Convert ASCII text to uppercase.
Convert characters with toupper while avoiding signed-char undefined behavior.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <errno.h>
#include <limits.h>
int main(void) {
char text[] = "hello";
for (size_t i = 0; text[i]; ++i) text[i] = (char)toupper((unsigned char)text[i]);
printf("%s\n", text);
return 0;
}Expected output EXAMPLE
HELLO
Details
Save as snippet.c. Compile with gcc -std=c17 -Wall -Wextra snippet.c -o snippet, then run ./snippet. Uses the C standard library.
This byte-oriented example uses the C locale, not Unicode case conversion.
Tested on Ubuntu 24.04.5 LTS · · gcc (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0 · gcc 4:13.2.0-7ubuntu1
Test inputs, results and environment
The exact published code ran in a dedicated Linux test container with prepared inputs. Unprivileged snippet process, external networking disabled, read-only root filesystem, and a fresh temporary working directory. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.
- Architecture / host kernel
- x86_64 / 6.18.54.1-microsoft-standard-WSL2
- Snippet identity / locale
- uid=65534, gid=65534 (unprivileged) / C.UTF-8
- Interpreter invocation
gcc -std=c17 -Wall -Wextra -Werror snippet.c -lm -o snippet && ./snippet- Container image ID
sha256:32dc2d998158b6554c99b73b533f423696a0f3e31e8cbea9db10b5eee9a2fadd- Code SHA-256
58c0d80966fed0f98cc0916acc80c62e483afb5722224d0dd109f3cf42046aa3- Dependencies
- gcc 4:13.2.0-7ubuntu1
Test input setup
These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.
# No input files required.
Observed standard output
HELLO
Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.