Why enclose blocks of C code in curly braces?

Question

I am looking at some C code, and have noticed it is full of these curly braces surrounding blocks of code without any sort of control structure. Take a look-see:

//do some stuff . . . fprintf(stderr, "%.2f sec\n", (float)(clock() - t) / CLOCKS_PER_SEC); {     //a block! why not?     char *tmp_argv[3];     tmp_argv[0] = argv[0]; tmp_argv[1] = str; tmp_argv[2] = prefix;     t = clock();     fprintf(stderr, "[bwa_index] Convert nucleotide PAC to color PAC... ");     bwa_pac2cspac(3, tmp_argv);     fprintf(stderr, "%.2f sec\n", (float)(clock() - t) / CLOCKS_PER_SEC); } 

Why would you insert blocks like this in the code? It is chock full of 'em. Is there some kind of performance benefit? Some mystical C thing? Why???

edit: This code if from BWA, a bioinformatics program that aligns small sequences to large reference ones using the Burrows-Wheeler transform, in case any of you were wondering. This code example isn't particularly relevant to the functionality of the application.

Answer 1

Legacy code needed { } in order to do declarations at all

In C89, you couldn't just do int i; anywhere; declarations were only valid at the beginning of blocks.

So:

a = 1; int i; /* error */ i = 2; 

...wasn't valid, but

a = 1 if (e) {   int i; 

...was fine, as was a plain block.

The resulting style continued even after declarations became valid (C99) block-item(s), partly by inertia, partly for backwards portability, and also because it makes sense to establish a scope for new declarations.

Answer 2

To scope variables. E.g. the variable tmp_argv will only be valid between the braces.