C Prog fprintf not generating output-CodePudding

I've written the following code below and ran it without errors on both xcode and vscode. However, I wasn't able to get any output filename.txt. It wasn't in any of my folders.

Appreciate if anyone could help.

#include <stdio.h>
#include <string.h>

int main() {    

    FILE *fp=NULL;
    fp = fopen("filename.txt","w ");

    if (fp!= NULL){
    fprintf(fp,"%s %d","Hello",555);
    }

    fclose(fp);

    return 0;
}

CodePudding user response：

ran it without errors

fclose(NULL) is undefined behavior (UB), so it is not clear that there was no error when file failed to open.

Print something in both cases of opening success/failure - and with a '\n'. Useful to add error info.

Robust code checks all I/O operations.

#include <stdio.h>
#include <string.h>

int main() {    
  const char *filename = "filename.txt";
  FILE *fp = fopen(filename,"w ");

  if (fp == NULL) {
    fprintf(stderr, "Unable to open <%s>\n", filename);
    perror("fopen()");
  } else { 
    printf("Success opening <%s>\n", filename);
    if (fprintf(fp,"%s %d","Hello", 555) < 0) {
      fprintf(stderr, "Print failure with <%s>\n", filename);
      perror("fprintf()");
    }
    if (fclose(fp) == EOF) {
      fprintf(stderr, "Failed to close <%s>\n", filename);
      perror("fclose()");
    }
  }

  return 0;
}

I've also tried the perror method and it shows filename.txt: Permission denied. Later.

Check if filename.txt is read-only, or in use by another application (editor?), or other permission limitations.

CodePudding user response：

If the file wasn't successfully opened, then the code does nothing (apart from closing a null FILE-pointer, which is undefined). You should use perror() to indicate why it couldn't be opened:

    const char *const filename = "filename.txt";
    FILE *const fp = fopen(filename, "w ");
    if (!fp) {
        perror(filename);
        return EXIT_FAILURE;
    }

    fprintf(fp, "%s %d", "Hello", 555);

There's a good chance that you have an existing filename.txt that isn't writable by you, but we'll need the error message to actually determine why it wasn't opened.

Alternatively, you're running in a different working directory to where you thought you were - that's something you should investigate (perhaps produce some logging to stderr to indicate where the file is being created).

CodePudding user response：

I ran your code and it works just finecheck this image

but, how are you compiling it and did you remember to run the a.out/execution?