diff options
| -rw-r--r-- | ChangeLog | 7 | ||||
| -rw-r--r-- | include/alloc_buffer.h | 31 |
2 files changed, 33 insertions, 5 deletions
diff --git a/ChangeLog b/ChangeLog index fe40b9568f..1d0ba7165c 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,10 @@ +2019-04-11 Florian Weimer <fweimer@redhat.com> + + * include/alloc_buffer.h (alloc_buffer_alloc_bytes): Update + comment. + (alloc_buffer_next): Change return type to non-const. Update + comment. + 2019-04-10 TAMUKI Shoichi <tamuki@linet.gr.jp> * manual/time.texi (Formatting Calendar Time): Add missing percent diff --git a/include/alloc_buffer.h b/include/alloc_buffer.h index f27cbb65ca..9c469b9e8b 100644 --- a/include/alloc_buffer.h +++ b/include/alloc_buffer.h @@ -183,7 +183,7 @@ alloc_buffer_add_byte (struct alloc_buffer *buf, unsigned char b) NULL is returned if there is not enough room, and the buffer is marked as failed, or if the buffer has already failed. (Zero-length allocations from an empty buffer which has not yet - failed succeed.) */ + failed succeed.) The buffer contents is not modified. */ static inline __attribute__ ((nonnull (1))) void * alloc_buffer_alloc_bytes (struct alloc_buffer *buf, size_t length) { @@ -300,11 +300,32 @@ __alloc_buffer_next (struct alloc_buffer *buf, size_t align) /* Like alloc_buffer_alloc, but do not advance the pointer beyond the object (so a subseqent call to alloc_buffer_next or alloc_buffer_alloc returns the same pointer). Note that the buffer - is still aligned according to the requirements of TYPE. The effect - of this function is similar to allocating a zero-length array from - the buffer. */ + is still aligned according to the requirements of TYPE, potentially + consuming buffer space. The effect of this function is similar to + allocating a zero-length array from the buffer. + + It is possible to use the return pointer to write to the buffer and + consume the written bytes using alloc_buffer_alloc_bytes (which + does not change the buffer contents), but the calling code needs to + perform manual length checks using alloc_buffer_size. For example, + to read as many int32_t values that are available in the input file + and can fit into the remaining buffer space, you can use this: + + int32_t array = alloc_buffer_next (buf, int32_t); + size_t ret = fread (array, sizeof (int32_t), + alloc_buffer_size (buf) / sizeof (int32_t), fp); + if (ferror (fp)) + handle_error (); + alloc_buffer_alloc_array (buf, int32_t, ret); + + The alloc_buffer_alloc_array call makes the actually-used part of + the buffer permanent. The remaining part of the buffer (not filled + with data from the file) can be used for something else. + + This manual length checking can easily introduce errors, so this + coding style is not recommended. */ #define alloc_buffer_next(buf, type) \ - ((const type *) __alloc_buffer_next \ + ((type *) __alloc_buffer_next \ (buf, __alloc_buffer_assert_align (__alignof__ (type)))) /* Internal function. Allocate an array. */ |