The bam format contains a field for tracking the insert size of paired end alignments:

1. TLEN: signed observed Template LENgth. If all segments are mapped to the same reference, the unsigned observed template length equals the number of bases from the leftmost mapped base to the rightmost mapped base. The leftmost segment has a plus sign and the rightmost has a minus sign. The sign of segments in the middle is undefined. It is set as 0 for single-segment template or when the information is unavailable.

— from the SAM specification v1.4

To create a histogram of all template lengths below some maximal length, one could simply pipe the sam format data into a script (awk, perl, python), but for large bam files this is much too slow. A faster alternative is to use the samtools library. The following short program is also a template on how to use the high level samtools API for the task of looping through all alignments in a bam file:

#include <stdio.h>
#include "sam.h"
 
void usage(const char *msg) {
        fputs(msg, stderr);
        fputs("\nUSAGE: insert_size_hist bam_file\n", stderr);
}
 
int main(int argc, char **argv) {
        if (argc != 2) {
                usage("missing bam_file argument");
                return 2;
        }
        samfile_t *bam = samopen(argv[1], "rb", NULL);
        if (bam == NULL) {
                fprintf(stderr, "Could not open %s\n", argv[1]);
                return 2;
        }
        bam1_t *aln = bam_init1();
        int hist[1001] = {0};
        while (samread(bam, aln) >= 0) {
                const bam1_core_t *c = &aln->core;
                if ((c->flag & BAM_FPROPER_PAIR) == BAM_FPROPER_PAIR &&
                    (c->flag & BAM_FSECONDARY) != BAM_FSECONDARY &&
                    c->qual > 30 &&
                    c->isize > 0) {
                        if (c->isize > 1000) {
                                hist[1000]++;
                        } else {
                                hist[c->isize]++;
                        }
                }
        }
        int i = 0;
        for (i=0; i < 1001; i++) {
                printf("%i\t%i\n", i, hist[i]);
        }
        samclose(bam);
        return 0;
}

The bam file is opened with samopen and samread is used to load each alignment. For each alignment it checks that (1) the alignment is from a proper pair and (2) is not secondary and (3) has a mapping qual greater than 30 and (4) a positive TLEN. Together these ensure that each high quality pair alignment is considered exactly once. Since I ensured that no pairs with a tlen of greater than 1000 were reported in the bam file, the same cutoff is hardcoded in this program, though in reality this should be a configurable option given on the command line (as should the quality cutoff). The output contains a single line for each possible length from 0 to 1000 (inclusive) and the frequency with which the length was observed.