Reading parameter from file and creating filenames - c

Question

I want to read a name from a file (for example config_file.txt with only one entry like run)
and then create filenames with that, like run0.txt, run1.txt and so on.
But I get something like run..0.txt with two black dots.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXCHAR 1000
void generate(char const *fileName);
int main(int argc, char** argv) {
generate("config_file.txt");
}
void generate(char const *fileName) {
char id[MAXCHAR];
FILE *fp;
char str[MAXCHAR];
fp = fopen(fileName, "r");
if (fp == NULL) {
printf("Could not open file %s", fileName);
return 1;
}
while (fgets(str, MAXCHAR, fp) != NULL) {
strcpy(id, str);
}
fclose(fp);
FILE *filePtr;
char filename[100];
for(int i = 0;i < 8;i++){
sprintf(filename, "%s%d.txt", id,i);
filePtr = fopen(filename, "w");
}
fclose(filePtr);
}

Answer 1

As I noted in the comments, your code does not zap the newline that fgets() normally preserves as it reads lines before trying to add the extension to it.
The simple and reliable method for zapping the newline is:
str[stcspn(str, "\n")] = '\0';
There are alternatives that might be more efficient (though efficiency is probably a red herring here — creating files takes a lot longer than reading through a short line of characters), but you have to get a variety of conditions right (empty buffer, buffer with no newline, etc).
You also have:
if (fp == NULL) {
printf("Could not open file %s", fileName);
return 1;
}
You should report errors on stderr instead of stdout.
You might consider including the error number and/or error message.
You should finish the message with a newline.
You can't write return 1; in a function returning void — the compiler must complain about that.
C11 §6.8.6.4 The return statement:
¶1 A return statement with an expression shall not appear in a function whose return type is void. A return statement without an expression shall only appear in a function whose return type is void.
Hence, you should consider writing:
if (fp == NULL)
{
fprintf(stderr, "Could not open file %s for reading: %s\n", fileName, strerror(errno));
return;
}
I normally use a set of error reporting functions that I wrote, one of which tells the library the name of the program (err_setarg0(argv[0]); in main()) and the others of which produce error messages as desired. This code is available in my SOQ (Stack Overflow Questions) repository on GitHub as files stderr.c and stderr.h in the src/libsoq sub-directory.
I'd write:
if (fp == NULL)
err_syserr("failed to open file '%s' for reading: ", fileName);
The function doesn't return. If I wanted to return, I'd use err_sysrem() ('remark') and arrange a return. The sys part of the name means that the error number and message are automatically reported too. I prefer these to perror() because perror() doesn't make it easy to get the program name etc into the error message.
There are analogous libraries available on some systems — err(3) on macOS, and also available on Linux (err(3), does roughly the same job.