error while using fopen frequently - c

Question

I have this function
char* Readfiletobuffer(char* file, FILE* fp){
char * buffer;
int file_size;
fp = fopen(file, "r");
if (fp != NULL) {
fseek(fp, 0, SEEK_END);
file_size = ftell(fp);
buffer = (char*) malloc((file_size + 1) * sizeof(char));
fseek(fp, 0, SEEK_SET);
fread(buffer, file_size, 1, fp);
buffer[file_size] = '\0';
return buffer;
} else {
printf("error loading file");
}
fclose(fp);
}
which I call 1050 times in my program and at the 1019th time fopen() returns a NULL pointer.
It doesn't depend on the file, it's always the 1019th time so I think it's something with freeing memory but why isn't the fclose() call enough?
Does someone have an idea?

Answer 1

Your program can tell you with errno, the global variable where many functions assign their error code to when they fail. Combined with strerror to provide a human readable error message, you'd change your error handling to something like this.
#include <errno.h>
#include <string.h>
...
fp = fopen(file, "r");
if (fp == NULL) {
fprintf(stderr, "Could not open '%s': %s", file, strerror(errno));
exit(1);
}
fseek(fp, 0, SEEK_END);
file_size = ftell(fp);
...
Note the use of early exit to eliminate having nest the whole function in an if/else block.
Also note that you're failing to check the rest of your file operations. fseek, ftell, and fread can all fail. You need similar checks for all of them. Rather than littering your code with error handling, and probably forgetting to do it in a few places, I recommend writing little wrappers.
FILE *open_file(const char *filename, const char *mode) {
FILE *fp = fopen(filename, mode);
if( fp == NULL ) {
fprintf(
stderr, "Could not open '%s' for '%s': %s\n",
filename, mode, strerror(errno)
);
exit(1);
}
return fp;
}
Note that this isn't the best error handling, it simply exits on error. At this stage in your learning C, it's probably best to just bail out on an error. If you did something like return NULL odds are you won't have the error handling to handle a null pointer and it will just bounce around causing mysterious problems and crashes later in the code. For now it's best to halt and catch fire as close to the error as possible.
Spoiler alert: your process ran out of file handles because you're not closing your files. As #BLUEPIXY correctly points out in the comments your fclose is after you return normally and will only happen if the file fails to open.
Since you're passing in the file pointer, maybe you intend to use it later? In that case you can't hold onto that many open files and you'll have to redesign your code. If not, there's no reason to pass it in since the function is opening it.
You should have gotten a warning like this, if you had warnings turned on with -Wall.
test.c:23:1: warning: control may reach end of non-void function [-Wreturn-type]
}
If the file fails to open, nothing gets returned, and that's not ok.
Don't ignore your warnings, fix all of them. Investigating this warning would have pointed you at the problem.
Check all your file operations to make sure they succeeded.
Include strerror(errno) in your error messages so you know why it failed.
Investigate and fix all your warnings.