fopen a file and skip character - c

Question

Is there a clean way to open a file like this without system calls:
ID*_LogConfig.csv
I tried the following but it didn't worked.
/*Read setup file*/
getcwd(cwd, sizeof(cwd));
snprintf(source, sizeof(source),"%s/ID%*d_LogConfig.csv",cwd);
if( NULL == (input = fopen(source,"r")))
{
snprintf(errbuffer,sizeof(errbuffer), "Could not open file %s - check existence/rights\n", source);
exitHandler(1, errbuffer);
}
It outputs:
/mnt/dataflash/canfilter.d/ID*_LogConfig.csv not found
But with e.g. cat /mnt/dataflash/canfilter.d/ID*_LogConfig.csv it shows the file content.
My compromise solution would be a system call ll ID*_LogConfig.csv and using the output as filename.

Answer 1

This line
snprintf(source, sizeof(source),"%s/ID%*d_LogConfig.csv",cwd);
likely does not produce what you think it does.
The %*d portion is an format specifier with a field-width, per the POSIX printf() documentation
A field width, or precision, or both, may be indicated by an
( '*' ). In this case an argument of type int supplies the
field width or precision. Applications shall ensure that arguments
specifying field width, or precision, or both appear in that order
before the argument, if any, to be converted. A negative field width
is taken as a '-' flag followed by a positive field width. A negative
precision is taken as if the precision were omitted. In format strings
containing the "%n$" form of a conversion specification, a field width
or precision may be indicated by the sequence "*m$", where m is a
decimal integer in the range [1,{NL_ARGMAX}] giving the position in
the argument list (after the format argument) of an integer argument
containing the field width or precision, for example:
printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);
So, this line
snprintf(source, sizeof(source),"%s/ID%*d_LogConfig.csv",cwd);
expects two more integer arguments to be passed. Since you don't pass them, you invoke undefined behavior.
See this answer: https://stackoverflow.com/a/19897395/4756299

Answer 2

Is there a clean way to open a file like this without system calls
No.
fopen() makes use of a system call. You cannot 'open' a file without a system call.
If you're referring to the system(3) function, then you're probably in for some pain - it's best to avoid it if possible, from a performance, reliability and security point of view.
If you want to open 'all files that match the pattern', then look at glob(3), which is likely what your shell is using to handle such wildcards.
You will need to iterate over each of the resulting paths, calling fopen(), fread() and fclose() on each.
Example usage of glob(3):
#include <stdio.h>
#include <glob.h>
void main(void) {
int ret;
int i;
glob_t glob_info;
ret = glob("*.csv", 0, NULL, &glob_info);
if (ret != 0)
{
fprintf(stderr, "glob() failed: %d\n", ret);
return;
}
for (i = 0; i < glob_info.gl_pathc; i++) {
printf("%d: %s\n", i, glob_info.gl_pathv[i]);
}
globfree(&glob_info);
}
It is not really a good idea to open lots of files and treat the stream as a single 'thing' (as you are doing with your cat example).
As #Andrew has pointed out, you must be careful with your use of printf() format strings...
You have provided the following: %s/ID%*d_LogConfig.csv. A % denotes the beginning of a format specifier, you have thus given the following:
%s - a char * (string) parameter follows
%*d - similar to %d, but the * means that the precision is provided as an int parameter, followed by the number itself.
For example:
printf(">%s< >%*d<\n", "Hello", 5, 3);
Will output: (note the 5 characters that the %d outputs)
>Hello< > 3<
If you are after a *, then just put a * in the format string.
If you are after a %, then you need to escape the % but putting %% in the format string.

Answer 3

Ok I solved the "problem" by using the following:
(Processing the output of ls -t and use the newest file as config-file)
/*Search for config-file*/
FILE *file_config;
file_config = popen("ls -t ID*_LogConfig.csv","r");
if (file_config == NULL) {
exitHandler(1, "Error opening date pipe.");
}
fgets(configfile, sizeof(configfile), file_config);
if (strlen(configfile) > 0)
configfile[strlen(configfile)-1] = '\0';
else {
exitHandler(1, "Could not find a ID*_LogConfig.csv\n");
}
getcwd(cwd, sizeof(cwd));
snprintf(source, sizeof(source),"%s/%s",cwd,configfile);
/*Read setup file*/
if( NULL == (input = fopen(source,"r")))
{
snprintf(errbuffer,sizeof(errbuffer), "Could not open file |%s| - check existence/rights\n", source);
exitHandler(1, errbuffer);
}
It seems that this is the only simple way.
Thanks to all.