variable-length-array

I am experimenting with the use of Variable Length Arrays (VLAs) in my C code and trying to iron out my understanding of what they should and shouldn't do. I have the following snippet from my function: void get_pdw_frame_usb(pdws_t *pdw_frame, pdw_io_t *pdw_io) { ... unsigned char buf[pdw_io->packet_size]; unsigned char *pdw_buf; memset(buf, '\0', sizeof(buf)); pdw_io is a data structure containing, amongst other things, packet_size , which is of type size_t the char array buf is to be used to store the contents of a usb bulk transfer packet I'm trying to instantiate it here as an automatic

C has a really cool feature called variable length arrays. Its available in C90 and above, and it allows deferring the size of the array until runtime. See GCC's manual 6.19 Arrays of Variable Length . I'm working in C++. At std=c++11 , I'm catching a compile failure due to the use of alloca under Cygwin. I want to switch to variable length arrays, if possible. I also want to try and avoid std::vector and std::array because I want to stay out of the memory manager. I believe every little bit helps, so I'm happy to take these opportunities (that some folks consider peepholes). Can I use a

I tried to write a function that returns a random array of pixel colors, so when I call randomPalette(i) , the function will create a random array of i colors. Below are my codes. It says error at random[colors] that expression must have constant value. I don't know why. How to fix it? pixel* randomPalette(int colors){ pixel random[colors] ; int i, x; srand(time(NULL)); //generate a random seed for (i = 0; i < colors; i++){ x = rand() % 256; random[i].r = x; random[i].g = x; random[i].b = x; } return random; } In your code, firstly pixel random[colors] ; syntax is called variable length array

This question already has an answer here: declaring a variable-length array as a global variable in C 4 answers Cannot compile static array with fixed size [duplicate] 1 answer If I write variable length array as a locally, like this: #include <stdio.h> int main() { int i=1; int a[i]; printf("%zu",sizeof(a)); } It working fine in GCC compiler. But, If I write Variable length array as a globally, like this: #include <stdio.h> int i=1; int a[i]; int main() { printf("%zu",sizeof(a)); } Then, GCC compiler gives following an error: prog.c:4:5: error: variably modified 'a' at file scope int a[i];

Is this undefined behavior? The relevant parts of the standard don't say much. size_t n = SIZE_MAX / sizeof(double) + 1; size_t m = sizeof(double[n]); The C standard does not explicitly state that the size_t type is sufficient for working with the sizes of all objects or types, especially for hypothetical types that are not actually instantiated. In C 2018 7.19 2, the standard says that size_t “is the unsigned integer type of the result of the sizeof operator”. That tells us about the type size_t but not about the values that may arise during computation. In 5.2.4, the standard recognizes that