc.c
+++ b/src/internal/shgetc.c
@@ -22,5 +22,6 @@ int __shgetc(FILE *f)
f->shend = f->rend;
if (f->rend) f->shcnt += f->rend - f->rpos + 1;
+ if (f->rpos[-1] != c) f->rpos[-1] = c;
return c;